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# The Rustonomicon
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#### The Dark Arts of Advanced and Unsafe Rust Programming
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# NOTE: This is a draft document, and may contain serious errors
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> Instead of the programs I had hoped for, there came only a shuddering blackness
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and ineffable loneliness; and I saw at last a fearful truth which no one had
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ever dared to breathe before — the unwhisperable secret of secrets — The fact
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that this language of stone and stridor is not a sentient perpetuation of Rust
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as London is of Old London and Paris of Old Paris, but that it is in fact
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quite unsafe, its sprawling body imperfectly embalmed and infested with queer
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animate things which have nothing to do with it as it was in compilation.
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This book digs into all the awful details that are necessary to understand in
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order to write correct Unsafe Rust programs. Due to the nature of this problem,
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it may lead to unleashing untold horrors that shatter your psyche into a billion
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infinitesimal fragments of despair.
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Should you wish a long and happy career of writing Rust programs, you should
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turn back now and forget you ever saw this book. It is not necessary. However
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if you intend to write unsafe code -- or just want to dig into the guts of the
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language -- this book contains invaluable information.
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Unlike [The Book][trpl] we will be assuming considerable prior knowledge. In
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particular, you should be comfortable with basic systems programming and Rust.
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If you don't feel comfortable with these topics, you should consider [reading
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The Book][trpl] first. Though we will not be assuming that you have, and will
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take care to occasionally give a refresher on the basics where appropriate. You
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can skip straight to this book if you want; just know that we won't be
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explaining everything from the ground up.
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To be clear, this book goes into deep detail. We're going to dig into
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exception-safety, pointer aliasing, memory models, and even some type-theory.
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We will also be spending a lot of time talking about the different kinds
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of safety and guarantees.
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[trpl]: ../book/index.html
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@ -1,55 +0,0 @@
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# Summary
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[Introduction](README.md)
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* [Meet Safe and Unsafe](meet-safe-and-unsafe.md)
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* [How Safe and Unsafe Interact](safe-unsafe-meaning.md)
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* [Working with Unsafe](working-with-unsafe.md)
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* [Data Layout](data.md)
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* [repr(Rust)](repr-rust.md)
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* [Exotically Sized Types](exotic-sizes.md)
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* [Other reprs](other-reprs.md)
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* [Ownership](ownership.md)
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* [References](references.md)
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* [Lifetimes](lifetimes.md)
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* [Limits of Lifetimes](lifetime-mismatch.md)
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* [Lifetime Elision](lifetime-elision.md)
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* [Unbounded Lifetimes](unbounded-lifetimes.md)
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* [Higher-Rank Trait Bounds](hrtb.md)
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* [Subtyping and Variance](subtyping.md)
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* [Drop Check](dropck.md)
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* [PhantomData](phantom-data.md)
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* [Splitting Borrows](borrow-splitting.md)
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* [Type Conversions](conversions.md)
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* [Coercions](coercions.md)
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* [The Dot Operator](dot-operator.md)
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* [Casts](casts.md)
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* [Transmutes](transmutes.md)
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* [Uninitialized Memory](uninitialized.md)
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* [Checked](checked-uninit.md)
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* [Drop Flags](drop-flags.md)
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* [Unchecked](unchecked-uninit.md)
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* [Ownership Based Resource Management](obrm.md)
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* [Constructors](constructors.md)
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* [Destructors](destructors.md)
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* [Leaking](leaking.md)
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* [Unwinding](unwinding.md)
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* [Exception Safety](exception-safety.md)
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* [Poisoning](poisoning.md)
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* [Concurrency](concurrency.md)
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* [Races](races.md)
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* [Send and Sync](send-and-sync.md)
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* [Atomics](atomics.md)
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* [Implementing Vec](vec.md)
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* [Layout](vec-layout.md)
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* [Allocating](vec-alloc.md)
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* [Push and Pop](vec-push-pop.md)
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* [Deallocating](vec-dealloc.md)
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* [Deref](vec-deref.md)
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* [Insert and Remove](vec-insert-remove.md)
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* [IntoIter](vec-into-iter.md)
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* [RawVec](vec-raw.md)
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* [Drain](vec-drain.md)
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* [Handling Zero-Sized Types](vec-zsts.md)
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* [Final Code](vec-final.md)
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* [Implementing Arc and Mutex](arc-and-mutex.md)
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@ -1,7 +0,0 @@
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# Implementing Arc and Mutex
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Knowing the theory is all fine and good, but the *best* way to understand
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something is to use it. To better understand atomics and interior mutability,
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we'll be implementing versions of the standard library's Arc and Mutex types.
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TODO: ALL OF THIS OMG
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@ -1,255 +0,0 @@
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# Atomics
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Rust pretty blatantly just inherits C11's memory model for atomics. This is not
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due to this model being particularly excellent or easy to understand. Indeed,
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this model is quite complex and known to have [several flaws][C11-busted].
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Rather, it is a pragmatic concession to the fact that *everyone* is pretty bad
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at modeling atomics. At very least, we can benefit from existing tooling and
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research around C.
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Trying to fully explain the model in this book is fairly hopeless. It's defined
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in terms of madness-inducing causality graphs that require a full book to
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properly understand in a practical way. If you want all the nitty-gritty
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details, you should check out [C's specification (Section 7.17)][C11-model].
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Still, we'll try to cover the basics and some of the problems Rust developers
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face.
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The C11 memory model is fundamentally about trying to bridge the gap between the
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semantics we want, the optimizations compilers want, and the inconsistent chaos
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our hardware wants. *We* would like to just write programs and have them do
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exactly what we said but, you know, fast. Wouldn't that be great?
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# Compiler Reordering
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Compilers fundamentally want to be able to do all sorts of complicated
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transformations to reduce data dependencies and eliminate dead code. In
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particular, they may radically change the actual order of events, or make events
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never occur! If we write something like
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```rust,ignore
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x = 1;
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y = 3;
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x = 2;
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```
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The compiler may conclude that it would be best if your program did
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```rust,ignore
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x = 2;
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y = 3;
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```
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This has inverted the order of events and completely eliminated one event.
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From a single-threaded perspective this is completely unobservable: after all
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the statements have executed we are in exactly the same state. But if our
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program is multi-threaded, we may have been relying on `x` to actually be
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assigned to 1 before `y` was assigned. We would like the compiler to be
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able to make these kinds of optimizations, because they can seriously improve
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performance. On the other hand, we'd also like to be able to depend on our
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program *doing the thing we said*.
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# Hardware Reordering
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On the other hand, even if the compiler totally understood what we wanted and
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respected our wishes, our hardware might instead get us in trouble. Trouble
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comes from CPUs in the form of memory hierarchies. There is indeed a global
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shared memory space somewhere in your hardware, but from the perspective of each
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CPU core it is *so very far away* and *so very slow*. Each CPU would rather work
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with its local cache of the data and only go through all the anguish of
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talking to shared memory only when it doesn't actually have that memory in
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cache.
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After all, that's the whole point of the cache, right? If every read from the
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cache had to run back to shared memory to double check that it hadn't changed,
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what would the point be? The end result is that the hardware doesn't guarantee
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that events that occur in the same order on *one* thread, occur in the same
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order on *another* thread. To guarantee this, we must issue special instructions
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to the CPU telling it to be a bit less smart.
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For instance, say we convince the compiler to emit this logic:
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```text
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initial state: x = 0, y = 1
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THREAD 1 THREAD2
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y = 3; if x == 1 {
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x = 1; y *= 2;
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}
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```
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Ideally this program has 2 possible final states:
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* `y = 3`: (thread 2 did the check before thread 1 completed)
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* `y = 6`: (thread 2 did the check after thread 1 completed)
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However there's a third potential state that the hardware enables:
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* `y = 2`: (thread 2 saw `x = 1`, but not `y = 3`, and then overwrote `y = 3`)
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It's worth noting that different kinds of CPU provide different guarantees. It
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is common to separate hardware into two categories: strongly-ordered and weakly-
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ordered. Most notably x86/64 provides strong ordering guarantees, while ARM
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provides weak ordering guarantees. This has two consequences for concurrent
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programming:
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* Asking for stronger guarantees on strongly-ordered hardware may be cheap or
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even free because they already provide strong guarantees unconditionally.
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Weaker guarantees may only yield performance wins on weakly-ordered hardware.
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* Asking for guarantees that are too weak on strongly-ordered hardware is
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more likely to *happen* to work, even though your program is strictly
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incorrect. If possible, concurrent algorithms should be tested on
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weakly-ordered hardware.
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# Data Accesses
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The C11 memory model attempts to bridge the gap by allowing us to talk about the
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*causality* of our program. Generally, this is by establishing a *happens
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before* relationship between parts of the program and the threads that are
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running them. This gives the hardware and compiler room to optimize the program
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more aggressively where a strict happens-before relationship isn't established,
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but forces them to be more careful where one is established. The way we
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communicate these relationships are through *data accesses* and *atomic
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accesses*.
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Data accesses are the bread-and-butter of the programming world. They are
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fundamentally unsynchronized and compilers are free to aggressively optimize
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them. In particular, data accesses are free to be reordered by the compiler on
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the assumption that the program is single-threaded. The hardware is also free to
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propagate the changes made in data accesses to other threads as lazily and
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inconsistently as it wants. Most critically, data accesses are how data races
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happen. Data accesses are very friendly to the hardware and compiler, but as
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we've seen they offer *awful* semantics to try to write synchronized code with.
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Actually, that's too weak.
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**It is literally impossible to write correct synchronized code using only data
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accesses.**
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Atomic accesses are how we tell the hardware and compiler that our program is
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multi-threaded. Each atomic access can be marked with an *ordering* that
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specifies what kind of relationship it establishes with other accesses. In
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practice, this boils down to telling the compiler and hardware certain things
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they *can't* do. For the compiler, this largely revolves around re-ordering of
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instructions. For the hardware, this largely revolves around how writes are
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propagated to other threads. The set of orderings Rust exposes are:
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* Sequentially Consistent (SeqCst)
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* Release
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* Acquire
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* Relaxed
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(Note: We explicitly do not expose the C11 *consume* ordering)
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TODO: negative reasoning vs positive reasoning? TODO: "can't forget to
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synchronize"
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# Sequentially Consistent
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Sequentially Consistent is the most powerful of all, implying the restrictions
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of all other orderings. Intuitively, a sequentially consistent operation
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cannot be reordered: all accesses on one thread that happen before and after a
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SeqCst access stay before and after it. A data-race-free program that uses
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only sequentially consistent atomics and data accesses has the very nice
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property that there is a single global execution of the program's instructions
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that all threads agree on. This execution is also particularly nice to reason
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about: it's just an interleaving of each thread's individual executions. This
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does not hold if you start using the weaker atomic orderings.
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The relative developer-friendliness of sequential consistency doesn't come for
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free. Even on strongly-ordered platforms sequential consistency involves
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emitting memory fences.
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In practice, sequential consistency is rarely necessary for program correctness.
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However sequential consistency is definitely the right choice if you're not
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confident about the other memory orders. Having your program run a bit slower
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than it needs to is certainly better than it running incorrectly! It's also
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mechanically trivial to downgrade atomic operations to have a weaker
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consistency later on. Just change `SeqCst` to `Relaxed` and you're done! Of
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course, proving that this transformation is *correct* is a whole other matter.
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# Acquire-Release
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Acquire and Release are largely intended to be paired. Their names hint at their
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use case: they're perfectly suited for acquiring and releasing locks, and
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ensuring that critical sections don't overlap.
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Intuitively, an acquire access ensures that every access after it stays after
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it. However operations that occur before an acquire are free to be reordered to
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occur after it. Similarly, a release access ensures that every access before it
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stays before it. However operations that occur after a release are free to be
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reordered to occur before it.
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When thread A releases a location in memory and then thread B subsequently
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acquires *the same* location in memory, causality is established. Every write
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that happened before A's release will be observed by B after its release.
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However no causality is established with any other threads. Similarly, no
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causality is established if A and B access *different* locations in memory.
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Basic use of release-acquire is therefore simple: you acquire a location of
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memory to begin the critical section, and then release that location to end it.
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For instance, a simple spinlock might look like:
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|
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```rust
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use std::sync::Arc;
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use std::sync::atomic::{AtomicBool, Ordering};
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use std::thread;
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fn main() {
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let lock = Arc::new(AtomicBool::new(false)); // value answers "am I locked?"
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// ... distribute lock to threads somehow ...
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// Try to acquire the lock by setting it to true
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while lock.compare_and_swap(false, true, Ordering::Acquire) { }
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// broke out of the loop, so we successfully acquired the lock!
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// ... scary data accesses ...
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// ok we're done, release the lock
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lock.store(false, Ordering::Release);
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}
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```
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On strongly-ordered platforms most accesses have release or acquire semantics,
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making release and acquire often totally free. This is not the case on
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weakly-ordered platforms.
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# Relaxed
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Relaxed accesses are the absolute weakest. They can be freely re-ordered and
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provide no happens-before relationship. Still, relaxed operations are still
|
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atomic. That is, they don't count as data accesses and any read-modify-write
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operations done to them occur atomically. Relaxed operations are appropriate for
|
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things that you definitely want to happen, but don't particularly otherwise care
|
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about. For instance, incrementing a counter can be safely done by multiple
|
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threads using a relaxed `fetch_add` if you're not using the counter to
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synchronize any other accesses.
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|
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There's rarely a benefit in making an operation relaxed on strongly-ordered
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platforms, since they usually provide release-acquire semantics anyway. However
|
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relaxed operations can be cheaper on weakly-ordered platforms.
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|
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|
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|
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|
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|
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[C11-busted]: http://plv.mpi-sws.org/c11comp/popl15.pdf
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[C11-model]: http://www.open-std.org/jtc1/sc22/wg14/www/standards.html#9899
|
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@ -1,291 +0,0 @@
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# Splitting Borrows
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|
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The mutual exclusion property of mutable references can be very limiting when
|
||||
working with a composite structure. The borrow checker understands some basic
|
||||
stuff, but will fall over pretty easily. It does understand structs
|
||||
sufficiently to know that it's possible to borrow disjoint fields of a struct
|
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simultaneously. So this works today:
|
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|
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```rust
|
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struct Foo {
|
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a: i32,
|
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b: i32,
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c: i32,
|
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}
|
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|
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let mut x = Foo {a: 0, b: 0, c: 0};
|
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let a = &mut x.a;
|
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let b = &mut x.b;
|
||||
let c = &x.c;
|
||||
*b += 1;
|
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let c2 = &x.c;
|
||||
*a += 10;
|
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println!("{} {} {} {}", a, b, c, c2);
|
||||
```
|
||||
|
||||
However borrowck doesn't understand arrays or slices in any way, so this doesn't
|
||||
work:
|
||||
|
||||
```rust,ignore
|
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let mut x = [1, 2, 3];
|
||||
let a = &mut x[0];
|
||||
let b = &mut x[1];
|
||||
println!("{} {}", a, b);
|
||||
```
|
||||
|
||||
```text
|
||||
<anon>:4:14: 4:18 error: cannot borrow `x[..]` as mutable more than once at a time
|
||||
<anon>:4 let b = &mut x[1];
|
||||
^~~~
|
||||
<anon>:3:14: 3:18 note: previous borrow of `x[..]` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x[..]` until the borrow ends
|
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<anon>:3 let a = &mut x[0];
|
||||
^~~~
|
||||
<anon>:6:2: 6:2 note: previous borrow ends here
|
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<anon>:1 fn main() {
|
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<anon>:2 let mut x = [1, 2, 3];
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<anon>:3 let a = &mut x[0];
|
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<anon>:4 let b = &mut x[1];
|
||||
<anon>:5 println!("{} {}", a, b);
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<anon>:6 }
|
||||
^
|
||||
error: aborting due to 2 previous errors
|
||||
```
|
||||
|
||||
While it was plausible that borrowck could understand this simple case, it's
|
||||
pretty clearly hopeless for borrowck to understand disjointness in general
|
||||
container types like a tree, especially if distinct keys actually *do* map
|
||||
to the same value.
|
||||
|
||||
In order to "teach" borrowck that what we're doing is ok, we need to drop down
|
||||
to unsafe code. For instance, mutable slices expose a `split_at_mut` function
|
||||
that consumes the slice and returns two mutable slices. One for everything to
|
||||
the left of the index, and one for everything to the right. Intuitively we know
|
||||
this is safe because the slices don't overlap, and therefore alias. However
|
||||
the implementation requires some unsafety:
|
||||
|
||||
```rust,ignore
|
||||
fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
|
||||
let len = self.len();
|
||||
let ptr = self.as_mut_ptr();
|
||||
assert!(mid <= len);
|
||||
unsafe {
|
||||
(from_raw_parts_mut(ptr, mid),
|
||||
from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
This is actually a bit subtle. So as to avoid ever making two `&mut`'s to the
|
||||
same value, we explicitly construct brand-new slices through raw pointers.
|
||||
|
||||
However more subtle is how iterators that yield mutable references work.
|
||||
The iterator trait is defined as follows:
|
||||
|
||||
```rust
|
||||
trait Iterator {
|
||||
type Item;
|
||||
|
||||
fn next(&mut self) -> Option<Self::Item>;
|
||||
}
|
||||
```
|
||||
|
||||
Given this definition, Self::Item has *no* connection to `self`. This means that
|
||||
we can call `next` several times in a row, and hold onto all the results
|
||||
*concurrently*. This is perfectly fine for by-value iterators, which have
|
||||
exactly these semantics. It's also actually fine for shared references, as they
|
||||
admit arbitrarily many references to the same thing (although the iterator needs
|
||||
to be a separate object from the thing being shared).
|
||||
|
||||
But mutable references make this a mess. At first glance, they might seem
|
||||
completely incompatible with this API, as it would produce multiple mutable
|
||||
references to the same object!
|
||||
|
||||
However it actually *does* work, exactly because iterators are one-shot objects.
|
||||
Everything an IterMut yields will be yielded at most once, so we don't
|
||||
actually ever yield multiple mutable references to the same piece of data.
|
||||
|
||||
Perhaps surprisingly, mutable iterators don't require unsafe code to be
|
||||
implemented for many types!
|
||||
|
||||
For instance here's a singly linked list:
|
||||
|
||||
```rust
|
||||
# fn main() {}
|
||||
type Link<T> = Option<Box<Node<T>>>;
|
||||
|
||||
struct Node<T> {
|
||||
elem: T,
|
||||
next: Link<T>,
|
||||
}
|
||||
|
||||
pub struct LinkedList<T> {
|
||||
head: Link<T>,
|
||||
}
|
||||
|
||||
pub struct IterMut<'a, T: 'a>(Option<&'a mut Node<T>>);
|
||||
|
||||
impl<T> LinkedList<T> {
|
||||
fn iter_mut(&mut self) -> IterMut<T> {
|
||||
IterMut(self.head.as_mut().map(|node| &mut **node))
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> Iterator for IterMut<'a, T> {
|
||||
type Item = &'a mut T;
|
||||
|
||||
fn next(&mut self) -> Option<Self::Item> {
|
||||
self.0.take().map(|node| {
|
||||
self.0 = node.next.as_mut().map(|node| &mut **node);
|
||||
&mut node.elem
|
||||
})
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Here's a mutable slice:
|
||||
|
||||
```rust
|
||||
# fn main() {}
|
||||
use std::mem;
|
||||
|
||||
pub struct IterMut<'a, T: 'a>(&'a mut[T]);
|
||||
|
||||
impl<'a, T> Iterator for IterMut<'a, T> {
|
||||
type Item = &'a mut T;
|
||||
|
||||
fn next(&mut self) -> Option<Self::Item> {
|
||||
let slice = mem::replace(&mut self.0, &mut []);
|
||||
if slice.is_empty() { return None; }
|
||||
|
||||
let (l, r) = slice.split_at_mut(1);
|
||||
self.0 = r;
|
||||
l.get_mut(0)
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
|
||||
fn next_back(&mut self) -> Option<Self::Item> {
|
||||
let slice = mem::replace(&mut self.0, &mut []);
|
||||
if slice.is_empty() { return None; }
|
||||
|
||||
let new_len = slice.len() - 1;
|
||||
let (l, r) = slice.split_at_mut(new_len);
|
||||
self.0 = l;
|
||||
r.get_mut(0)
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And here's a binary tree:
|
||||
|
||||
```rust
|
||||
# fn main() {}
|
||||
use std::collections::VecDeque;
|
||||
|
||||
type Link<T> = Option<Box<Node<T>>>;
|
||||
|
||||
struct Node<T> {
|
||||
elem: T,
|
||||
left: Link<T>,
|
||||
right: Link<T>,
|
||||
}
|
||||
|
||||
pub struct Tree<T> {
|
||||
root: Link<T>,
|
||||
}
|
||||
|
||||
struct NodeIterMut<'a, T: 'a> {
|
||||
elem: Option<&'a mut T>,
|
||||
left: Option<&'a mut Node<T>>,
|
||||
right: Option<&'a mut Node<T>>,
|
||||
}
|
||||
|
||||
enum State<'a, T: 'a> {
|
||||
Elem(&'a mut T),
|
||||
Node(&'a mut Node<T>),
|
||||
}
|
||||
|
||||
pub struct IterMut<'a, T: 'a>(VecDeque<NodeIterMut<'a, T>>);
|
||||
|
||||
impl<T> Tree<T> {
|
||||
pub fn iter_mut(&mut self) -> IterMut<T> {
|
||||
let mut deque = VecDeque::new();
|
||||
self.root.as_mut().map(|root| deque.push_front(root.iter_mut()));
|
||||
IterMut(deque)
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Node<T> {
|
||||
pub fn iter_mut(&mut self) -> NodeIterMut<T> {
|
||||
NodeIterMut {
|
||||
elem: Some(&mut self.elem),
|
||||
left: self.left.as_mut().map(|node| &mut **node),
|
||||
right: self.right.as_mut().map(|node| &mut **node),
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
impl<'a, T> Iterator for NodeIterMut<'a, T> {
|
||||
type Item = State<'a, T>;
|
||||
|
||||
fn next(&mut self) -> Option<Self::Item> {
|
||||
match self.left.take() {
|
||||
Some(node) => Some(State::Node(node)),
|
||||
None => match self.elem.take() {
|
||||
Some(elem) => Some(State::Elem(elem)),
|
||||
None => match self.right.take() {
|
||||
Some(node) => Some(State::Node(node)),
|
||||
None => None,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> DoubleEndedIterator for NodeIterMut<'a, T> {
|
||||
fn next_back(&mut self) -> Option<Self::Item> {
|
||||
match self.right.take() {
|
||||
Some(node) => Some(State::Node(node)),
|
||||
None => match self.elem.take() {
|
||||
Some(elem) => Some(State::Elem(elem)),
|
||||
None => match self.left.take() {
|
||||
Some(node) => Some(State::Node(node)),
|
||||
None => None,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> Iterator for IterMut<'a, T> {
|
||||
type Item = &'a mut T;
|
||||
fn next(&mut self) -> Option<Self::Item> {
|
||||
loop {
|
||||
match self.0.front_mut().and_then(|node_it| node_it.next()) {
|
||||
Some(State::Elem(elem)) => return Some(elem),
|
||||
Some(State::Node(node)) => self.0.push_front(node.iter_mut()),
|
||||
None => if let None = self.0.pop_front() { return None },
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
|
||||
fn next_back(&mut self) -> Option<Self::Item> {
|
||||
loop {
|
||||
match self.0.back_mut().and_then(|node_it| node_it.next_back()) {
|
||||
Some(State::Elem(elem)) => return Some(elem),
|
||||
Some(State::Node(node)) => self.0.push_back(node.iter_mut()),
|
||||
None => if let None = self.0.pop_back() { return None },
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
All of these are completely safe and work on stable Rust! This ultimately
|
||||
falls out of the simple struct case we saw before: Rust understands that you
|
||||
can safely split a mutable reference into subfields. We can then encode
|
||||
permanently consuming a reference via Options (or in the case of slices,
|
||||
replacing with an empty slice).
|
|
@ -1,71 +0,0 @@
|
|||
# Casts
|
||||
|
||||
Casts are a superset of coercions: every coercion can be explicitly
|
||||
invoked via a cast. However some conversions require a cast.
|
||||
While coercions are pervasive and largely harmless, these "true casts"
|
||||
are rare and potentially dangerous. As such, casts must be explicitly invoked
|
||||
using the `as` keyword: `expr as Type`.
|
||||
|
||||
True casts generally revolve around raw pointers and the primitive numeric
|
||||
types. Even though they're dangerous, these casts are infallible at runtime.
|
||||
If a cast triggers some subtle corner case no indication will be given that
|
||||
this occurred. The cast will simply succeed. That said, casts must be valid
|
||||
at the type level, or else they will be prevented statically. For instance,
|
||||
`7u8 as bool` will not compile.
|
||||
|
||||
That said, casts aren't `unsafe` because they generally can't violate memory
|
||||
safety *on their own*. For instance, converting an integer to a raw pointer can
|
||||
very easily lead to terrible things. However the act of creating the pointer
|
||||
itself is safe, because actually using a raw pointer is already marked as
|
||||
`unsafe`.
|
||||
|
||||
Here's an exhaustive list of all the true casts. For brevity, we will use `*`
|
||||
to denote either a `*const` or `*mut`, and `integer` to denote any integral
|
||||
primitive:
|
||||
|
||||
* `*T as *U` where `T, U: Sized`
|
||||
* `*T as *U` TODO: explain unsized situation
|
||||
* `*T as integer`
|
||||
* `integer as *T`
|
||||
* `number as number`
|
||||
* `C-like-enum as integer`
|
||||
* `bool as integer`
|
||||
* `char as integer`
|
||||
* `u8 as char`
|
||||
* `&[T; n] as *const T`
|
||||
* `fn as *T` where `T: Sized`
|
||||
* `fn as integer`
|
||||
|
||||
Note that lengths are not adjusted when casting raw slices -
|
||||
`*const [u16] as *const [u8]` creates a slice that only includes
|
||||
half of the original memory.
|
||||
|
||||
Casting is not transitive, that is, even if `e as U1 as U2` is a valid
|
||||
expression, `e as U2` is not necessarily so.
|
||||
|
||||
For numeric casts, there are quite a few cases to consider:
|
||||
|
||||
* casting between two integers of the same size (e.g. i32 -> u32) is a no-op
|
||||
* casting from a larger integer to a smaller integer (e.g. u32 -> u8) will
|
||||
truncate
|
||||
* casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
|
||||
* zero-extend if the source is unsigned
|
||||
* sign-extend if the source is signed
|
||||
* casting from a float to an integer will round the float towards zero
|
||||
* **[NOTE: currently this will cause Undefined Behavior if the rounded
|
||||
value cannot be represented by the target integer type][float-int]**.
|
||||
This includes Inf and NaN. This is a bug and will be fixed.
|
||||
* casting from an integer to float will produce the floating point
|
||||
representation of the integer, rounded if necessary (rounding strategy
|
||||
unspecified)
|
||||
* casting from an f32 to an f64 is perfect and lossless
|
||||
* casting from an f64 to an f32 will produce the closest possible value
|
||||
(rounding strategy unspecified)
|
||||
* **[NOTE: currently this will cause Undefined Behavior if the value
|
||||
is finite but larger or smaller than the largest or smallest finite
|
||||
value representable by f32][float-float]**. This is a bug and will
|
||||
be fixed.
|
||||
|
||||
|
||||
[float-int]: https://github.com/rust-lang/rust/issues/10184
|
||||
[float-float]: https://github.com/rust-lang/rust/issues/15536
|
|
@ -1 +0,0 @@
|
|||
# Chapter 1
|
|
@ -1,117 +0,0 @@
|
|||
# Checked Uninitialized Memory
|
||||
|
||||
Like C, all stack variables in Rust are uninitialized until a value is
|
||||
explicitly assigned to them. Unlike C, Rust statically prevents you from ever
|
||||
reading them until you do:
|
||||
|
||||
```rust,ignore
|
||||
fn main() {
|
||||
let x: i32;
|
||||
println!("{}", x);
|
||||
}
|
||||
```
|
||||
|
||||
```text
|
||||
src/main.rs:3:20: 3:21 error: use of possibly uninitialized variable: `x`
|
||||
src/main.rs:3 println!("{}", x);
|
||||
^
|
||||
```
|
||||
|
||||
This is based off of a basic branch analysis: every branch must assign a value
|
||||
to `x` before it is first used. Interestingly, Rust doesn't require the variable
|
||||
to be mutable to perform a delayed initialization if every branch assigns
|
||||
exactly once. However the analysis does not take advantage of constant analysis
|
||||
or anything like that. So this compiles:
|
||||
|
||||
```rust
|
||||
fn main() {
|
||||
let x: i32;
|
||||
|
||||
if true {
|
||||
x = 1;
|
||||
} else {
|
||||
x = 2;
|
||||
}
|
||||
|
||||
println!("{}", x);
|
||||
}
|
||||
```
|
||||
|
||||
but this doesn't:
|
||||
|
||||
```rust,ignore
|
||||
fn main() {
|
||||
let x: i32;
|
||||
if true {
|
||||
x = 1;
|
||||
}
|
||||
println!("{}", x);
|
||||
}
|
||||
```
|
||||
|
||||
```text
|
||||
src/main.rs:6:17: 6:18 error: use of possibly uninitialized variable: `x`
|
||||
src/main.rs:6 println!("{}", x);
|
||||
```
|
||||
|
||||
while this does:
|
||||
|
||||
```rust
|
||||
fn main() {
|
||||
let x: i32;
|
||||
if true {
|
||||
x = 1;
|
||||
println!("{}", x);
|
||||
}
|
||||
// Don't care that there are branches where it's not initialized
|
||||
// since we don't use the value in those branches
|
||||
}
|
||||
```
|
||||
|
||||
Of course, while the analysis doesn't consider actual values, it does
|
||||
have a relatively sophisticated understanding of dependencies and control
|
||||
flow. For instance, this works:
|
||||
|
||||
```rust
|
||||
let x: i32;
|
||||
|
||||
loop {
|
||||
// Rust doesn't understand that this branch will be taken unconditionally,
|
||||
// because it relies on actual values.
|
||||
if true {
|
||||
// But it does understand that it will only be taken once because
|
||||
// we unconditionally break out of it. Therefore `x` doesn't
|
||||
// need to be marked as mutable.
|
||||
x = 0;
|
||||
break;
|
||||
}
|
||||
}
|
||||
// It also knows that it's impossible to get here without reaching the break.
|
||||
// And therefore that `x` must be initialized here!
|
||||
println!("{}", x);
|
||||
```
|
||||
|
||||
If a value is moved out of a variable, that variable becomes logically
|
||||
uninitialized if the type of the value isn't Copy. That is:
|
||||
|
||||
```rust
|
||||
fn main() {
|
||||
let x = 0;
|
||||
let y = Box::new(0);
|
||||
let z1 = x; // x is still valid because i32 is Copy
|
||||
let z2 = y; // y is now logically uninitialized because Box isn't Copy
|
||||
}
|
||||
```
|
||||
|
||||
However reassigning `y` in this example *would* require `y` to be marked as
|
||||
mutable, as a Safe Rust program could observe that the value of `y` changed:
|
||||
|
||||
```rust
|
||||
fn main() {
|
||||
let mut y = Box::new(0);
|
||||
let z = y; // y is now logically uninitialized because Box isn't Copy
|
||||
y = Box::new(1); // reinitialize y
|
||||
}
|
||||
```
|
||||
|
||||
Otherwise it's like `y` is a brand new variable.
|
|
@ -1,72 +0,0 @@
|
|||
# Coercions
|
||||
|
||||
Types can implicitly be coerced to change in certain contexts. These changes are
|
||||
generally just *weakening* of types, largely focused around pointers and
|
||||
lifetimes. They mostly exist to make Rust "just work" in more cases, and are
|
||||
largely harmless.
|
||||
|
||||
Here's all the kinds of coercion:
|
||||
|
||||
Coercion is allowed between the following types:
|
||||
|
||||
* Transitivity: `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to
|
||||
`T_3`
|
||||
* Pointer Weakening:
|
||||
* `&mut T` to `&T`
|
||||
* `*mut T` to `*const T`
|
||||
* `&T` to `*const T`
|
||||
* `&mut T` to `*mut T`
|
||||
* Unsizing: `T` to `U` if `T` implements `CoerceUnsized<U>`
|
||||
* Deref coercion: Expression `&x` of type `&T` to `&*x` of type `&U` if `T` derefs to `U` (i.e. `T: Deref<Target=U>`)
|
||||
|
||||
`CoerceUnsized<Pointer<U>> for Pointer<T> where T: Unsize<U>` is implemented
|
||||
for all pointer types (including smart pointers like Box and Rc). Unsize is
|
||||
only implemented automatically, and enables the following transformations:
|
||||
|
||||
* `[T; n]` => `[T]`
|
||||
* `T` => `Trait` where `T: Trait`
|
||||
* `Foo<..., T, ...>` => `Foo<..., U, ...>` where:
|
||||
* `T: Unsize<U>`
|
||||
* `Foo` is a struct
|
||||
* Only the last field of `Foo` has type involving `T`
|
||||
* `T` is not part of the type of any other fields
|
||||
* `Bar<T>: Unsize<Bar<U>>`, if the last field of `Foo` has type `Bar<T>`
|
||||
|
||||
Coercions occur at a *coercion site*. Any location that is explicitly typed
|
||||
will cause a coercion to its type. If inference is necessary, the coercion will
|
||||
not be performed. Exhaustively, the coercion sites for an expression `e` to
|
||||
type `U` are:
|
||||
|
||||
* let statements, statics, and consts: `let x: U = e`
|
||||
* Arguments to functions: `takes_a_U(e)`
|
||||
* Any expression that will be returned: `fn foo() -> U { e }`
|
||||
* Struct literals: `Foo { some_u: e }`
|
||||
* Array literals: `let x: [U; 10] = [e, ..]`
|
||||
* Tuple literals: `let x: (U, ..) = (e, ..)`
|
||||
* The last expression in a block: `let x: U = { ..; e }`
|
||||
|
||||
Note that we do not perform coercions when matching traits (except for
|
||||
receivers, see below). If there is an impl for some type `U` and `T` coerces to
|
||||
`U`, that does not constitute an implementation for `T`. For example, the
|
||||
following will not type check, even though it is OK to coerce `t` to `&T` and
|
||||
there is an impl for `&T`:
|
||||
|
||||
```rust,ignore
|
||||
trait Trait {}
|
||||
|
||||
fn foo<X: Trait>(t: X) {}
|
||||
|
||||
impl<'a> Trait for &'a i32 {}
|
||||
|
||||
|
||||
fn main() {
|
||||
let t: &mut i32 = &mut 0;
|
||||
foo(t);
|
||||
}
|
||||
```
|
||||
|
||||
```text
|
||||
<anon>:10:5: 10:8 error: the trait bound `&mut i32 : Trait` is not satisfied [E0277]
|
||||
<anon>:10 foo(t);
|
||||
^~~
|
||||
```
|
|
@ -1,13 +0,0 @@
|
|||
# Concurrency and Parallelism
|
||||
|
||||
Rust as a language doesn't *really* have an opinion on how to do concurrency or
|
||||
parallelism. The standard library exposes OS threads and blocking sys-calls
|
||||
because everyone has those, and they're uniform enough that you can provide
|
||||
an abstraction over them in a relatively uncontroversial way. Message passing,
|
||||
green threads, and async APIs are all diverse enough that any abstraction over
|
||||
them tends to involve trade-offs that we weren't willing to commit to for 1.0.
|
||||
|
||||
However the way Rust models concurrency makes it relatively easy to design your own
|
||||
concurrency paradigm as a library and have everyone else's code Just Work
|
||||
with yours. Just require the right lifetimes and Send and Sync where appropriate
|
||||
and you're off to the races. Or rather, off to the... not... having... races.
|
|
@ -1,59 +0,0 @@
|
|||
# Constructors
|
||||
|
||||
There is exactly one way to create an instance of a user-defined type: name it,
|
||||
and initialize all its fields at once:
|
||||
|
||||
```rust
|
||||
struct Foo {
|
||||
a: u8,
|
||||
b: u32,
|
||||
c: bool,
|
||||
}
|
||||
|
||||
enum Bar {
|
||||
X(u32),
|
||||
Y(bool),
|
||||
}
|
||||
|
||||
struct Unit;
|
||||
|
||||
let foo = Foo { a: 0, b: 1, c: false };
|
||||
let bar = Bar::X(0);
|
||||
let empty = Unit;
|
||||
```
|
||||
|
||||
That's it. Every other way you make an instance of a type is just calling a
|
||||
totally vanilla function that does some stuff and eventually bottoms out to The
|
||||
One True Constructor.
|
||||
|
||||
Unlike C++, Rust does not come with a slew of built-in kinds of constructor.
|
||||
There are no Copy, Default, Assignment, Move, or whatever constructors. The
|
||||
reasons for this are varied, but it largely boils down to Rust's philosophy of
|
||||
*being explicit*.
|
||||
|
||||
Move constructors are meaningless in Rust because we don't enable types to
|
||||
"care" about their location in memory. Every type must be ready for it to be
|
||||
blindly memcopied to somewhere else in memory. This means pure on-the-stack-but-
|
||||
still-movable intrusive linked lists are simply not happening in Rust (safely).
|
||||
|
||||
Assignment and copy constructors similarly don't exist because move semantics
|
||||
are the only semantics in Rust. At most `x = y` just moves the bits of y into
|
||||
the x variable. Rust does provide two facilities for providing C++'s copy-
|
||||
oriented semantics: `Copy` and `Clone`. Clone is our moral equivalent of a copy
|
||||
constructor, but it's never implicitly invoked. You have to explicitly call
|
||||
`clone` on an element you want to be cloned. Copy is a special case of Clone
|
||||
where the implementation is just "copy the bits". Copy types *are* implicitly
|
||||
cloned whenever they're moved, but because of the definition of Copy this just
|
||||
means not treating the old copy as uninitialized -- a no-op.
|
||||
|
||||
While Rust provides a `Default` trait for specifying the moral equivalent of a
|
||||
default constructor, it's incredibly rare for this trait to be used. This is
|
||||
because variables [aren't implicitly initialized][uninit]. Default is basically
|
||||
only useful for generic programming. In concrete contexts, a type will provide a
|
||||
static `new` method for any kind of "default" constructor. This has no relation
|
||||
to `new` in other languages and has no special meaning. It's just a naming
|
||||
convention.
|
||||
|
||||
TODO: talk about "placement new"?
|
||||
|
||||
[uninit]: uninitialized.html
|
|
@ -1,34 +0,0 @@
|
|||
# Type Conversions
|
||||
|
||||
At the end of the day, everything is just a pile of bits somewhere, and type
|
||||
systems are just there to help us use those bits right. There are two common
|
||||
problems with typing bits: needing to reinterpret those exact bits as a
|
||||
different type, and needing to change the bits to have equivalent meaning for
|
||||
a different type. Because Rust encourages encoding important properties in the
|
||||
type system, these problems are incredibly pervasive. As such, Rust
|
||||
consequently gives you several ways to solve them.
|
||||
|
||||
First we'll look at the ways that Safe Rust gives you to reinterpret values.
|
||||
The most trivial way to do this is to just destructure a value into its
|
||||
constituent parts and then build a new type out of them. e.g.
|
||||
|
||||
```rust
|
||||
struct Foo {
|
||||
x: u32,
|
||||
y: u16,
|
||||
}
|
||||
|
||||
struct Bar {
|
||||
a: u32,
|
||||
b: u16,
|
||||
}
|
||||
|
||||
fn reinterpret(foo: Foo) -> Bar {
|
||||
let Foo { x, y } = foo;
|
||||
Bar { a: x, b: y }
|
||||
}
|
||||
```
|
||||
|
||||
But this is, at best, annoying. For common conversions, Rust provides
|
||||
more ergonomic alternatives.
|
||||
|
|
@ -1,5 +0,0 @@
|
|||
# Data Representation in Rust
|
||||
|
||||
Low-level programming cares a lot about data layout. It's a big deal. It also
|
||||
pervasively influences the rest of the language, so we're going to start by
|
||||
digging into how data is represented in Rust.
|
|
@ -1,190 +0,0 @@
|
|||
# Destructors
|
||||
|
||||
What the language *does* provide is full-blown automatic destructors through the
|
||||
`Drop` trait, which provides the following method:
|
||||
|
||||
```rust,ignore
|
||||
fn drop(&mut self);
|
||||
```
|
||||
|
||||
This method gives the type time to somehow finish what it was doing.
|
||||
|
||||
**After `drop` is run, Rust will recursively try to drop all of the fields
|
||||
of `self`.**
|
||||
|
||||
This is a convenience feature so that you don't have to write "destructor
|
||||
boilerplate" to drop children. If a struct has no special logic for being
|
||||
dropped other than dropping its children, then it means `Drop` doesn't need to
|
||||
be implemented at all!
|
||||
|
||||
**There is no stable way to prevent this behavior in Rust 1.0.**
|
||||
|
||||
Note that taking `&mut self` means that even if you could suppress recursive
|
||||
Drop, Rust will prevent you from e.g. moving fields out of self. For most types,
|
||||
this is totally fine.
|
||||
|
||||
For instance, a custom implementation of `Box` might write `Drop` like this:
|
||||
|
||||
```rust
|
||||
#![feature(alloc, heap_api, unique)]
|
||||
|
||||
extern crate alloc;
|
||||
|
||||
use std::ptr::{drop_in_place, Unique};
|
||||
use std::mem;
|
||||
|
||||
use alloc::heap;
|
||||
|
||||
struct Box<T>{ ptr: Unique<T> }
|
||||
|
||||
impl<T> Drop for Box<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
drop_in_place(*self.ptr);
|
||||
heap::deallocate((*self.ptr) as *mut u8,
|
||||
mem::size_of::<T>(),
|
||||
mem::align_of::<T>());
|
||||
}
|
||||
}
|
||||
}
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
and this works fine because when Rust goes to drop the `ptr` field it just sees
|
||||
a [Unique] that has no actual `Drop` implementation. Similarly nothing can
|
||||
use-after-free the `ptr` because when drop exits, it becomes inaccessible.
|
||||
|
||||
However this wouldn't work:
|
||||
|
||||
```rust
|
||||
#![feature(alloc, heap_api, unique)]
|
||||
|
||||
extern crate alloc;
|
||||
|
||||
use std::ptr::{drop_in_place, Unique};
|
||||
use std::mem;
|
||||
|
||||
use alloc::heap;
|
||||
|
||||
struct Box<T>{ ptr: Unique<T> }
|
||||
|
||||
impl<T> Drop for Box<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
drop_in_place(*self.ptr);
|
||||
heap::deallocate((*self.ptr) as *mut u8,
|
||||
mem::size_of::<T>(),
|
||||
mem::align_of::<T>());
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
struct SuperBox<T> { my_box: Box<T> }
|
||||
|
||||
impl<T> Drop for SuperBox<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
// Hyper-optimized: deallocate the box's contents for it
|
||||
// without `drop`ing the contents
|
||||
heap::deallocate((*self.my_box.ptr) as *mut u8,
|
||||
mem::size_of::<T>(),
|
||||
mem::align_of::<T>());
|
||||
}
|
||||
}
|
||||
}
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
After we deallocate the `box`'s ptr in SuperBox's destructor, Rust will
|
||||
happily proceed to tell the box to Drop itself and everything will blow up with
|
||||
use-after-frees and double-frees.
|
||||
|
||||
Note that the recursive drop behavior applies to all structs and enums
|
||||
regardless of whether they implement Drop. Therefore something like
|
||||
|
||||
```rust
|
||||
struct Boxy<T> {
|
||||
data1: Box<T>,
|
||||
data2: Box<T>,
|
||||
info: u32,
|
||||
}
|
||||
```
|
||||
|
||||
will have its data1 and data2's fields destructors whenever it "would" be
|
||||
dropped, even though it itself doesn't implement Drop. We say that such a type
|
||||
*needs Drop*, even though it is not itself Drop.
|
||||
|
||||
Similarly,
|
||||
|
||||
```rust
|
||||
enum Link {
|
||||
Next(Box<Link>),
|
||||
None,
|
||||
}
|
||||
```
|
||||
|
||||
will have its inner Box field dropped if and only if an instance stores the
|
||||
Next variant.
|
||||
|
||||
In general this works really nicely because you don't need to worry about
|
||||
adding/removing drops when you refactor your data layout. Still there's
|
||||
certainly many valid usecases for needing to do trickier things with
|
||||
destructors.
|
||||
|
||||
The classic safe solution to overriding recursive drop and allowing moving out
|
||||
of Self during `drop` is to use an Option:
|
||||
|
||||
```rust
|
||||
#![feature(alloc, heap_api, unique)]
|
||||
|
||||
extern crate alloc;
|
||||
|
||||
use std::ptr::{drop_in_place, Unique};
|
||||
use std::mem;
|
||||
|
||||
use alloc::heap;
|
||||
|
||||
struct Box<T>{ ptr: Unique<T> }
|
||||
|
||||
impl<T> Drop for Box<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
drop_in_place(*self.ptr);
|
||||
heap::deallocate((*self.ptr) as *mut u8,
|
||||
mem::size_of::<T>(),
|
||||
mem::align_of::<T>());
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
struct SuperBox<T> { my_box: Option<Box<T>> }
|
||||
|
||||
impl<T> Drop for SuperBox<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
// Hyper-optimized: deallocate the box's contents for it
|
||||
// without `drop`ing the contents. Need to set the `box`
|
||||
// field as `None` to prevent Rust from trying to Drop it.
|
||||
let my_box = self.my_box.take().unwrap();
|
||||
heap::deallocate((*my_box.ptr) as *mut u8,
|
||||
mem::size_of::<T>(),
|
||||
mem::align_of::<T>());
|
||||
mem::forget(my_box);
|
||||
}
|
||||
}
|
||||
}
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
However this has fairly odd semantics: you're saying that a field that *should*
|
||||
always be Some *may* be None, just because that happens in the destructor. Of
|
||||
course this conversely makes a lot of sense: you can call arbitrary methods on
|
||||
self during the destructor, and this should prevent you from ever doing so after
|
||||
deinitializing the field. Not that it will prevent you from producing any other
|
||||
arbitrarily invalid state in there.
|
||||
|
||||
On balance this is an ok choice. Certainly what you should reach for by default.
|
||||
However, in the future we expect there to be a first-class way to announce that
|
||||
a field shouldn't be automatically dropped.
|
||||
|
||||
[Unique]: phantom-data.html
|
|
@ -1,6 +0,0 @@
|
|||
# The Dot Operator
|
||||
|
||||
The dot operator will perform a lot of magic to convert types. It will perform
|
||||
auto-referencing, auto-dereferencing, and coercion until types match.
|
||||
|
||||
TODO: steal information from http://stackoverflow.com/questions/28519997/what-are-rusts-exact-auto-dereferencing-rules/28552082#28552082
|
|
@ -1,83 +0,0 @@
|
|||
# Drop Flags
|
||||
|
||||
The examples in the previous section introduce an interesting problem for Rust.
|
||||
We have seen that it's possible to conditionally initialize, deinitialize, and
|
||||
reinitialize locations of memory totally safely. For Copy types, this isn't
|
||||
particularly notable since they're just a random pile of bits. However types
|
||||
with destructors are a different story: Rust needs to know whether to call a
|
||||
destructor whenever a variable is assigned to, or a variable goes out of scope.
|
||||
How can it do this with conditional initialization?
|
||||
|
||||
Note that this is not a problem that all assignments need worry about. In
|
||||
particular, assigning through a dereference unconditionally drops, and assigning
|
||||
in a `let` unconditionally doesn't drop:
|
||||
|
||||
```
|
||||
let mut x = Box::new(0); // let makes a fresh variable, so never need to drop
|
||||
let y = &mut x;
|
||||
*y = Box::new(1); // Deref assumes the referent is initialized, so always drops
|
||||
```
|
||||
|
||||
This is only a problem when overwriting a previously initialized variable or
|
||||
one of its subfields.
|
||||
|
||||
It turns out that Rust actually tracks whether a type should be dropped or not
|
||||
*at runtime*. As a variable becomes initialized and uninitialized, a *drop flag*
|
||||
for that variable is toggled. When a variable might need to be dropped, this
|
||||
flag is evaluated to determine if it should be dropped.
|
||||
|
||||
Of course, it is often the case that a value's initialization state can be
|
||||
statically known at every point in the program. If this is the case, then the
|
||||
compiler can theoretically generate more efficient code! For instance, straight-
|
||||
line code has such *static drop semantics*:
|
||||
|
||||
```rust
|
||||
let mut x = Box::new(0); // x was uninit; just overwrite.
|
||||
let mut y = x; // y was uninit; just overwrite and make x uninit.
|
||||
x = Box::new(0); // x was uninit; just overwrite.
|
||||
y = x; // y was init; Drop y, overwrite it, and make x uninit!
|
||||
// y goes out of scope; y was init; Drop y!
|
||||
// x goes out of scope; x was uninit; do nothing.
|
||||
```
|
||||
|
||||
Similarly, branched code where all branches have the same behavior with respect
|
||||
to initialization has static drop semantics:
|
||||
|
||||
```rust
|
||||
# let condition = true;
|
||||
let mut x = Box::new(0); // x was uninit; just overwrite.
|
||||
if condition {
|
||||
drop(x) // x gets moved out; make x uninit.
|
||||
} else {
|
||||
println!("{}", x);
|
||||
drop(x) // x gets moved out; make x uninit.
|
||||
}
|
||||
x = Box::new(0); // x was uninit; just overwrite.
|
||||
// x goes out of scope; x was init; Drop x!
|
||||
```
|
||||
|
||||
However code like this *requires* runtime information to correctly Drop:
|
||||
|
||||
```rust
|
||||
# let condition = true;
|
||||
let x;
|
||||
if condition {
|
||||
x = Box::new(0); // x was uninit; just overwrite.
|
||||
println!("{}", x);
|
||||
}
|
||||
// x goes out of scope; x might be uninit;
|
||||
// check the flag!
|
||||
```
|
||||
|
||||
Of course, in this case it's trivial to retrieve static drop semantics:
|
||||
|
||||
```rust
|
||||
# let condition = true;
|
||||
if condition {
|
||||
let x = Box::new(0);
|
||||
println!("{}", x);
|
||||
}
|
||||
```
|
||||
|
||||
The drop flags are tracked on the stack and no longer stashed in types that
|
||||
implement drop.
|
|
@ -1,295 +0,0 @@
|
|||
# Drop Check
|
||||
|
||||
We have seen how lifetimes provide us some fairly simple rules for ensuring
|
||||
that we never read dangling references. However up to this point we have only ever
|
||||
interacted with the *outlives* relationship in an inclusive manner. That is,
|
||||
when we talked about `'a: 'b`, it was ok for `'a` to live *exactly* as long as
|
||||
`'b`. At first glance, this seems to be a meaningless distinction. Nothing ever
|
||||
gets dropped at the same time as another, right? This is why we used the
|
||||
following desugaring of `let` statements:
|
||||
|
||||
```rust,ignore
|
||||
let x;
|
||||
let y;
|
||||
```
|
||||
|
||||
```rust,ignore
|
||||
{
|
||||
let x;
|
||||
{
|
||||
let y;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Each creates its own scope, clearly establishing that one drops before the
|
||||
other. However, what if we do the following?
|
||||
|
||||
```rust,ignore
|
||||
let (x, y) = (vec![], vec![]);
|
||||
```
|
||||
|
||||
Does either value strictly outlive the other? The answer is in fact *no*,
|
||||
neither value strictly outlives the other. Of course, one of x or y will be
|
||||
dropped before the other, but the actual order is not specified. Tuples aren't
|
||||
special in this regard; composite structures just don't guarantee their
|
||||
destruction order as of Rust 1.0.
|
||||
|
||||
We *could* specify this for the fields of built-in composites like tuples and
|
||||
structs. However, what about something like Vec? Vec has to manually drop its
|
||||
elements via pure-library code. In general, anything that implements Drop has
|
||||
a chance to fiddle with its innards during its final death knell. Therefore
|
||||
the compiler can't sufficiently reason about the actual destruction order
|
||||
of the contents of any type that implements Drop.
|
||||
|
||||
So why do we care? We care because if the type system isn't careful, it could
|
||||
accidentally make dangling pointers. Consider the following simple program:
|
||||
|
||||
```rust
|
||||
struct Inspector<'a>(&'a u8);
|
||||
|
||||
fn main() {
|
||||
let (inspector, days);
|
||||
days = Box::new(1);
|
||||
inspector = Inspector(&days);
|
||||
}
|
||||
```
|
||||
|
||||
This program is totally sound and compiles today. The fact that `days` does
|
||||
not *strictly* outlive `inspector` doesn't matter. As long as the `inspector`
|
||||
is alive, so is days.
|
||||
|
||||
However if we add a destructor, the program will no longer compile!
|
||||
|
||||
```rust,ignore
|
||||
struct Inspector<'a>(&'a u8);
|
||||
|
||||
impl<'a> Drop for Inspector<'a> {
|
||||
fn drop(&mut self) {
|
||||
println!("I was only {} days from retirement!", self.0);
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
let (inspector, days);
|
||||
days = Box::new(1);
|
||||
inspector = Inspector(&days);
|
||||
// Let's say `days` happens to get dropped first.
|
||||
// Then when Inspector is dropped, it will try to read free'd memory!
|
||||
}
|
||||
```
|
||||
|
||||
```text
|
||||
<anon>:12:28: 12:32 error: `days` does not live long enough
|
||||
<anon>:12 inspector = Inspector(&days);
|
||||
^~~~
|
||||
<anon>:9:11: 15:2 note: reference must be valid for the block at 9:10...
|
||||
<anon>:9 fn main() {
|
||||
<anon>:10 let (inspector, days);
|
||||
<anon>:11 days = Box::new(1);
|
||||
<anon>:12 inspector = Inspector(&days);
|
||||
<anon>:13 // Let's say `days` happens to get dropped first.
|
||||
<anon>:14 // Then when Inspector is dropped, it will try to read free'd memory!
|
||||
...
|
||||
<anon>:10:27: 15:2 note: ...but borrowed value is only valid for the block suffix following statement 0 at 10:26
|
||||
<anon>:10 let (inspector, days);
|
||||
<anon>:11 days = Box::new(1);
|
||||
<anon>:12 inspector = Inspector(&days);
|
||||
<anon>:13 // Let's say `days` happens to get dropped first.
|
||||
<anon>:14 // Then when Inspector is dropped, it will try to read free'd memory!
|
||||
<anon>:15 }
|
||||
```
|
||||
|
||||
Implementing Drop lets the Inspector execute some arbitrary code during its
|
||||
death. This means it can potentially observe that types that are supposed to
|
||||
live as long as it does actually were destroyed first.
|
||||
|
||||
Interestingly, only generic types need to worry about this. If they aren't
|
||||
generic, then the only lifetimes they can harbor are `'static`, which will truly
|
||||
live *forever*. This is why this problem is referred to as *sound generic drop*.
|
||||
Sound generic drop is enforced by the *drop checker*. As of this writing, some
|
||||
of the finer details of how the drop checker validates types is totally up in
|
||||
the air. However The Big Rule is the subtlety that we have focused on this whole
|
||||
section:
|
||||
|
||||
**For a generic type to soundly implement drop, its generics arguments must
|
||||
strictly outlive it.**
|
||||
|
||||
Obeying this rule is (usually) necessary to satisfy the borrow
|
||||
checker; obeying it is sufficient but not necessary to be
|
||||
sound. That is, if your type obeys this rule then it's definitely
|
||||
sound to drop.
|
||||
|
||||
The reason that it is not always necessary to satisfy the above rule
|
||||
is that some Drop implementations will not access borrowed data even
|
||||
though their type gives them the capability for such access.
|
||||
|
||||
For example, this variant of the above `Inspector` example will never
|
||||
access borrowed data:
|
||||
|
||||
```rust,ignore
|
||||
struct Inspector<'a>(&'a u8, &'static str);
|
||||
|
||||
impl<'a> Drop for Inspector<'a> {
|
||||
fn drop(&mut self) {
|
||||
println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
let (inspector, days);
|
||||
days = Box::new(1);
|
||||
inspector = Inspector(&days, "gadget");
|
||||
// Let's say `days` happens to get dropped first.
|
||||
// Even when Inspector is dropped, its destructor will not access the
|
||||
// borrowed `days`.
|
||||
}
|
||||
```
|
||||
|
||||
Likewise, this variant will also never access borrowed data:
|
||||
|
||||
```rust,ignore
|
||||
use std::fmt;
|
||||
|
||||
struct Inspector<T: fmt::Display>(T, &'static str);
|
||||
|
||||
impl<T: fmt::Display> Drop for Inspector<T> {
|
||||
fn drop(&mut self) {
|
||||
println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
let (inspector, days): (Inspector<&u8>, Box<u8>);
|
||||
days = Box::new(1);
|
||||
inspector = Inspector(&days, "gadget");
|
||||
// Let's say `days` happens to get dropped first.
|
||||
// Even when Inspector is dropped, its destructor will not access the
|
||||
// borrowed `days`.
|
||||
}
|
||||
```
|
||||
|
||||
However, *both* of the above variants are rejected by the borrow
|
||||
checker during the analysis of `fn main`, saying that `days` does not
|
||||
live long enough.
|
||||
|
||||
The reason is that the borrow checking analysis of `main` does not
|
||||
know about the internals of each Inspector's Drop implementation. As
|
||||
far as the borrow checker knows while it is analyzing `main`, the body
|
||||
of an inspector's destructor might access that borrowed data.
|
||||
|
||||
Therefore, the drop checker forces all borrowed data in a value to
|
||||
strictly outlive that value.
|
||||
|
||||
# An Escape Hatch
|
||||
|
||||
The precise rules that govern drop checking may be less restrictive in
|
||||
the future.
|
||||
|
||||
The current analysis is deliberately conservative and trivial; it forces all
|
||||
borrowed data in a value to outlive that value, which is certainly sound.
|
||||
|
||||
Future versions of the language may make the analysis more precise, to
|
||||
reduce the number of cases where sound code is rejected as unsafe.
|
||||
This would help address cases such as the two Inspectors above that
|
||||
know not to inspect during destruction.
|
||||
|
||||
In the meantime, there is an unstable attribute that one can use to
|
||||
assert (unsafely) that a generic type's destructor is *guaranteed* to
|
||||
not access any expired data, even if its type gives it the capability
|
||||
to do so.
|
||||
|
||||
That attribute is called `may_dangle` and was introduced in [RFC 1327]
|
||||
(https://github.com/rust-lang/rfcs/blob/master/text/1327-dropck-param-eyepatch.md).
|
||||
To deploy it on the Inspector example from above, we would write:
|
||||
|
||||
```rust,ignore
|
||||
struct Inspector<'a>(&'a u8, &'static str);
|
||||
|
||||
unsafe impl<#[may_dangle] 'a> Drop for Inspector<'a> {
|
||||
fn drop(&mut self) {
|
||||
println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Use of this attribute requires the `Drop` impl to be marked `unsafe` because the
|
||||
compiler is not checking the implicit assertion that no potentially expired data
|
||||
(e.g. `self.0` above) is accessed.
|
||||
|
||||
The attribute can be applied to any number of lifetime and type parameters. In
|
||||
the following example, we assert that we access no data behind a reference of
|
||||
lifetime `'b` and that the only uses of `T` will be moves or drops, but omit
|
||||
the attribute from `'a` and `U`, because we do access data with that lifetime
|
||||
and that type:
|
||||
|
||||
```rust,ignore
|
||||
use std::fmt::Display;
|
||||
|
||||
struct Inspector<'a, 'b, T, U: Display>(&'a u8, &'b u8, T, U);
|
||||
|
||||
unsafe impl<'a, #[may_dangle] 'b, #[may_dangle] T, U: Display> Drop for Inspector<'a, 'b, T, U> {
|
||||
fn drop(&mut self) {
|
||||
println!("Inspector({}, _, _, {})", self.0, self.3);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
It is sometimes obvious that no such access can occur, like the case above.
|
||||
However, when dealing with a generic type parameter, such access can
|
||||
occur indirectly. Examples of such indirect access are:
|
||||
|
||||
* invoking a callback,
|
||||
* via a trait method call.
|
||||
|
||||
(Future changes to the language, such as impl specialization, may add
|
||||
other avenues for such indirect access.)
|
||||
|
||||
Here is an example of invoking a callback:
|
||||
|
||||
```rust,ignore
|
||||
struct Inspector<T>(T, &'static str, Box<for <'r> fn(&'r T) -> String>);
|
||||
|
||||
impl<T> Drop for Inspector<T> {
|
||||
fn drop(&mut self) {
|
||||
// The `self.2` call could access a borrow e.g. if `T` is `&'a _`.
|
||||
println!("Inspector({}, {}) unwittingly inspects expired data.",
|
||||
(self.2)(&self.0), self.1);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Here is an example of a trait method call:
|
||||
|
||||
```rust,ignore
|
||||
use std::fmt;
|
||||
|
||||
struct Inspector<T: fmt::Display>(T, &'static str);
|
||||
|
||||
impl<T: fmt::Display> Drop for Inspector<T> {
|
||||
fn drop(&mut self) {
|
||||
// There is a hidden call to `<T as Display>::fmt` below, which
|
||||
// could access a borrow e.g. if `T` is `&'a _`
|
||||
println!("Inspector({}, {}) unwittingly inspects expired data.",
|
||||
self.0, self.1);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And of course, all of these accesses could be further hidden within
|
||||
some other method invoked by the destructor, rather than being written
|
||||
directly within it.
|
||||
|
||||
In all of the above cases where the `&'a u8` is accessed in the
|
||||
destructor, adding the `#[may_dangle]`
|
||||
attribute makes the type vulnerable to misuse that the borrower
|
||||
checker will not catch, inviting havoc. It is better to avoid adding
|
||||
the attribute.
|
||||
|
||||
# Is that all about drop checker?
|
||||
|
||||
It turns out that when writing unsafe code, we generally don't need to
|
||||
worry at all about doing the right thing for the drop checker. However there
|
||||
is one special case that you need to worry about, which we will look at in
|
||||
the next section.
|
||||
|
|
@ -1,217 +0,0 @@
|
|||
# Exception Safety
|
||||
|
||||
Although programs should use unwinding sparingly, there's a lot of code that
|
||||
*can* panic. If you unwrap a None, index out of bounds, or divide by 0, your
|
||||
program will panic. On debug builds, every arithmetic operation can panic
|
||||
if it overflows. Unless you are very careful and tightly control what code runs,
|
||||
pretty much everything can unwind, and you need to be ready for it.
|
||||
|
||||
Being ready for unwinding is often referred to as *exception safety*
|
||||
in the broader programming world. In Rust, there are two levels of exception
|
||||
safety that one may concern themselves with:
|
||||
|
||||
* In unsafe code, we *must* be exception safe to the point of not violating
|
||||
memory safety. We'll call this *minimal* exception safety.
|
||||
|
||||
* In safe code, it is *good* to be exception safe to the point of your program
|
||||
doing the right thing. We'll call this *maximal* exception safety.
|
||||
|
||||
As is the case in many places in Rust, Unsafe code must be ready to deal with
|
||||
bad Safe code when it comes to unwinding. Code that transiently creates
|
||||
unsound states must be careful that a panic does not cause that state to be
|
||||
used. Generally this means ensuring that only non-panicking code is run while
|
||||
these states exist, or making a guard that cleans up the state in the case of
|
||||
a panic. This does not necessarily mean that the state a panic witnesses is a
|
||||
fully coherent state. We need only guarantee that it's a *safe* state.
|
||||
|
||||
Most Unsafe code is leaf-like, and therefore fairly easy to make exception-safe.
|
||||
It controls all the code that runs, and most of that code can't panic. However
|
||||
it is not uncommon for Unsafe code to work with arrays of temporarily
|
||||
uninitialized data while repeatedly invoking caller-provided code. Such code
|
||||
needs to be careful and consider exception safety.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
## Vec::push_all
|
||||
|
||||
`Vec::push_all` is a temporary hack to get extending a Vec by a slice reliably
|
||||
efficient without specialization. Here's a simple implementation:
|
||||
|
||||
```rust,ignore
|
||||
impl<T: Clone> Vec<T> {
|
||||
fn push_all(&mut self, to_push: &[T]) {
|
||||
self.reserve(to_push.len());
|
||||
unsafe {
|
||||
// can't overflow because we just reserved this
|
||||
self.set_len(self.len() + to_push.len());
|
||||
|
||||
for (i, x) in to_push.iter().enumerate() {
|
||||
self.ptr().offset(i as isize).write(x.clone());
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
We bypass `push` in order to avoid redundant capacity and `len` checks on the
|
||||
Vec that we definitely know has capacity. The logic is totally correct, except
|
||||
there's a subtle problem with our code: it's not exception-safe! `set_len`,
|
||||
`offset`, and `write` are all fine; `clone` is the panic bomb we over-looked.
|
||||
|
||||
Clone is completely out of our control, and is totally free to panic. If it
|
||||
does, our function will exit early with the length of the Vec set too large. If
|
||||
the Vec is looked at or dropped, uninitialized memory will be read!
|
||||
|
||||
The fix in this case is fairly simple. If we want to guarantee that the values
|
||||
we *did* clone are dropped, we can set the `len` every loop iteration. If we
|
||||
just want to guarantee that uninitialized memory can't be observed, we can set
|
||||
the `len` after the loop.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
## BinaryHeap::sift_up
|
||||
|
||||
Bubbling an element up a heap is a bit more complicated than extending a Vec.
|
||||
The pseudocode is as follows:
|
||||
|
||||
```text
|
||||
bubble_up(heap, index):
|
||||
while index != 0 && heap[index] < heap[parent(index)]:
|
||||
heap.swap(index, parent(index))
|
||||
index = parent(index)
|
||||
|
||||
```
|
||||
|
||||
A literal transcription of this code to Rust is totally fine, but has an annoying
|
||||
performance characteristic: the `self` element is swapped over and over again
|
||||
uselessly. We would rather have the following:
|
||||
|
||||
```text
|
||||
bubble_up(heap, index):
|
||||
let elem = heap[index]
|
||||
while index != 0 && elem < heap[parent(index)]:
|
||||
heap[index] = heap[parent(index)]
|
||||
index = parent(index)
|
||||
heap[index] = elem
|
||||
```
|
||||
|
||||
This code ensures that each element is copied as little as possible (it is in
|
||||
fact necessary that elem be copied twice in general). However it now exposes
|
||||
some exception safety trouble! At all times, there exists two copies of one
|
||||
value. If we panic in this function something will be double-dropped.
|
||||
Unfortunately, we also don't have full control of the code: that comparison is
|
||||
user-defined!
|
||||
|
||||
Unlike Vec, the fix isn't as easy here. One option is to break the user-defined
|
||||
code and the unsafe code into two separate phases:
|
||||
|
||||
```text
|
||||
bubble_up(heap, index):
|
||||
let end_index = index;
|
||||
while end_index != 0 && heap[end_index] < heap[parent(end_index)]:
|
||||
end_index = parent(end_index)
|
||||
|
||||
let elem = heap[index]
|
||||
while index != end_index:
|
||||
heap[index] = heap[parent(index)]
|
||||
index = parent(index)
|
||||
heap[index] = elem
|
||||
```
|
||||
|
||||
If the user-defined code blows up, that's no problem anymore, because we haven't
|
||||
actually touched the state of the heap yet. Once we do start messing with the
|
||||
heap, we're working with only data and functions that we trust, so there's no
|
||||
concern of panics.
|
||||
|
||||
Perhaps you're not happy with this design. Surely it's cheating! And we have
|
||||
to do the complex heap traversal *twice*! Alright, let's bite the bullet. Let's
|
||||
intermix untrusted and unsafe code *for reals*.
|
||||
|
||||
If Rust had `try` and `finally` like in Java, we could do the following:
|
||||
|
||||
```text
|
||||
bubble_up(heap, index):
|
||||
let elem = heap[index]
|
||||
try:
|
||||
while index != 0 && elem < heap[parent(index)]:
|
||||
heap[index] = heap[parent(index)]
|
||||
index = parent(index)
|
||||
finally:
|
||||
heap[index] = elem
|
||||
```
|
||||
|
||||
The basic idea is simple: if the comparison panics, we just toss the loose
|
||||
element in the logically uninitialized index and bail out. Anyone who observes
|
||||
the heap will see a potentially *inconsistent* heap, but at least it won't
|
||||
cause any double-drops! If the algorithm terminates normally, then this
|
||||
operation happens to coincide precisely with the how we finish up regardless.
|
||||
|
||||
Sadly, Rust has no such construct, so we're going to need to roll our own! The
|
||||
way to do this is to store the algorithm's state in a separate struct with a
|
||||
destructor for the "finally" logic. Whether we panic or not, that destructor
|
||||
will run and clean up after us.
|
||||
|
||||
```rust,ignore
|
||||
struct Hole<'a, T: 'a> {
|
||||
data: &'a mut [T],
|
||||
/// `elt` is always `Some` from new until drop.
|
||||
elt: Option<T>,
|
||||
pos: usize,
|
||||
}
|
||||
|
||||
impl<'a, T> Hole<'a, T> {
|
||||
fn new(data: &'a mut [T], pos: usize) -> Self {
|
||||
unsafe {
|
||||
let elt = ptr::read(&data[pos]);
|
||||
Hole {
|
||||
data: data,
|
||||
elt: Some(elt),
|
||||
pos: pos,
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
fn pos(&self) -> usize { self.pos }
|
||||
|
||||
fn removed(&self) -> &T { self.elt.as_ref().unwrap() }
|
||||
|
||||
unsafe fn get(&self, index: usize) -> &T { &self.data[index] }
|
||||
|
||||
unsafe fn move_to(&mut self, index: usize) {
|
||||
let index_ptr: *const _ = &self.data[index];
|
||||
let hole_ptr = &mut self.data[self.pos];
|
||||
ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
|
||||
self.pos = index;
|
||||
}
|
||||
}
|
||||
|
||||
impl<'a, T> Drop for Hole<'a, T> {
|
||||
fn drop(&mut self) {
|
||||
// fill the hole again
|
||||
unsafe {
|
||||
let pos = self.pos;
|
||||
ptr::write(&mut self.data[pos], self.elt.take().unwrap());
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T: Ord> BinaryHeap<T> {
|
||||
fn sift_up(&mut self, pos: usize) {
|
||||
unsafe {
|
||||
// Take out the value at `pos` and create a hole.
|
||||
let mut hole = Hole::new(&mut self.data, pos);
|
||||
|
||||
while hole.pos() != 0 {
|
||||
let parent = parent(hole.pos());
|
||||
if hole.removed() <= hole.get(parent) { break }
|
||||
hole.move_to(parent);
|
||||
}
|
||||
// Hole will be unconditionally filled here; panic or not!
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,137 +0,0 @@
|
|||
# Exotically Sized Types
|
||||
|
||||
Most of the time, we think in terms of types with a fixed, positive size. This
|
||||
is not always the case, however.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
# Dynamically Sized Types (DSTs)
|
||||
|
||||
Rust in fact supports Dynamically Sized Types (DSTs): types without a statically
|
||||
known size or alignment. On the surface, this is a bit nonsensical: Rust *must*
|
||||
know the size and alignment of something in order to correctly work with it! In
|
||||
this regard, DSTs are not normal types. Due to their lack of a statically known
|
||||
size, these types can only exist behind some kind of pointer. Any pointer to a
|
||||
DST consequently becomes a *fat* pointer consisting of the pointer and the
|
||||
information that "completes" them (more on this below).
|
||||
|
||||
There are two major DSTs exposed by the language: trait objects, and slices.
|
||||
|
||||
A trait object represents some type that implements the traits it specifies.
|
||||
The exact original type is *erased* in favor of runtime reflection
|
||||
with a vtable containing all the information necessary to use the type.
|
||||
This is the information that completes a trait object: a pointer to its vtable.
|
||||
|
||||
A slice is simply a view into some contiguous storage -- typically an array or
|
||||
`Vec`. The information that completes a slice is just the number of elements
|
||||
it points to.
|
||||
|
||||
Structs can actually store a single DST directly as their last field, but this
|
||||
makes them a DST as well:
|
||||
|
||||
```rust
|
||||
// Can't be stored on the stack directly
|
||||
struct Foo {
|
||||
info: u32,
|
||||
data: [u8],
|
||||
}
|
||||
```
|
||||
|
||||
**NOTE: [As of Rust 1.0 struct DSTs are broken if the last field has
|
||||
a variable position based on its alignment][dst-issue].**
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
# Zero Sized Types (ZSTs)
|
||||
|
||||
Rust actually allows types to be specified that occupy no space:
|
||||
|
||||
```rust
|
||||
struct Foo; // No fields = no size
|
||||
|
||||
// All fields have no size = no size
|
||||
struct Baz {
|
||||
foo: Foo,
|
||||
qux: (), // empty tuple has no size
|
||||
baz: [u8; 0], // empty array has no size
|
||||
}
|
||||
```
|
||||
|
||||
On their own, Zero Sized Types (ZSTs) are, for obvious reasons, pretty useless.
|
||||
However as with many curious layout choices in Rust, their potential is realized
|
||||
in a generic context: Rust largely understands that any operation that produces
|
||||
or stores a ZST can be reduced to a no-op. First off, storing it doesn't even
|
||||
make sense -- it doesn't occupy any space. Also there's only one value of that
|
||||
type, so anything that loads it can just produce it from the aether -- which is
|
||||
also a no-op since it doesn't occupy any space.
|
||||
|
||||
One of the most extreme example's of this is Sets and Maps. Given a
|
||||
`Map<Key, Value>`, it is common to implement a `Set<Key>` as just a thin wrapper
|
||||
around `Map<Key, UselessJunk>`. In many languages, this would necessitate
|
||||
allocating space for UselessJunk and doing work to store and load UselessJunk
|
||||
only to discard it. Proving this unnecessary would be a difficult analysis for
|
||||
the compiler.
|
||||
|
||||
However in Rust, we can just say that `Set<Key> = Map<Key, ()>`. Now Rust
|
||||
statically knows that every load and store is useless, and no allocation has any
|
||||
size. The result is that the monomorphized code is basically a custom
|
||||
implementation of a HashSet with none of the overhead that HashMap would have to
|
||||
support values.
|
||||
|
||||
Safe code need not worry about ZSTs, but *unsafe* code must be careful about the
|
||||
consequence of types with no size. In particular, pointer offsets are no-ops,
|
||||
and standard allocators (including jemalloc, the one used by default in Rust)
|
||||
may return `nullptr` when a zero-sized allocation is requested, which is
|
||||
indistinguishable from out of memory.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
# Empty Types
|
||||
|
||||
Rust also enables types to be declared that *cannot even be instantiated*. These
|
||||
types can only be talked about at the type level, and never at the value level.
|
||||
Empty types can be declared by specifying an enum with no variants:
|
||||
|
||||
```rust
|
||||
enum Void {} // No variants = EMPTY
|
||||
```
|
||||
|
||||
Empty types are even more marginal than ZSTs. The primary motivating example for
|
||||
Void types is type-level unreachability. For instance, suppose an API needs to
|
||||
return a Result in general, but a specific case actually is infallible. It's
|
||||
actually possible to communicate this at the type level by returning a
|
||||
`Result<T, Void>`. Consumers of the API can confidently unwrap such a Result
|
||||
knowing that it's *statically impossible* for this value to be an `Err`, as
|
||||
this would require providing a value of type `Void`.
|
||||
|
||||
In principle, Rust can do some interesting analyses and optimizations based
|
||||
on this fact. For instance, `Result<T, Void>` could be represented as just `T`,
|
||||
because the `Err` case doesn't actually exist. The following *could* also
|
||||
compile:
|
||||
|
||||
```rust,ignore
|
||||
enum Void {}
|
||||
|
||||
let res: Result<u32, Void> = Ok(0);
|
||||
|
||||
// Err doesn't exist anymore, so Ok is actually irrefutable.
|
||||
let Ok(num) = res;
|
||||
```
|
||||
|
||||
But neither of these tricks work today, so all Void types get you is
|
||||
the ability to be confident that certain situations are statically impossible.
|
||||
|
||||
One final subtle detail about empty types is that raw pointers to them are
|
||||
actually valid to construct, but dereferencing them is Undefined Behavior
|
||||
because that doesn't actually make sense. That is, you could model C's `void *`
|
||||
type with `*const Void`, but this doesn't necessarily gain anything over using
|
||||
e.g. `*const ()`, which *is* safe to randomly dereference.
|
||||
|
||||
|
||||
[dst-issue]: https://github.com/rust-lang/rust/issues/26403
|
|
@ -1,73 +0,0 @@
|
|||
# Higher-Rank Trait Bounds (HRTBs)
|
||||
|
||||
Rust's `Fn` traits are a little bit magic. For instance, we can write the
|
||||
following code:
|
||||
|
||||
```rust
|
||||
struct Closure<F> {
|
||||
data: (u8, u16),
|
||||
func: F,
|
||||
}
|
||||
|
||||
impl<F> Closure<F>
|
||||
where F: Fn(&(u8, u16)) -> &u8,
|
||||
{
|
||||
fn call(&self) -> &u8 {
|
||||
(self.func)(&self.data)
|
||||
}
|
||||
}
|
||||
|
||||
fn do_it(data: &(u8, u16)) -> &u8 { &data.0 }
|
||||
|
||||
fn main() {
|
||||
let clo = Closure { data: (0, 1), func: do_it };
|
||||
println!("{}", clo.call());
|
||||
}
|
||||
```
|
||||
|
||||
If we try to naively desugar this code in the same way that we did in the
|
||||
lifetimes section, we run into some trouble:
|
||||
|
||||
```rust,ignore
|
||||
struct Closure<F> {
|
||||
data: (u8, u16),
|
||||
func: F,
|
||||
}
|
||||
|
||||
impl<F> Closure<F>
|
||||
// where F: Fn(&'??? (u8, u16)) -> &'??? u8,
|
||||
{
|
||||
fn call<'a>(&'a self) -> &'a u8 {
|
||||
(self.func)(&self.data)
|
||||
}
|
||||
}
|
||||
|
||||
fn do_it<'b>(data: &'b (u8, u16)) -> &'b u8 { &'b data.0 }
|
||||
|
||||
fn main() {
|
||||
'x: {
|
||||
let clo = Closure { data: (0, 1), func: do_it };
|
||||
println!("{}", clo.call());
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
How on earth are we supposed to express the lifetimes on `F`'s trait bound? We
|
||||
need to provide some lifetime there, but the lifetime we care about can't be
|
||||
named until we enter the body of `call`! Also, that isn't some fixed lifetime;
|
||||
`call` works with *any* lifetime `&self` happens to have at that point.
|
||||
|
||||
This job requires The Magic of Higher-Rank Trait Bounds (HRTBs). The way we
|
||||
desugar this is as follows:
|
||||
|
||||
```rust,ignore
|
||||
where for<'a> F: Fn(&'a (u8, u16)) -> &'a u8,
|
||||
```
|
||||
|
||||
(Where `Fn(a, b, c) -> d` is itself just sugar for the unstable *real* `Fn`
|
||||
trait)
|
||||
|
||||
`for<'a>` can be read as "for all choices of `'a`", and basically produces an
|
||||
*infinite list* of trait bounds that F must satisfy. Intense. There aren't many
|
||||
places outside of the `Fn` traits where we encounter HRTBs, and even for
|
||||
those we have a nice magic sugar for the common cases.
|
|
@ -1,251 +0,0 @@
|
|||
# Leaking
|
||||
|
||||
Ownership-based resource management is intended to simplify composition. You
|
||||
acquire resources when you create the object, and you release the resources when
|
||||
it gets destroyed. Since destruction is handled for you, it means you can't
|
||||
forget to release the resources, and it happens as soon as possible! Surely this
|
||||
is perfect and all of our problems are solved.
|
||||
|
||||
Everything is terrible and we have new and exotic problems to try to solve.
|
||||
|
||||
Many people like to believe that Rust eliminates resource leaks. In practice,
|
||||
this is basically true. You would be surprised to see a Safe Rust program
|
||||
leak resources in an uncontrolled way.
|
||||
|
||||
However from a theoretical perspective this is absolutely not the case, no
|
||||
matter how you look at it. In the strictest sense, "leaking" is so abstract as
|
||||
to be unpreventable. It's quite trivial to initialize a collection at the start
|
||||
of a program, fill it with tons of objects with destructors, and then enter an
|
||||
infinite event loop that never refers to it. The collection will sit around
|
||||
uselessly, holding on to its precious resources until the program terminates (at
|
||||
which point all those resources would have been reclaimed by the OS anyway).
|
||||
|
||||
We may consider a more restricted form of leak: failing to drop a value that is
|
||||
unreachable. Rust also doesn't prevent this. In fact Rust *has a function for
|
||||
doing this*: `mem::forget`. This function consumes the value it is passed *and
|
||||
then doesn't run its destructor*.
|
||||
|
||||
In the past `mem::forget` was marked as unsafe as a sort of lint against using
|
||||
it, since failing to call a destructor is generally not a well-behaved thing to
|
||||
do (though useful for some special unsafe code). However this was generally
|
||||
determined to be an untenable stance to take: there are many ways to fail to
|
||||
call a destructor in safe code. The most famous example is creating a cycle of
|
||||
reference-counted pointers using interior mutability.
|
||||
|
||||
It is reasonable for safe code to assume that destructor leaks do not happen, as
|
||||
any program that leaks destructors is probably wrong. However *unsafe* code
|
||||
cannot rely on destructors to be run in order to be safe. For most types this
|
||||
doesn't matter: if you leak the destructor then the type is by definition
|
||||
inaccessible, so it doesn't matter, right? For instance, if you leak a `Box<u8>`
|
||||
then you waste some memory but that's hardly going to violate memory-safety.
|
||||
|
||||
However where we must be careful with destructor leaks are *proxy* types. These
|
||||
are types which manage access to a distinct object, but don't actually own it.
|
||||
Proxy objects are quite rare. Proxy objects you'll need to care about are even
|
||||
rarer. However we'll focus on three interesting examples in the standard
|
||||
library:
|
||||
|
||||
* `vec::Drain`
|
||||
* `Rc`
|
||||
* `thread::scoped::JoinGuard`
|
||||
|
||||
|
||||
|
||||
## Drain
|
||||
|
||||
`drain` is a collections API that moves data out of the container without
|
||||
consuming the container. This enables us to reuse the allocation of a `Vec`
|
||||
after claiming ownership over all of its contents. It produces an iterator
|
||||
(Drain) that returns the contents of the Vec by-value.
|
||||
|
||||
Now, consider Drain in the middle of iteration: some values have been moved out,
|
||||
and others haven't. This means that part of the Vec is now full of logically
|
||||
uninitialized data! We could backshift all the elements in the Vec every time we
|
||||
remove a value, but this would have pretty catastrophic performance
|
||||
consequences.
|
||||
|
||||
Instead, we would like Drain to fix the Vec's backing storage when it is
|
||||
dropped. It should run itself to completion, backshift any elements that weren't
|
||||
removed (drain supports subranges), and then fix Vec's `len`. It's even
|
||||
unwinding-safe! Easy!
|
||||
|
||||
Now consider the following:
|
||||
|
||||
```rust,ignore
|
||||
let mut vec = vec![Box::new(0); 4];
|
||||
|
||||
{
|
||||
// start draining, vec can no longer be accessed
|
||||
let mut drainer = vec.drain(..);
|
||||
|
||||
// pull out two elements and immediately drop them
|
||||
drainer.next();
|
||||
drainer.next();
|
||||
|
||||
// get rid of drainer, but don't call its destructor
|
||||
mem::forget(drainer);
|
||||
}
|
||||
|
||||
// Oops, vec[0] was dropped, we're reading a pointer into free'd memory!
|
||||
println!("{}", vec[0]);
|
||||
```
|
||||
|
||||
This is pretty clearly Not Good. Unfortunately, we're kind of stuck between a
|
||||
rock and a hard place: maintaining consistent state at every step has an
|
||||
enormous cost (and would negate any benefits of the API). Failing to maintain
|
||||
consistent state gives us Undefined Behavior in safe code (making the API
|
||||
unsound).
|
||||
|
||||
So what can we do? Well, we can pick a trivially consistent state: set the Vec's
|
||||
len to be 0 when we start the iteration, and fix it up if necessary in the
|
||||
destructor. That way, if everything executes like normal we get the desired
|
||||
behavior with minimal overhead. But if someone has the *audacity* to
|
||||
mem::forget us in the middle of the iteration, all that does is *leak even more*
|
||||
(and possibly leave the Vec in an unexpected but otherwise consistent state).
|
||||
Since we've accepted that mem::forget is safe, this is definitely safe. We call
|
||||
leaks causing more leaks a *leak amplification*.
|
||||
|
||||
|
||||
|
||||
|
||||
## Rc
|
||||
|
||||
Rc is an interesting case because at first glance it doesn't appear to be a
|
||||
proxy value at all. After all, it manages the data it points to, and dropping
|
||||
all the Rcs for a value will drop that value. Leaking an Rc doesn't seem like it
|
||||
would be particularly dangerous. It will leave the refcount permanently
|
||||
incremented and prevent the data from being freed or dropped, but that seems
|
||||
just like Box, right?
|
||||
|
||||
Nope.
|
||||
|
||||
Let's consider a simplified implementation of Rc:
|
||||
|
||||
```rust,ignore
|
||||
struct Rc<T> {
|
||||
ptr: *mut RcBox<T>,
|
||||
}
|
||||
|
||||
struct RcBox<T> {
|
||||
data: T,
|
||||
ref_count: usize,
|
||||
}
|
||||
|
||||
impl<T> Rc<T> {
|
||||
fn new(data: T) -> Self {
|
||||
unsafe {
|
||||
// Wouldn't it be nice if heap::allocate worked like this?
|
||||
let ptr = heap::allocate::<RcBox<T>>();
|
||||
ptr::write(ptr, RcBox {
|
||||
data: data,
|
||||
ref_count: 1,
|
||||
});
|
||||
Rc { ptr: ptr }
|
||||
}
|
||||
}
|
||||
|
||||
fn clone(&self) -> Self {
|
||||
unsafe {
|
||||
(*self.ptr).ref_count += 1;
|
||||
}
|
||||
Rc { ptr: self.ptr }
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Drop for Rc<T> {
|
||||
fn drop(&mut self) {
|
||||
unsafe {
|
||||
(*self.ptr).ref_count -= 1;
|
||||
if (*self.ptr).ref_count == 0 {
|
||||
// drop the data and then free it
|
||||
ptr::read(self.ptr);
|
||||
heap::deallocate(self.ptr);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
This code contains an implicit and subtle assumption: `ref_count` can fit in a
|
||||
`usize`, because there can't be more than `usize::MAX` Rcs in memory. However
|
||||
this itself assumes that the `ref_count` accurately reflects the number of Rcs
|
||||
in memory, which we know is false with `mem::forget`. Using `mem::forget` we can
|
||||
overflow the `ref_count`, and then get it down to 0 with outstanding Rcs. Then
|
||||
we can happily use-after-free the inner data. Bad Bad Not Good.
|
||||
|
||||
This can be solved by just checking the `ref_count` and doing *something*. The
|
||||
standard library's stance is to just abort, because your program has become
|
||||
horribly degenerate. Also *oh my gosh* it's such a ridiculous corner case.
|
||||
|
||||
|
||||
|
||||
|
||||
## thread::scoped::JoinGuard
|
||||
|
||||
The thread::scoped API intends to allow threads to be spawned that reference
|
||||
data on their parent's stack without any synchronization over that data by
|
||||
ensuring the parent joins the thread before any of the shared data goes out
|
||||
of scope.
|
||||
|
||||
```rust,ignore
|
||||
pub fn scoped<'a, F>(f: F) -> JoinGuard<'a>
|
||||
where F: FnOnce() + Send + 'a
|
||||
```
|
||||
|
||||
Here `f` is some closure for the other thread to execute. Saying that
|
||||
`F: Send +'a` is saying that it closes over data that lives for `'a`, and it
|
||||
either owns that data or the data was Sync (implying `&data` is Send).
|
||||
|
||||
Because JoinGuard has a lifetime, it keeps all the data it closes over
|
||||
borrowed in the parent thread. This means the JoinGuard can't outlive
|
||||
the data that the other thread is working on. When the JoinGuard *does* get
|
||||
dropped it blocks the parent thread, ensuring the child terminates before any
|
||||
of the closed-over data goes out of scope in the parent.
|
||||
|
||||
Usage looked like:
|
||||
|
||||
```rust,ignore
|
||||
let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
|
||||
{
|
||||
let guards = vec![];
|
||||
for x in &mut data {
|
||||
// Move the mutable reference into the closure, and execute
|
||||
// it on a different thread. The closure has a lifetime bound
|
||||
// by the lifetime of the mutable reference `x` we store in it.
|
||||
// The guard that is returned is in turn assigned the lifetime
|
||||
// of the closure, so it also mutably borrows `data` as `x` did.
|
||||
// This means we cannot access `data` until the guard goes away.
|
||||
let guard = thread::scoped(move || {
|
||||
*x *= 2;
|
||||
});
|
||||
// store the thread's guard for later
|
||||
guards.push(guard);
|
||||
}
|
||||
// All guards are dropped here, forcing the threads to join
|
||||
// (this thread blocks here until the others terminate).
|
||||
// Once the threads join, the borrow expires and the data becomes
|
||||
// accessible again in this thread.
|
||||
}
|
||||
// data is definitely mutated here.
|
||||
```
|
||||
|
||||
In principle, this totally works! Rust's ownership system perfectly ensures it!
|
||||
...except it relies on a destructor being called to be safe.
|
||||
|
||||
```rust,ignore
|
||||
let mut data = Box::new(0);
|
||||
{
|
||||
let guard = thread::scoped(|| {
|
||||
// This is at best a data race. At worst, it's also a use-after-free.
|
||||
*data += 1;
|
||||
});
|
||||
// Because the guard is forgotten, expiring the loan without blocking this
|
||||
// thread.
|
||||
mem::forget(guard);
|
||||
}
|
||||
// So the Box is dropped here while the scoped thread may or may not be trying
|
||||
// to access it.
|
||||
```
|
||||
|
||||
Dang. Here the destructor running was pretty fundamental to the API, and it had
|
||||
to be scrapped in favor of a completely different design.
|
|
@ -1,64 +0,0 @@
|
|||
# Lifetime Elision
|
||||
|
||||
In order to make common patterns more ergonomic, Rust allows lifetimes to be
|
||||
*elided* in function signatures.
|
||||
|
||||
A *lifetime position* is anywhere you can write a lifetime in a type:
|
||||
|
||||
```rust,ignore
|
||||
&'a T
|
||||
&'a mut T
|
||||
T<'a>
|
||||
```
|
||||
|
||||
Lifetime positions can appear as either "input" or "output":
|
||||
|
||||
* For `fn` definitions, input refers to the types of the formal arguments
|
||||
in the `fn` definition, while output refers to
|
||||
result types. So `fn foo(s: &str) -> (&str, &str)` has elided one lifetime in
|
||||
input position and two lifetimes in output position.
|
||||
Note that the input positions of a `fn` method definition do not
|
||||
include the lifetimes that occur in the method's `impl` header
|
||||
(nor lifetimes that occur in the trait header, for a default method).
|
||||
|
||||
* In the future, it should be possible to elide `impl` headers in the same manner.
|
||||
|
||||
Elision rules are as follows:
|
||||
|
||||
* Each elided lifetime in input position becomes a distinct lifetime
|
||||
parameter.
|
||||
|
||||
* If there is exactly one input lifetime position (elided or not), that lifetime
|
||||
is assigned to *all* elided output lifetimes.
|
||||
|
||||
* If there are multiple input lifetime positions, but one of them is `&self` or
|
||||
`&mut self`, the lifetime of `self` is assigned to *all* elided output lifetimes.
|
||||
|
||||
* Otherwise, it is an error to elide an output lifetime.
|
||||
|
||||
Examples:
|
||||
|
||||
```rust,ignore
|
||||
fn print(s: &str); // elided
|
||||
fn print<'a>(s: &'a str); // expanded
|
||||
|
||||
fn debug(lvl: uint, s: &str); // elided
|
||||
fn debug<'a>(lvl: uint, s: &'a str); // expanded
|
||||
|
||||
fn substr(s: &str, until: uint) -> &str; // elided
|
||||
fn substr<'a>(s: &'a str, until: uint) -> &'a str; // expanded
|
||||
|
||||
fn get_str() -> &str; // ILLEGAL
|
||||
|
||||
fn frob(s: &str, t: &str) -> &str; // ILLEGAL
|
||||
|
||||
fn get_mut(&mut self) -> &mut T; // elided
|
||||
fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded
|
||||
|
||||
fn args<T: ToCStr>(&mut self, args: &[T]) -> &mut Command // elided
|
||||
fn args<'a, 'b, T: ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded
|
||||
|
||||
fn new(buf: &mut [u8]) -> BufWriter; // elided
|
||||
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a> // expanded
|
||||
|
||||
```
|
|
@ -1,81 +0,0 @@
|
|||
# Limits of Lifetimes
|
||||
|
||||
Given the following code:
|
||||
|
||||
```rust,ignore
|
||||
struct Foo;
|
||||
|
||||
impl Foo {
|
||||
fn mutate_and_share(&mut self) -> &Self { &*self }
|
||||
fn share(&self) {}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
let mut foo = Foo;
|
||||
let loan = foo.mutate_and_share();
|
||||
foo.share();
|
||||
}
|
||||
```
|
||||
|
||||
One might expect it to compile. We call `mutate_and_share`, which mutably borrows
|
||||
`foo` temporarily, but then returns only a shared reference. Therefore we
|
||||
would expect `foo.share()` to succeed as `foo` shouldn't be mutably borrowed.
|
||||
|
||||
However when we try to compile it:
|
||||
|
||||
```text
|
||||
<anon>:11:5: 11:8 error: cannot borrow `foo` as immutable because it is also borrowed as mutable
|
||||
<anon>:11 foo.share();
|
||||
^~~
|
||||
<anon>:10:16: 10:19 note: previous borrow of `foo` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `foo` until the borrow ends
|
||||
<anon>:10 let loan = foo.mutate_and_share();
|
||||
^~~
|
||||
<anon>:12:2: 12:2 note: previous borrow ends here
|
||||
<anon>:8 fn main() {
|
||||
<anon>:9 let mut foo = Foo;
|
||||
<anon>:10 let loan = foo.mutate_and_share();
|
||||
<anon>:11 foo.share();
|
||||
<anon>:12 }
|
||||
^
|
||||
```
|
||||
|
||||
What happened? Well, we got the exact same reasoning as we did for
|
||||
[Example 2 in the previous section][ex2]. We desugar the program and we get
|
||||
the following:
|
||||
|
||||
```rust,ignore
|
||||
struct Foo;
|
||||
|
||||
impl Foo {
|
||||
fn mutate_and_share<'a>(&'a mut self) -> &'a Self { &'a *self }
|
||||
fn share<'a>(&'a self) {}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
'b: {
|
||||
let mut foo: Foo = Foo;
|
||||
'c: {
|
||||
let loan: &'c Foo = Foo::mutate_and_share::<'c>(&'c mut foo);
|
||||
'd: {
|
||||
Foo::share::<'d>(&'d foo);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
The lifetime system is forced to extend the `&mut foo` to have lifetime `'c`,
|
||||
due to the lifetime of `loan` and mutate_and_share's signature. Then when we
|
||||
try to call `share`, and it sees we're trying to alias that `&'c mut foo` and
|
||||
blows up in our face!
|
||||
|
||||
This program is clearly correct according to the reference semantics we actually
|
||||
care about, but the lifetime system is too coarse-grained to handle that.
|
||||
|
||||
|
||||
TODO: other common problems? SEME regions stuff, mostly?
|
||||
|
||||
|
||||
|
||||
|
||||
[ex2]: lifetimes.html#example-aliasing-a-mutable-reference
|
|
@ -1,215 +0,0 @@
|
|||
# Lifetimes
|
||||
|
||||
Rust enforces these rules through *lifetimes*. Lifetimes are effectively
|
||||
just names for scopes somewhere in the program. Each reference,
|
||||
and anything that contains a reference, is tagged with a lifetime specifying
|
||||
the scope it's valid for.
|
||||
|
||||
Within a function body, Rust generally doesn't let you explicitly name the
|
||||
lifetimes involved. This is because it's generally not really necessary
|
||||
to talk about lifetimes in a local context; Rust has all the information and
|
||||
can work out everything as optimally as possible. Many anonymous scopes and
|
||||
temporaries that you would otherwise have to write are often introduced to
|
||||
make your code Just Work.
|
||||
|
||||
However once you cross the function boundary, you need to start talking about
|
||||
lifetimes. Lifetimes are denoted with an apostrophe: `'a`, `'static`. To dip
|
||||
our toes with lifetimes, we're going to pretend that we're actually allowed
|
||||
to label scopes with lifetimes, and desugar the examples from the start of
|
||||
this chapter.
|
||||
|
||||
Originally, our examples made use of *aggressive* sugar -- high fructose corn
|
||||
syrup even -- around scopes and lifetimes, because writing everything out
|
||||
explicitly is *extremely noisy*. All Rust code relies on aggressive inference
|
||||
and elision of "obvious" things.
|
||||
|
||||
One particularly interesting piece of sugar is that each `let` statement implicitly
|
||||
introduces a scope. For the most part, this doesn't really matter. However it
|
||||
does matter for variables that refer to each other. As a simple example, let's
|
||||
completely desugar this simple piece of Rust code:
|
||||
|
||||
```rust
|
||||
let x = 0;
|
||||
let y = &x;
|
||||
let z = &y;
|
||||
```
|
||||
|
||||
The borrow checker always tries to minimize the extent of a lifetime, so it will
|
||||
likely desugar to the following:
|
||||
|
||||
```rust,ignore
|
||||
// NOTE: `'a: {` and `&'b x` is not valid syntax!
|
||||
'a: {
|
||||
let x: i32 = 0;
|
||||
'b: {
|
||||
// lifetime used is 'b because that's good enough.
|
||||
let y: &'b i32 = &'b x;
|
||||
'c: {
|
||||
// ditto on 'c
|
||||
let z: &'c &'b i32 = &'c y;
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Wow. That's... awful. Let's all take a moment to thank Rust for making this easier.
|
||||
|
||||
Actually passing references to outer scopes will cause Rust to infer
|
||||
a larger lifetime:
|
||||
|
||||
```rust
|
||||
let x = 0;
|
||||
let z;
|
||||
let y = &x;
|
||||
z = y;
|
||||
```
|
||||
|
||||
```rust,ignore
|
||||
'a: {
|
||||
let x: i32 = 0;
|
||||
'b: {
|
||||
let z: &'b i32;
|
||||
'c: {
|
||||
// Must use 'b here because this reference is
|
||||
// being passed to that scope.
|
||||
let y: &'b i32 = &'b x;
|
||||
z = y;
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
|
||||
|
||||
# Example: references that outlive referents
|
||||
|
||||
Alright, let's look at some of those examples from before:
|
||||
|
||||
```rust,ignore
|
||||
fn as_str(data: &u32) -> &str {
|
||||
let s = format!("{}", data);
|
||||
&s
|
||||
}
|
||||
```
|
||||
|
||||
desugars to:
|
||||
|
||||
```rust,ignore
|
||||
fn as_str<'a>(data: &'a u32) -> &'a str {
|
||||
'b: {
|
||||
let s = format!("{}", data);
|
||||
return &'a s;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
This signature of `as_str` takes a reference to a u32 with *some* lifetime, and
|
||||
promises that it can produce a reference to a str that can live *just as long*.
|
||||
Already we can see why this signature might be trouble. That basically implies
|
||||
that we're going to find a str somewhere in the scope the reference
|
||||
to the u32 originated in, or somewhere *even earlier*. That's a bit of a tall
|
||||
order.
|
||||
|
||||
We then proceed to compute the string `s`, and return a reference to it. Since
|
||||
the contract of our function says the reference must outlive `'a`, that's the
|
||||
lifetime we infer for the reference. Unfortunately, `s` was defined in the
|
||||
scope `'b`, so the only way this is sound is if `'b` contains `'a` -- which is
|
||||
clearly false since `'a` must contain the function call itself. We have therefore
|
||||
created a reference whose lifetime outlives its referent, which is *literally*
|
||||
the first thing we said that references can't do. The compiler rightfully blows
|
||||
up in our face.
|
||||
|
||||
To make this more clear, we can expand the example:
|
||||
|
||||
```rust,ignore
|
||||
fn as_str<'a>(data: &'a u32) -> &'a str {
|
||||
'b: {
|
||||
let s = format!("{}", data);
|
||||
return &'a s
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
'c: {
|
||||
let x: u32 = 0;
|
||||
'd: {
|
||||
// An anonymous scope is introduced because the borrow does not
|
||||
// need to last for the whole scope x is valid for. The return
|
||||
// of as_str must find a str somewhere before this function
|
||||
// call. Obviously not happening.
|
||||
println!("{}", as_str::<'d>(&'d x));
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Shoot!
|
||||
|
||||
Of course, the right way to write this function is as follows:
|
||||
|
||||
```rust
|
||||
fn to_string(data: &u32) -> String {
|
||||
format!("{}", data)
|
||||
}
|
||||
```
|
||||
|
||||
We must produce an owned value inside the function to return it! The only way
|
||||
we could have returned an `&'a str` would have been if it was in a field of the
|
||||
`&'a u32`, which is obviously not the case.
|
||||
|
||||
(Actually we could have also just returned a string literal, which as a global
|
||||
can be considered to reside at the bottom of the stack; though this limits
|
||||
our implementation *just a bit*.)
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
# Example: aliasing a mutable reference
|
||||
|
||||
How about the other example:
|
||||
|
||||
```rust,ignore
|
||||
let mut data = vec![1, 2, 3];
|
||||
let x = &data[0];
|
||||
data.push(4);
|
||||
println!("{}", x);
|
||||
```
|
||||
|
||||
```rust,ignore
|
||||
'a: {
|
||||
let mut data: Vec<i32> = vec![1, 2, 3];
|
||||
'b: {
|
||||
// 'b is as big as we need this borrow to be
|
||||
// (just need to get to `println!`)
|
||||
let x: &'b i32 = Index::index::<'b>(&'b data, 0);
|
||||
'c: {
|
||||
// Temporary scope because we don't need the
|
||||
// &mut to last any longer.
|
||||
Vec::push(&'c mut data, 4);
|
||||
}
|
||||
println!("{}", x);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
The problem here is a bit more subtle and interesting. We want Rust to
|
||||
reject this program for the following reason: We have a live shared reference `x`
|
||||
to a descendant of `data` when we try to take a mutable reference to `data`
|
||||
to `push`. This would create an aliased mutable reference, which would
|
||||
violate the *second* rule of references.
|
||||
|
||||
However this is *not at all* how Rust reasons that this program is bad. Rust
|
||||
doesn't understand that `x` is a reference to a subpath of `data`. It doesn't
|
||||
understand Vec at all. What it *does* see is that `x` has to live for `'b` to
|
||||
be printed. The signature of `Index::index` subsequently demands that the
|
||||
reference we take to `data` has to survive for `'b`. When we try to call `push`,
|
||||
it then sees us try to make an `&'c mut data`. Rust knows that `'c` is contained
|
||||
within `'b`, and rejects our program because the `&'b data` must still be live!
|
||||
|
||||
Here we see that the lifetime system is much more coarse than the reference
|
||||
semantics we're actually interested in preserving. For the most part, *that's
|
||||
totally ok*, because it keeps us from spending all day explaining our program
|
||||
to the compiler. However it does mean that several programs that are totally
|
||||
correct with respect to Rust's *true* semantics are rejected because lifetimes
|
||||
are too dumb.
|
|
@ -1,98 +0,0 @@
|
|||
# Meet Safe and Unsafe
|
||||
|
||||
Programmers in safe "high-level" languages face a fundamental dilemma. On one
|
||||
hand, it would be *really* great to just say what you want and not worry about
|
||||
how it's done. On the other hand, that can lead to unacceptably poor
|
||||
performance. It may be necessary to drop down to less clear or idiomatic
|
||||
practices to get the performance characteristics you want. Or maybe you just
|
||||
throw up your hands in disgust and decide to shell out to an implementation in
|
||||
a less sugary-wonderful *unsafe* language.
|
||||
|
||||
Worse, when you want to talk directly to the operating system, you *have* to
|
||||
talk to an unsafe language: *C*. C is ever-present and unavoidable. It's the
|
||||
lingua-franca of the programming world.
|
||||
Even other safe languages generally expose C interfaces for the world at large!
|
||||
Regardless of why you're doing it, as soon as your program starts talking to
|
||||
C it stops being safe.
|
||||
|
||||
With that said, Rust is *totally* a safe programming language.
|
||||
|
||||
Well, Rust *has* a safe programming language. Let's step back a bit.
|
||||
|
||||
Rust can be thought of as being composed of two programming languages: *Safe
|
||||
Rust* and *Unsafe Rust*. Safe Rust is For Reals Totally Safe. Unsafe Rust,
|
||||
unsurprisingly, is *not* For Reals Totally Safe. In fact, Unsafe Rust lets you
|
||||
do some really, *really* unsafe things.
|
||||
|
||||
Safe Rust is the *true* Rust programming language. If all you do is write Safe
|
||||
Rust, you will never have to worry about type-safety or memory-safety. You will
|
||||
never endure a null or dangling pointer, or any of that Undefined Behavior
|
||||
nonsense.
|
||||
|
||||
*That's totally awesome.*
|
||||
|
||||
The standard library also gives you enough utilities out-of-the-box that you'll
|
||||
be able to write awesome high-performance applications and libraries in pure
|
||||
idiomatic Safe Rust.
|
||||
|
||||
But maybe you want to talk to another language. Maybe you're writing a
|
||||
low-level abstraction not exposed by the standard library. Maybe you're
|
||||
*writing* the standard library (which is written entirely in Rust). Maybe you
|
||||
need to do something the type-system doesn't understand and just *frob some dang
|
||||
bits*. Maybe you need Unsafe Rust.
|
||||
|
||||
Unsafe Rust is exactly like Safe Rust with all the same rules and semantics.
|
||||
However Unsafe Rust lets you do some *extra* things that are Definitely Not Safe.
|
||||
|
||||
The only things that are different in Unsafe Rust are that you can:
|
||||
|
||||
* Dereference raw pointers
|
||||
* Call `unsafe` functions (including C functions, intrinsics, and the raw allocator)
|
||||
* Implement `unsafe` traits
|
||||
* Mutate statics
|
||||
|
||||
That's it. The reason these operations are relegated to Unsafe is that misusing
|
||||
any of these things will cause the ever dreaded Undefined Behavior. Invoking
|
||||
Undefined Behavior gives the compiler full rights to do arbitrarily bad things
|
||||
to your program. You definitely *should not* invoke Undefined Behavior.
|
||||
|
||||
Unlike C, Undefined Behavior is pretty limited in scope in Rust. All the core
|
||||
language cares about is preventing the following things:
|
||||
|
||||
* Dereferencing null or dangling pointers
|
||||
* Reading [uninitialized memory]
|
||||
* Breaking the [pointer aliasing rules]
|
||||
* Producing invalid primitive values:
|
||||
* dangling/null references
|
||||
* a `bool` that isn't 0 or 1
|
||||
* an undefined `enum` discriminant
|
||||
* a `char` outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
|
||||
* A non-utf8 `str`
|
||||
* Unwinding into another language
|
||||
* Causing a [data race][race]
|
||||
|
||||
That's it. That's all the causes of Undefined Behavior baked into Rust. Of
|
||||
course, unsafe functions and traits are free to declare arbitrary other
|
||||
constraints that a program must maintain to avoid Undefined Behavior. However,
|
||||
generally violations of these constraints will just transitively lead to one of
|
||||
the above problems. Some additional constraints may also derive from compiler
|
||||
intrinsics that make special assumptions about how code can be optimized.
|
||||
|
||||
Rust is otherwise quite permissive with respect to other dubious operations.
|
||||
Rust considers it "safe" to:
|
||||
|
||||
* Deadlock
|
||||
* Have a [race condition][race]
|
||||
* Leak memory
|
||||
* Fail to call destructors
|
||||
* Overflow integers
|
||||
* Abort the program
|
||||
* Delete the production database
|
||||
|
||||
However any program that actually manages to do such a thing is *probably*
|
||||
incorrect. Rust provides lots of tools to make these things rare, but
|
||||
these problems are considered impractical to categorically prevent.
|
||||
|
||||
[pointer aliasing rules]: references.html
|
||||
[uninitialized memory]: uninitialized.html
|
||||
[race]: races.html
|
|
@ -1,14 +0,0 @@
|
|||
# The Perils Of Ownership Based Resource Management (OBRM)
|
||||
|
||||
OBRM (AKA RAII: Resource Acquisition Is Initialization) is something you'll
|
||||
interact with a lot in Rust. Especially if you use the standard library.
|
||||
|
||||
Roughly speaking the pattern is as follows: to acquire a resource, you create an
|
||||
object that manages it. To release the resource, you simply destroy the object,
|
||||
and it cleans up the resource for you. The most common "resource" this pattern
|
||||
manages is simply *memory*. `Box`, `Rc`, and basically everything in
|
||||
`std::collections` is a convenience to enable correctly managing memory. This is
|
||||
particularly important in Rust because we have no pervasive GC to rely on for
|
||||
memory management. Which is the point, really: Rust is about control. However we
|
||||
are not limited to just memory. Pretty much every other system resource like a
|
||||
thread, file, or socket is exposed through this kind of API.
|
|
@ -1,79 +0,0 @@
|
|||
# Alternative representations
|
||||
|
||||
Rust allows you to specify alternative data layout strategies from the default.
|
||||
|
||||
|
||||
|
||||
|
||||
# repr(C)
|
||||
|
||||
This is the most important `repr`. It has fairly simple intent: do what C does.
|
||||
The order, size, and alignment of fields is exactly what you would expect from C
|
||||
or C++. Any type you expect to pass through an FFI boundary should have
|
||||
`repr(C)`, as C is the lingua-franca of the programming world. This is also
|
||||
necessary to soundly do more elaborate tricks with data layout such as
|
||||
reinterpreting values as a different type.
|
||||
|
||||
However, the interaction with Rust's more exotic data layout features must be
|
||||
kept in mind. Due to its dual purpose as "for FFI" and "for layout control",
|
||||
`repr(C)` can be applied to types that will be nonsensical or problematic if
|
||||
passed through the FFI boundary.
|
||||
|
||||
* ZSTs are still zero-sized, even though this is not a standard behavior in
|
||||
C, and is explicitly contrary to the behavior of an empty type in C++, which
|
||||
still consumes a byte of space.
|
||||
|
||||
* DSTs, tuples, and tagged unions are not a concept in C and as such are never
|
||||
FFI safe.
|
||||
|
||||
* Tuple structs are like structs with regards to `repr(C)`, as the only
|
||||
difference from a struct is that the fields aren’t named.
|
||||
|
||||
* **If the type would have any [drop flags], they will still be added**
|
||||
|
||||
* This is equivalent to one of `repr(u*)` (see the next section) for enums. The
|
||||
chosen size is the default enum size for the target platform's C ABI. Note that
|
||||
enum representation in C is implementation defined, so this is really a "best
|
||||
guess". In particular, this may be incorrect when the C code of interest is
|
||||
compiled with certain flags.
|
||||
|
||||
|
||||
|
||||
# repr(u8), repr(u16), repr(u32), repr(u64)
|
||||
|
||||
These specify the size to make a C-like enum. If the discriminant overflows the
|
||||
integer it has to fit in, it will produce a compile-time error. You can manually
|
||||
ask Rust to allow this by setting the overflowing element to explicitly be 0.
|
||||
However Rust will not allow you to create an enum where two variants have the
|
||||
same discriminant.
|
||||
|
||||
On non-C-like enums, this will inhibit certain optimizations like the null-
|
||||
pointer optimization.
|
||||
|
||||
These reprs have no effect on a struct.
|
||||
|
||||
|
||||
|
||||
|
||||
# repr(packed)
|
||||
|
||||
`repr(packed)` forces Rust to strip any padding, and only align the type to a
|
||||
byte. This may improve the memory footprint, but will likely have other negative
|
||||
side-effects.
|
||||
|
||||
In particular, most architectures *strongly* prefer values to be aligned. This
|
||||
may mean the unaligned loads are penalized (x86), or even fault (some ARM
|
||||
chips). For simple cases like directly loading or storing a packed field, the
|
||||
compiler might be able to paper over alignment issues with shifts and masks.
|
||||
However if you take a reference to a packed field, it's unlikely that the
|
||||
compiler will be able to emit code to avoid an unaligned load.
|
||||
|
||||
**[As of Rust 1.0 this can cause undefined behavior.][ub loads]**
|
||||
|
||||
`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
|
||||
this should not be used.
|
||||
|
||||
This repr is a modifier on `repr(C)` and `repr(rust)`.
|
||||
|
||||
[drop flags]: drop-flags.html
|
||||
[ub loads]: https://github.com/rust-lang/rust/issues/27060
|
|
@ -1,66 +0,0 @@
|
|||
# Ownership and Lifetimes
|
||||
|
||||
Ownership is the breakout feature of Rust. It allows Rust to be completely
|
||||
memory-safe and efficient, while avoiding garbage collection. Before getting
|
||||
into the ownership system in detail, we will consider the motivation of this
|
||||
design.
|
||||
|
||||
We will assume that you accept that garbage collection (GC) is not always an
|
||||
optimal solution, and that it is desirable to manually manage memory in some
|
||||
contexts. If you do not accept this, might I interest you in a different
|
||||
language?
|
||||
|
||||
Regardless of your feelings on GC, it is pretty clearly a *massive* boon to
|
||||
making code safe. You never have to worry about things going away *too soon*
|
||||
(although whether you still wanted to be pointing at that thing is a different
|
||||
issue...). This is a pervasive problem that C and C++ programs need to deal
|
||||
with. Consider this simple mistake that all of us who have used a non-GC'd
|
||||
language have made at one point:
|
||||
|
||||
```rust,ignore
|
||||
fn as_str(data: &u32) -> &str {
|
||||
// compute the string
|
||||
let s = format!("{}", data);
|
||||
|
||||
// OH NO! We returned a reference to something that
|
||||
// exists only in this function!
|
||||
// Dangling pointer! Use after free! Alas!
|
||||
// (this does not compile in Rust)
|
||||
&s
|
||||
}
|
||||
```
|
||||
|
||||
This is exactly what Rust's ownership system was built to solve.
|
||||
Rust knows the scope in which the `&s` lives, and as such can prevent it from
|
||||
escaping. However this is a simple case that even a C compiler could plausibly
|
||||
catch. Things get more complicated as code gets bigger and pointers get fed through
|
||||
various functions. Eventually, a C compiler will fall down and won't be able to
|
||||
perform sufficient escape analysis to prove your code unsound. It will consequently
|
||||
be forced to accept your program on the assumption that it is correct.
|
||||
|
||||
This will never happen to Rust. It's up to the programmer to prove to the
|
||||
compiler that everything is sound.
|
||||
|
||||
Of course, Rust's story around ownership is much more complicated than just
|
||||
verifying that references don't escape the scope of their referent. That's
|
||||
because ensuring pointers are always valid is much more complicated than this.
|
||||
For instance in this code,
|
||||
|
||||
```rust,ignore
|
||||
let mut data = vec![1, 2, 3];
|
||||
// get an internal reference
|
||||
let x = &data[0];
|
||||
|
||||
// OH NO! `push` causes the backing storage of `data` to be reallocated.
|
||||
// Dangling pointer! Use after free! Alas!
|
||||
// (this does not compile in Rust)
|
||||
data.push(4);
|
||||
|
||||
println!("{}", x);
|
||||
```
|
||||
|
||||
naive scope analysis would be insufficient to prevent this bug, because `data`
|
||||
does in fact live as long as we needed. However it was *changed* while we had
|
||||
a reference into it. This is why Rust requires any references to freeze the
|
||||
referent and its owners.
|
||||
|
|
@ -1,104 +0,0 @@
|
|||
# PhantomData
|
||||
|
||||
When working with unsafe code, we can often end up in a situation where
|
||||
types or lifetimes are logically associated with a struct, but not actually
|
||||
part of a field. This most commonly occurs with lifetimes. For instance, the
|
||||
`Iter` for `&'a [T]` is (approximately) defined as follows:
|
||||
|
||||
```rust,ignore
|
||||
struct Iter<'a, T: 'a> {
|
||||
ptr: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
```
|
||||
|
||||
However because `'a` is unused within the struct's body, it's *unbounded*.
|
||||
Because of the troubles this has historically caused, unbounded lifetimes and
|
||||
types are *forbidden* in struct definitions. Therefore we must somehow refer
|
||||
to these types in the body. Correctly doing this is necessary to have
|
||||
correct variance and drop checking.
|
||||
|
||||
We do this using `PhantomData`, which is a special marker type. `PhantomData`
|
||||
consumes no space, but simulates a field of the given type for the purpose of
|
||||
static analysis. This was deemed to be less error-prone than explicitly telling
|
||||
the type-system the kind of variance that you want, while also providing other
|
||||
useful such as the information needed by drop check.
|
||||
|
||||
Iter logically contains a bunch of `&'a T`s, so this is exactly what we tell
|
||||
the PhantomData to simulate:
|
||||
|
||||
```
|
||||
use std::marker;
|
||||
|
||||
struct Iter<'a, T: 'a> {
|
||||
ptr: *const T,
|
||||
end: *const T,
|
||||
_marker: marker::PhantomData<&'a T>,
|
||||
}
|
||||
```
|
||||
|
||||
and that's it. The lifetime will be bounded, and your iterator will be variant
|
||||
over `'a` and `T`. Everything Just Works.
|
||||
|
||||
Another important example is Vec, which is (approximately) defined as follows:
|
||||
|
||||
```
|
||||
struct Vec<T> {
|
||||
data: *const T, // *const for variance!
|
||||
len: usize,
|
||||
cap: usize,
|
||||
}
|
||||
```
|
||||
|
||||
Unlike the previous example, it *appears* that everything is exactly as we
|
||||
want. Every generic argument to Vec shows up in at least one field.
|
||||
Good to go!
|
||||
|
||||
Nope.
|
||||
|
||||
The drop checker will generously determine that `Vec<T>` does not own any values
|
||||
of type T. This will in turn make it conclude that it doesn't need to worry
|
||||
about Vec dropping any T's in its destructor for determining drop check
|
||||
soundness. This will in turn allow people to create unsoundness using
|
||||
Vec's destructor.
|
||||
|
||||
In order to tell dropck that we *do* own values of type T, and therefore may
|
||||
drop some T's when *we* drop, we must add an extra PhantomData saying exactly
|
||||
that:
|
||||
|
||||
```
|
||||
use std::marker;
|
||||
|
||||
struct Vec<T> {
|
||||
data: *const T, // *const for covariance!
|
||||
len: usize,
|
||||
cap: usize,
|
||||
_marker: marker::PhantomData<T>,
|
||||
}
|
||||
```
|
||||
|
||||
Raw pointers that own an allocation is such a pervasive pattern that the
|
||||
standard library made a utility for itself called `Unique<T>` which:
|
||||
|
||||
* wraps a `*const T` for variance
|
||||
* includes a `PhantomData<T>`
|
||||
* auto-derives `Send`/`Sync` as if T was contained
|
||||
* marks the pointer as `NonZero` for the null-pointer optimization
|
||||
|
||||
## Table of `PhantomData` patterns
|
||||
|
||||
Here’s a table of all the wonderful ways `PhantomData` could be used:
|
||||
|
||||
| Phantom type | `'a` | `T` |
|
||||
|-----------------------------|-----------|---------------------------|
|
||||
| `PhantomData<T>` | - | variant (with drop check) |
|
||||
| `PhantomData<&'a T>` | variant | variant |
|
||||
| `PhantomData<&'a mut T>` | variant | invariant |
|
||||
| `PhantomData<*const T>` | - | variant |
|
||||
| `PhantomData<*mut T>` | - | invariant |
|
||||
| `PhantomData<fn(T)>` | - | contravariant (*) |
|
||||
| `PhantomData<fn() -> T>` | - | variant |
|
||||
| `PhantomData<fn(T) -> T>` | - | invariant |
|
||||
| `PhantomData<Cell<&'a ()>>` | invariant | - |
|
||||
|
||||
(*) If contravariance gets scrapped, this would be invariant.
|
|
@ -1,35 +0,0 @@
|
|||
# Poisoning
|
||||
|
||||
Although all unsafe code *must* ensure it has minimal exception safety, not all
|
||||
types ensure *maximal* exception safety. Even if the type does, your code may
|
||||
ascribe additional meaning to it. For instance, an integer is certainly
|
||||
exception-safe, but has no semantics on its own. It's possible that code that
|
||||
panics could fail to correctly update the integer, producing an inconsistent
|
||||
program state.
|
||||
|
||||
This is *usually* fine, because anything that witnesses an exception is about
|
||||
to get destroyed. For instance, if you send a Vec to another thread and that
|
||||
thread panics, it doesn't matter if the Vec is in a weird state. It will be
|
||||
dropped and go away forever. However some types are especially good at smuggling
|
||||
values across the panic boundary.
|
||||
|
||||
These types may choose to explicitly *poison* themselves if they witness a panic.
|
||||
Poisoning doesn't entail anything in particular. Generally it just means
|
||||
preventing normal usage from proceeding. The most notable example of this is the
|
||||
standard library's Mutex type. A Mutex will poison itself if one of its
|
||||
MutexGuards (the thing it returns when a lock is obtained) is dropped during a
|
||||
panic. Any future attempts to lock the Mutex will return an `Err` or panic.
|
||||
|
||||
Mutex poisons not for true safety in the sense that Rust normally cares about. It
|
||||
poisons as a safety-guard against blindly using the data that comes out of a Mutex
|
||||
that has witnessed a panic while locked. The data in such a Mutex was likely in the
|
||||
middle of being modified, and as such may be in an inconsistent or incomplete state.
|
||||
It is important to note that one cannot violate memory safety with such a type
|
||||
if it is correctly written. After all, it must be minimally exception-safe!
|
||||
|
||||
However if the Mutex contained, say, a BinaryHeap that does not actually have the
|
||||
heap property, it's unlikely that any code that uses it will do
|
||||
what the author intended. As such, the program should not proceed normally.
|
||||
Still, if you're double-plus-sure that you can do *something* with the value,
|
||||
the Mutex exposes a method to get the lock anyway. It *is* safe, after all.
|
||||
Just maybe nonsense.
|
|
@ -1,86 +0,0 @@
|
|||
# Data Races and Race Conditions
|
||||
|
||||
Safe Rust guarantees an absence of data races, which are defined as:
|
||||
|
||||
* two or more threads concurrently accessing a location of memory
|
||||
* one of them is a write
|
||||
* one of them is unsynchronized
|
||||
|
||||
A data race has Undefined Behavior, and is therefore impossible to perform
|
||||
in Safe Rust. Data races are *mostly* prevented through rust's ownership system:
|
||||
it's impossible to alias a mutable reference, so it's impossible to perform a
|
||||
data race. Interior mutability makes this more complicated, which is largely why
|
||||
we have the Send and Sync traits (see below).
|
||||
|
||||
**However Rust does not prevent general race conditions.**
|
||||
|
||||
This is pretty fundamentally impossible, and probably honestly undesirable. Your
|
||||
hardware is racy, your OS is racy, the other programs on your computer are racy,
|
||||
and the world this all runs in is racy. Any system that could genuinely claim to
|
||||
prevent *all* race conditions would be pretty awful to use, if not just
|
||||
incorrect.
|
||||
|
||||
So it's perfectly "fine" for a Safe Rust program to get deadlocked or do
|
||||
something nonsensical with incorrect synchronization. Obviously such a program
|
||||
isn't very good, but Rust can only hold your hand so far. Still, a race
|
||||
condition can't violate memory safety in a Rust program on its own. Only in
|
||||
conjunction with some other unsafe code can a race condition actually violate
|
||||
memory safety. For instance:
|
||||
|
||||
```rust,no_run
|
||||
use std::thread;
|
||||
use std::sync::atomic::{AtomicUsize, Ordering};
|
||||
use std::sync::Arc;
|
||||
|
||||
let data = vec![1, 2, 3, 4];
|
||||
// Arc so that the memory the AtomicUsize is stored in still exists for
|
||||
// the other thread to increment, even if we completely finish executing
|
||||
// before it. Rust won't compile the program without it, because of the
|
||||
// lifetime requirements of thread::spawn!
|
||||
let idx = Arc::new(AtomicUsize::new(0));
|
||||
let other_idx = idx.clone();
|
||||
|
||||
// `move` captures other_idx by-value, moving it into this thread
|
||||
thread::spawn(move || {
|
||||
// It's ok to mutate idx because this value
|
||||
// is an atomic, so it can't cause a Data Race.
|
||||
other_idx.fetch_add(10, Ordering::SeqCst);
|
||||
});
|
||||
|
||||
// Index with the value loaded from the atomic. This is safe because we
|
||||
// read the atomic memory only once, and then pass a copy of that value
|
||||
// to the Vec's indexing implementation. This indexing will be correctly
|
||||
// bounds checked, and there's no chance of the value getting changed
|
||||
// in the middle. However our program may panic if the thread we spawned
|
||||
// managed to increment before this ran. A race condition because correct
|
||||
// program execution (panicking is rarely correct) depends on order of
|
||||
// thread execution.
|
||||
println!("{}", data[idx.load(Ordering::SeqCst)]);
|
||||
```
|
||||
|
||||
```rust,no_run
|
||||
use std::thread;
|
||||
use std::sync::atomic::{AtomicUsize, Ordering};
|
||||
use std::sync::Arc;
|
||||
|
||||
let data = vec![1, 2, 3, 4];
|
||||
|
||||
let idx = Arc::new(AtomicUsize::new(0));
|
||||
let other_idx = idx.clone();
|
||||
|
||||
// `move` captures other_idx by-value, moving it into this thread
|
||||
thread::spawn(move || {
|
||||
// It's ok to mutate idx because this value
|
||||
// is an atomic, so it can't cause a Data Race.
|
||||
other_idx.fetch_add(10, Ordering::SeqCst);
|
||||
});
|
||||
|
||||
if idx.load(Ordering::SeqCst) < data.len() {
|
||||
unsafe {
|
||||
// Incorrectly loading the idx after we did the bounds check.
|
||||
// It could have changed. This is a race condition, *and dangerous*
|
||||
// because we decided to do `get_unchecked`, which is `unsafe`.
|
||||
println!("{}", data.get_unchecked(idx.load(Ordering::SeqCst)));
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,177 +0,0 @@
|
|||
# References
|
||||
|
||||
This section gives a high-level view of the memory model that *all* Rust
|
||||
programs must satisfy to be correct. Safe code is statically verified
|
||||
to obey this model by the borrow checker. Unsafe code may go above
|
||||
and beyond the borrow checker while still satisfying this model. The borrow
|
||||
checker may also be extended to allow more programs to compile, as long as
|
||||
this more fundamental model is satisfied.
|
||||
|
||||
There are two kinds of reference:
|
||||
|
||||
* Shared reference: `&`
|
||||
* Mutable reference: `&mut`
|
||||
|
||||
Which obey the following rules:
|
||||
|
||||
* A reference cannot outlive its referent
|
||||
* A mutable reference cannot be aliased
|
||||
|
||||
That's it. That's the whole model. Of course, we should probably define
|
||||
what *aliased* means. To define aliasing, we must define the notion of
|
||||
*paths* and *liveness*.
|
||||
|
||||
|
||||
**NOTE: The model that follows is generally agreed to be dubious and have
|
||||
issues. It's ok-ish as an intuitive model, but fails to capture the desired
|
||||
semantics. We leave this here to be able to use notions introduced here in later
|
||||
sections. This will be significantly changed in the future. TODO: do that.**
|
||||
|
||||
|
||||
# Paths
|
||||
|
||||
If all Rust had were values (no pointers), then every value would be uniquely
|
||||
owned by a variable or composite structure. From this we naturally derive a
|
||||
*tree* of ownership. The stack itself is the root of the tree, with every
|
||||
variable as its direct children. Each variable's direct children would be their
|
||||
fields (if any), and so on.
|
||||
|
||||
From this view, every value in Rust has a unique *path* in the tree of
|
||||
ownership. Of particular interest are *ancestors* and *descendants*: if `x` owns
|
||||
`y`, then `x` is an ancestor of `y`, and `y` is a descendant of `x`. Note
|
||||
that this is an inclusive relationship: `x` is a descendant and ancestor of
|
||||
itself.
|
||||
|
||||
We can then define references as simply *names* for paths. When you create a
|
||||
reference, you're declaring that an ownership path exists to this address
|
||||
of memory.
|
||||
|
||||
Tragically, plenty of data doesn't reside on the stack, and we must also
|
||||
accommodate this. Globals and thread-locals are simple enough to model as
|
||||
residing at the bottom of the stack (though we must be careful with mutable
|
||||
globals). Data on the heap poses a different problem.
|
||||
|
||||
If all Rust had on the heap was data uniquely owned by a pointer on the stack,
|
||||
then we could just treat such a pointer as a struct that owns the value on the
|
||||
heap. Box, Vec, String, and HashMap, are examples of types which uniquely
|
||||
own data on the heap.
|
||||
|
||||
Unfortunately, data on the heap is not *always* uniquely owned. Rc for instance
|
||||
introduces a notion of *shared* ownership. Shared ownership of a value means
|
||||
there is no unique path to it. A value with no unique path limits what we can do
|
||||
with it.
|
||||
|
||||
In general, only shared references can be created to non-unique paths. However
|
||||
mechanisms which ensure mutual exclusion may establish One True Owner
|
||||
temporarily, establishing a unique path to that value (and therefore all
|
||||
its children). If this is done, the value may be mutated. In particular, a
|
||||
mutable reference can be taken.
|
||||
|
||||
The most common way to establish such a path is through *interior mutability*,
|
||||
in contrast to the *inherited mutability* that everything in Rust normally uses.
|
||||
Cell, RefCell, Mutex, and RWLock are all examples of interior mutability types.
|
||||
These types provide exclusive access through runtime restrictions.
|
||||
|
||||
An interesting case of this effect is Rc itself: if an Rc has refcount 1,
|
||||
then it is safe to mutate or even move its internals. Note however that the
|
||||
refcount itself uses interior mutability.
|
||||
|
||||
In order to correctly communicate to the type system that a variable or field of
|
||||
a struct can have interior mutability, it must be wrapped in an UnsafeCell. This
|
||||
does not in itself make it safe to perform interior mutability operations on
|
||||
that value. You still must yourself ensure that mutual exclusion is upheld.
|
||||
|
||||
|
||||
|
||||
|
||||
# Liveness
|
||||
|
||||
Note: Liveness is not the same thing as a *lifetime*, which will be explained
|
||||
in detail in the next section of this chapter.
|
||||
|
||||
Roughly, a reference is *live* at some point in a program if it can be
|
||||
dereferenced. Shared references are always live unless they are literally
|
||||
unreachable (for instance, they reside in freed or leaked memory). Mutable
|
||||
references can be reachable but *not* live through the process of *reborrowing*.
|
||||
|
||||
A mutable reference can be reborrowed to either a shared or mutable reference to
|
||||
one of its descendants. A reborrowed reference will only be live again once all
|
||||
reborrows derived from it expire. For instance, a mutable reference can be
|
||||
reborrowed to point to a field of its referent:
|
||||
|
||||
```rust
|
||||
let x = &mut (1, 2);
|
||||
{
|
||||
// reborrow x to a subfield
|
||||
let y = &mut x.0;
|
||||
// y is now live, but x isn't
|
||||
*y = 3;
|
||||
}
|
||||
// y goes out of scope, so x is live again
|
||||
*x = (5, 7);
|
||||
```
|
||||
|
||||
It is also possible to reborrow into *multiple* mutable references, as long as
|
||||
they are *disjoint*: no reference is an ancestor of another. Rust
|
||||
explicitly enables this to be done with disjoint struct fields, because
|
||||
disjointness can be statically proven:
|
||||
|
||||
```rust
|
||||
let x = &mut (1, 2);
|
||||
{
|
||||
// reborrow x to two disjoint subfields
|
||||
let y = &mut x.0;
|
||||
let z = &mut x.1;
|
||||
|
||||
// y and z are now live, but x isn't
|
||||
*y = 3;
|
||||
*z = 4;
|
||||
}
|
||||
// y and z go out of scope, so x is live again
|
||||
*x = (5, 7);
|
||||
```
|
||||
|
||||
However it's often the case that Rust isn't sufficiently smart to prove that
|
||||
multiple borrows are disjoint. *This does not mean it is fundamentally illegal
|
||||
to make such a borrow*, just that Rust isn't as smart as you want.
|
||||
|
||||
To simplify things, we can model variables as a fake type of reference: *owned*
|
||||
references. Owned references have much the same semantics as mutable references:
|
||||
they can be re-borrowed in a mutable or shared manner, which makes them no
|
||||
longer live. Live owned references have the unique property that they can be
|
||||
moved out of (though mutable references *can* be swapped out of). This power is
|
||||
only given to *live* owned references because moving its referent would of
|
||||
course invalidate all outstanding references prematurely.
|
||||
|
||||
As a local lint against inappropriate mutation, only variables that are marked
|
||||
as `mut` can be borrowed mutably.
|
||||
|
||||
It is interesting to note that Box behaves exactly like an owned reference. It
|
||||
can be moved out of, and Rust understands it sufficiently to reason about its
|
||||
paths like a normal variable.
|
||||
|
||||
|
||||
|
||||
|
||||
# Aliasing
|
||||
|
||||
With liveness and paths defined, we can now properly define *aliasing*:
|
||||
|
||||
**A mutable reference is aliased if there exists another live reference to one
|
||||
of its ancestors or descendants.**
|
||||
|
||||
(If you prefer, you may also say the two live references alias *each other*.
|
||||
This has no semantic consequences, but is probably a more useful notion when
|
||||
verifying the soundness of a construct.)
|
||||
|
||||
That's it. Super simple right? Except for the fact that it took us two pages to
|
||||
define all of the terms in that definition. You know: Super. Simple.
|
||||
|
||||
Actually it's a bit more complicated than that. In addition to references, Rust
|
||||
has *raw pointers*: `*const T` and `*mut T`. Raw pointers have no inherent
|
||||
ownership or aliasing semantics. As a result, Rust makes absolutely no effort to
|
||||
track that they are used correctly, and they are wildly unsafe.
|
||||
|
||||
**It is an open question to what degree raw pointers have alias semantics.
|
||||
However it is important for these definitions to be sound that the existence of
|
||||
a raw pointer does not imply some kind of live path.**
|
|
@ -1,154 +0,0 @@
|
|||
# repr(Rust)
|
||||
|
||||
First and foremost, all types have an alignment specified in bytes. The
|
||||
alignment of a type specifies what addresses are valid to store the value at. A
|
||||
value of alignment `n` must only be stored at an address that is a multiple of
|
||||
`n`. So alignment 2 means you must be stored at an even address, and 1 means
|
||||
that you can be stored anywhere. Alignment is at least 1, and always a power of
|
||||
2. Most primitives are generally aligned to their size, although this is
|
||||
platform-specific behavior. In particular, on x86 `u64` and `f64` may be only
|
||||
aligned to 32 bits.
|
||||
|
||||
A type's size must always be a multiple of its alignment. This ensures that an
|
||||
array of that type may always be indexed by offsetting by a multiple of its
|
||||
size. Note that the size and alignment of a type may not be known
|
||||
statically in the case of [dynamically sized types][dst].
|
||||
|
||||
Rust gives you the following ways to lay out composite data:
|
||||
|
||||
* structs (named product types)
|
||||
* tuples (anonymous product types)
|
||||
* arrays (homogeneous product types)
|
||||
* enums (named sum types -- tagged unions)
|
||||
|
||||
An enum is said to be *C-like* if none of its variants have associated data.
|
||||
|
||||
Composite structures will have an alignment equal to the maximum
|
||||
of their fields' alignment. Rust will consequently insert padding where
|
||||
necessary to ensure that all fields are properly aligned and that the overall
|
||||
type's size is a multiple of its alignment. For instance:
|
||||
|
||||
```rust
|
||||
struct A {
|
||||
a: u8,
|
||||
b: u32,
|
||||
c: u16,
|
||||
}
|
||||
```
|
||||
|
||||
will be 32-bit aligned on an architecture that aligns these primitives to their
|
||||
respective sizes. The whole struct will therefore have a size that is a multiple
|
||||
of 32-bits. It will potentially become:
|
||||
|
||||
```rust
|
||||
struct A {
|
||||
a: u8,
|
||||
_pad1: [u8; 3], // to align `b`
|
||||
b: u32,
|
||||
c: u16,
|
||||
_pad2: [u8; 2], // to make overall size multiple of 4
|
||||
}
|
||||
```
|
||||
|
||||
There is *no indirection* for these types; all data is stored within the struct,
|
||||
as you would expect in C. However with the exception of arrays (which are
|
||||
densely packed and in-order), the layout of data is not by default specified in
|
||||
Rust. Given the two following struct definitions:
|
||||
|
||||
```rust
|
||||
struct A {
|
||||
a: i32,
|
||||
b: u64,
|
||||
}
|
||||
|
||||
struct B {
|
||||
a: i32,
|
||||
b: u64,
|
||||
}
|
||||
```
|
||||
|
||||
Rust *does* guarantee that two instances of A have their data laid out in
|
||||
exactly the same way. However Rust *does not* currently guarantee that an
|
||||
instance of A has the same field ordering or padding as an instance of B, though
|
||||
in practice there's no reason why they wouldn't.
|
||||
|
||||
With A and B as written, this point would seem to be pedantic, but several other
|
||||
features of Rust make it desirable for the language to play with data layout in
|
||||
complex ways.
|
||||
|
||||
For instance, consider this struct:
|
||||
|
||||
```rust
|
||||
struct Foo<T, U> {
|
||||
count: u16,
|
||||
data1: T,
|
||||
data2: U,
|
||||
}
|
||||
```
|
||||
|
||||
Now consider the monomorphizations of `Foo<u32, u16>` and `Foo<u16, u32>`. If
|
||||
Rust lays out the fields in the order specified, we expect it to pad the
|
||||
values in the struct to satisfy their alignment requirements. So if Rust
|
||||
didn't reorder fields, we would expect it to produce the following:
|
||||
|
||||
```rust,ignore
|
||||
struct Foo<u16, u32> {
|
||||
count: u16,
|
||||
data1: u16,
|
||||
data2: u32,
|
||||
}
|
||||
|
||||
struct Foo<u32, u16> {
|
||||
count: u16,
|
||||
_pad1: u16,
|
||||
data1: u32,
|
||||
data2: u16,
|
||||
_pad2: u16,
|
||||
}
|
||||
```
|
||||
|
||||
The latter case quite simply wastes space. An optimal use of space therefore
|
||||
requires different monomorphizations to have *different field orderings*.
|
||||
|
||||
**Note: this is a hypothetical optimization that is not yet implemented in Rust
|
||||
1.0**
|
||||
|
||||
Enums make this consideration even more complicated. Naively, an enum such as:
|
||||
|
||||
```rust
|
||||
enum Foo {
|
||||
A(u32),
|
||||
B(u64),
|
||||
C(u8),
|
||||
}
|
||||
```
|
||||
|
||||
would be laid out as:
|
||||
|
||||
```rust
|
||||
struct FooRepr {
|
||||
data: u64, // this is either a u64, u32, or u8 based on `tag`
|
||||
tag: u8, // 0 = A, 1 = B, 2 = C
|
||||
}
|
||||
```
|
||||
|
||||
And indeed this is approximately how it would be laid out in general (modulo the
|
||||
size and position of `tag`).
|
||||
|
||||
However there are several cases where such a representation is inefficient. The
|
||||
classic case of this is Rust's "null pointer optimization": an enum consisting
|
||||
of a single outer unit variant (e.g. `None`) and a (potentially nested) non-
|
||||
nullable pointer variant (e.g. `&T`) makes the tag unnecessary, because a null
|
||||
pointer value can safely be interpreted to mean that the unit variant is chosen
|
||||
instead. The net result is that, for example, `size_of::<Option<&T>>() ==
|
||||
size_of::<&T>()`.
|
||||
|
||||
There are many types in Rust that are, or contain, non-nullable pointers such as
|
||||
`Box<T>`, `Vec<T>`, `String`, `&T`, and `&mut T`. Similarly, one can imagine
|
||||
nested enums pooling their tags into a single discriminant, as they are by
|
||||
definition known to have a limited range of valid values. In principle enums could
|
||||
use fairly elaborate algorithms to cache bits throughout nested types with
|
||||
special constrained representations. As such it is *especially* desirable that
|
||||
we leave enum layout unspecified today.
|
||||
|
||||
[dst]: exotic-sizes.html#dynamically-sized-types-dsts
|
|
@ -1,123 +0,0 @@
|
|||
# How Safe and Unsafe Interact
|
||||
|
||||
What's the relationship between Safe Rust and Unsafe Rust? How do they
|
||||
interact?
|
||||
|
||||
The separation between Safe Rust and Unsafe Rust is controlled with the
|
||||
`unsafe` keyword, which acts as an interface from one to the other. This is
|
||||
why we can say Safe Rust is a safe language: all the unsafe parts are kept
|
||||
exclusively behind the boundary.
|
||||
|
||||
The `unsafe` keyword has two uses: to declare the existence of contracts the
|
||||
compiler can't check, and to declare that the adherence of some code to
|
||||
those contracts has been checked by the programmer.
|
||||
|
||||
You can use `unsafe` to indicate the existence of unchecked contracts on
|
||||
_functions_ and on _trait declarations_. On functions, `unsafe` means that
|
||||
users of the function must check that function's documentation to ensure
|
||||
they are using it in a way that maintains the contracts the function
|
||||
requires. On trait declarations, `unsafe` means that implementors of the
|
||||
trait must check the trait documentation to ensure their implementation
|
||||
maintains the contracts the trait requires.
|
||||
|
||||
You can use `unsafe` on a block to declare that all constraints required
|
||||
by an unsafe function within the block have been adhered to, and the code
|
||||
can therefore be trusted. You can use `unsafe` on a trait implementation
|
||||
to declare that the implementation of that trait has adhered to whatever
|
||||
contracts the trait's documentation requires.
|
||||
|
||||
The standard library has a number of unsafe functions, including:
|
||||
|
||||
* `slice::get_unchecked`, which performs unchecked indexing, allowing
|
||||
memory safety to be freely violated.
|
||||
* `mem::transmute` reinterprets some value as having a given type, bypassing
|
||||
type safety in arbitrary ways (see [conversions] for details).
|
||||
* Every raw pointer to a sized type has an intrinstic `offset` method that
|
||||
invokes Undefined Behavior if the passed offset is not "in bounds" as
|
||||
defined by LLVM.
|
||||
* All FFI functions are `unsafe` because the other language can do arbitrary
|
||||
operations that the Rust compiler can't check.
|
||||
|
||||
As of Rust 1.0 there are exactly two unsafe traits:
|
||||
|
||||
* `Send` is a marker trait (a trait with no API) that promises implementors are
|
||||
safe to send (move) to another thread.
|
||||
* `Sync` is a marker trait that promises threads can safely share implementors
|
||||
through a shared reference.
|
||||
|
||||
Much of the Rust standard library also uses Unsafe Rust internally, although
|
||||
these implementations are rigorously manually checked, and the Safe Rust
|
||||
interfaces provided on top of these implementations can be assumed to be safe.
|
||||
|
||||
The need for all of this separation boils down a single fundamental property
|
||||
of Safe Rust:
|
||||
|
||||
**No matter what, Safe Rust can't cause Undefined Behavior.**
|
||||
|
||||
The design of the safe/unsafe split means that Safe Rust inherently has to
|
||||
trust that any Unsafe Rust it touches has been written correctly (meaning
|
||||
the Unsafe Rust actually maintains whatever contracts it is supposed to
|
||||
maintain). On the other hand, Unsafe Rust has to be very careful about
|
||||
trusting Safe Rust.
|
||||
|
||||
As an example, Rust has the `PartialOrd` and `Ord` traits to differentiate
|
||||
between types which can "just" be compared, and those that provide a total
|
||||
ordering (where every value of the type is either equal to, greater than,
|
||||
or less than any other value of the same type). The sorted map type
|
||||
`BTreeMap` doesn't make sense for partially-ordered types, and so it
|
||||
requires that any key type for it implements the `Ord` trait. However,
|
||||
`BTreeMap` has Unsafe Rust code inside of its implementation, and this
|
||||
Unsafe Rust code cannot assume that any `Ord` implementation it gets makes
|
||||
sense. The unsafe portions of `BTreeMap`'s internals have to be careful to
|
||||
maintain all necessary contracts, even if a key type's `Ord` implementation
|
||||
does not implement a total ordering.
|
||||
|
||||
Unsafe Rust cannot automatically trust Safe Rust. When writing Unsafe Rust,
|
||||
you must be careful to only rely on specific Safe Rust code, and not make
|
||||
assumptions about potential future Safe Rust code providing the same
|
||||
guarantees.
|
||||
|
||||
This is the problem that `unsafe` traits exist to resolve. The `BTreeMap`
|
||||
type could theoretically require that keys implement a new trait called
|
||||
`UnsafeOrd`, rather than `Ord`, that might look like this:
|
||||
|
||||
```rust
|
||||
use std::cmp::Ordering;
|
||||
|
||||
unsafe trait UnsafeOrd {
|
||||
fn cmp(&self, other: &Self) -> Ordering;
|
||||
}
|
||||
```
|
||||
|
||||
Then, a type would use `unsafe` to implement `UnsafeOrd`, indicating that
|
||||
they've ensured their implementation maintains whatever contracts the
|
||||
trait expects. In this situation, the Unsafe Rust in the internals of
|
||||
`BTreeMap` could trust that the key type's `UnsafeOrd` implementation is
|
||||
correct. If it isn't, it's the fault of the unsafe trait implementation
|
||||
code, which is consistent with Rust's safety guarantees.
|
||||
|
||||
The decision of whether to mark a trait `unsafe` is an API design choice.
|
||||
Rust has traditionally avoided marking traits unsafe because it makes Unsafe
|
||||
Rust pervasive, which is not desirable. `Send` and `Sync` are marked unsafe
|
||||
because thread safety is a *fundamental property* that unsafe code can't
|
||||
possibly hope to defend against in the way it could defend against a bad
|
||||
`Ord` implementation. The decision of whether to mark your own traits `unsafe`
|
||||
depends on the same sort of consideration. If `unsafe` code cannot reasonably
|
||||
expect to defend against a bad implementation of the trait, then marking the
|
||||
trait `unsafe` is a reasonable choice.
|
||||
|
||||
As an aside, while `Send` and `Sync` are `unsafe` traits, they are
|
||||
automatically implemented for types when such derivations are provably safe
|
||||
to do. `Send` is automatically derived for all types composed only of values
|
||||
whose types also implement `Send`. `Sync` is automatically derived for all
|
||||
types composed only of values whose types also implement `Sync`.
|
||||
|
||||
This is the dance of Safe Rust and Unsafe Rust. It is designed to make using
|
||||
Safe Rust as ergonomic as possible, but requires extra effort and care when
|
||||
writing Unsafe Rust. The rest of the book is largely a discussion of the sort
|
||||
of care that must be taken, and what contracts it is expected of Unsafe Rust
|
||||
to uphold.
|
||||
|
||||
[drop flags]: drop-flags.html
|
||||
[conversions]: conversions.html
|
||||
|
|
@ -1,80 +0,0 @@
|
|||
# Send and Sync
|
||||
|
||||
Not everything obeys inherited mutability, though. Some types allow you to
|
||||
multiply alias a location in memory while mutating it. Unless these types use
|
||||
synchronization to manage this access, they are absolutely not thread safe. Rust
|
||||
captures this through the `Send` and `Sync` traits.
|
||||
|
||||
* A type is Send if it is safe to send it to another thread.
|
||||
* A type is Sync if it is safe to share between threads (`&T` is Send).
|
||||
|
||||
Send and Sync are fundamental to Rust's concurrency story. As such, a
|
||||
substantial amount of special tooling exists to make them work right. First and
|
||||
foremost, they're [unsafe traits]. This means that they are unsafe to
|
||||
implement, and other unsafe code can assume that they are correctly
|
||||
implemented. Since they're *marker traits* (they have no associated items like
|
||||
methods), correctly implemented simply means that they have the intrinsic
|
||||
properties an implementor should have. Incorrectly implementing Send or Sync can
|
||||
cause Undefined Behavior.
|
||||
|
||||
Send and Sync are also automatically derived traits. This means that, unlike
|
||||
every other trait, if a type is composed entirely of Send or Sync types, then it
|
||||
is Send or Sync. Almost all primitives are Send and Sync, and as a consequence
|
||||
pretty much all types you'll ever interact with are Send and Sync.
|
||||
|
||||
Major exceptions include:
|
||||
|
||||
* raw pointers are neither Send nor Sync (because they have no safety guards).
|
||||
* `UnsafeCell` isn't Sync (and therefore `Cell` and `RefCell` aren't).
|
||||
* `Rc` isn't Send or Sync (because the refcount is shared and unsynchronized).
|
||||
|
||||
`Rc` and `UnsafeCell` are very fundamentally not thread-safe: they enable
|
||||
unsynchronized shared mutable state. However raw pointers are, strictly
|
||||
speaking, marked as thread-unsafe as more of a *lint*. Doing anything useful
|
||||
with a raw pointer requires dereferencing it, which is already unsafe. In that
|
||||
sense, one could argue that it would be "fine" for them to be marked as thread
|
||||
safe.
|
||||
|
||||
However it's important that they aren't thread safe to prevent types that
|
||||
contain them from being automatically marked as thread safe. These types have
|
||||
non-trivial untracked ownership, and it's unlikely that their author was
|
||||
necessarily thinking hard about thread safety. In the case of Rc, we have a nice
|
||||
example of a type that contains a `*mut` that is definitely not thread safe.
|
||||
|
||||
Types that aren't automatically derived can simply implement them if desired:
|
||||
|
||||
```rust
|
||||
struct MyBox(*mut u8);
|
||||
|
||||
unsafe impl Send for MyBox {}
|
||||
unsafe impl Sync for MyBox {}
|
||||
```
|
||||
|
||||
In the *incredibly rare* case that a type is inappropriately automatically
|
||||
derived to be Send or Sync, then one can also unimplement Send and Sync:
|
||||
|
||||
```rust
|
||||
#![feature(optin_builtin_traits)]
|
||||
|
||||
// I have some magic semantics for some synchronization primitive!
|
||||
struct SpecialThreadToken(u8);
|
||||
|
||||
impl !Send for SpecialThreadToken {}
|
||||
impl !Sync for SpecialThreadToken {}
|
||||
```
|
||||
|
||||
Note that *in and of itself* it is impossible to incorrectly derive Send and
|
||||
Sync. Only types that are ascribed special meaning by other unsafe code can
|
||||
possible cause trouble by being incorrectly Send or Sync.
|
||||
|
||||
Most uses of raw pointers should be encapsulated behind a sufficient abstraction
|
||||
that Send and Sync can be derived. For instance all of Rust's standard
|
||||
collections are Send and Sync (when they contain Send and Sync types) in spite
|
||||
of their pervasive use of raw pointers to manage allocations and complex ownership.
|
||||
Similarly, most iterators into these collections are Send and Sync because they
|
||||
largely behave like an `&` or `&mut` into the collection.
|
||||
|
||||
TODO: better explain what can or can't be Send or Sync. Sufficient to appeal
|
||||
only to data races?
|
||||
|
||||
[unsafe traits]: safe-unsafe-meaning.html
|
|
@ -1,213 +0,0 @@
|
|||
# Subtyping and Variance
|
||||
|
||||
Although Rust doesn't have any notion of structural inheritance, it *does*
|
||||
include subtyping. In Rust, subtyping derives entirely from lifetimes. Since
|
||||
lifetimes are scopes, we can partially order them based on the *contains*
|
||||
(outlives) relationship. We can even express this as a generic bound.
|
||||
|
||||
Subtyping on lifetimes is in terms of that relationship: if `'a: 'b` ("a contains
|
||||
b" or "a outlives b"), then `'a` is a subtype of `'b`. This is a large source of
|
||||
confusion, because it seems intuitively backwards to many: the bigger scope is a
|
||||
*subtype* of the smaller scope.
|
||||
|
||||
This does in fact make sense, though. The intuitive reason for this is that if
|
||||
you expect an `&'a u8`, then it's totally fine for me to hand you an `&'static
|
||||
u8`, in the same way that if you expect an Animal in Java, it's totally fine for
|
||||
me to hand you a Cat. Cats are just Animals *and more*, just as `'static` is
|
||||
just `'a` *and more*.
|
||||
|
||||
(Note, the subtyping relationship and typed-ness of lifetimes is a fairly
|
||||
arbitrary construct that some disagree with. However it simplifies our analysis
|
||||
to treat lifetimes and types uniformly.)
|
||||
|
||||
Higher-ranked lifetimes are also subtypes of every concrete lifetime. This is
|
||||
because taking an arbitrary lifetime is strictly more general than taking a
|
||||
specific one.
|
||||
|
||||
|
||||
|
||||
# Variance
|
||||
|
||||
Variance is where things get a bit complicated.
|
||||
|
||||
Variance is a property that *type constructors* have with respect to their
|
||||
arguments. A type constructor in Rust is a generic type with unbound arguments.
|
||||
For instance `Vec` is a type constructor that takes a `T` and returns a
|
||||
`Vec<T>`. `&` and `&mut` are type constructors that take two inputs: a
|
||||
lifetime, and a type to point to.
|
||||
|
||||
A type constructor's *variance* is how the subtyping of its inputs affects the
|
||||
subtyping of its outputs. There are two kinds of variance in Rust:
|
||||
|
||||
* F is *variant* over `T` if `T` being a subtype of `U` implies
|
||||
`F<T>` is a subtype of `F<U>` (subtyping "passes through")
|
||||
* F is *invariant* over `T` otherwise (no subtyping relation can be derived)
|
||||
|
||||
(For those of you who are familiar with variance from other languages, what we
|
||||
refer to as "just" variance is in fact *covariance*. Rust has *contravariance*
|
||||
for functions. The future of contravariance is uncertain and it may be
|
||||
scrapped. For now, `fn(T)` is contravariant in `T`, which is used in matching
|
||||
methods in trait implementations to the trait definition. Traits don't have
|
||||
inferred variance, so `Fn(T)` is invariant in `T`).
|
||||
|
||||
Some important variances:
|
||||
|
||||
* `&'a T` is variant over `'a` and `T` (as is `*const T` by metaphor)
|
||||
* `&'a mut T` is variant over `'a` but invariant over `T`
|
||||
* `Fn(T) -> U` is invariant over `T`, but variant over `U`
|
||||
* `Box`, `Vec`, and all other collections are variant over the types of
|
||||
their contents
|
||||
* `UnsafeCell<T>`, `Cell<T>`, `RefCell<T>`, `Mutex<T>` and all other
|
||||
interior mutability types are invariant over T (as is `*mut T` by metaphor)
|
||||
|
||||
To understand why these variances are correct and desirable, we will consider
|
||||
several examples.
|
||||
|
||||
|
||||
We have already covered why `&'a T` should be variant over `'a` when
|
||||
introducing subtyping: it's desirable to be able to pass longer-lived things
|
||||
where shorter-lived things are needed.
|
||||
|
||||
Similar reasoning applies to why it should be variant over T. It is reasonable
|
||||
to be able to pass `&&'static str` where an `&&'a str` is expected. The
|
||||
additional level of indirection does not change the desire to be able to pass
|
||||
longer lived things where shorted lived things are expected.
|
||||
|
||||
However this logic doesn't apply to `&mut`. To see why `&mut` should
|
||||
be invariant over T, consider the following code:
|
||||
|
||||
```rust,ignore
|
||||
fn overwrite<T: Copy>(input: &mut T, new: &mut T) {
|
||||
*input = *new;
|
||||
}
|
||||
|
||||
fn main() {
|
||||
let mut forever_str: &'static str = "hello";
|
||||
{
|
||||
let string = String::from("world");
|
||||
overwrite(&mut forever_str, &mut &*string);
|
||||
}
|
||||
// Oops, printing free'd memory
|
||||
println!("{}", forever_str);
|
||||
}
|
||||
```
|
||||
|
||||
The signature of `overwrite` is clearly valid: it takes mutable references to
|
||||
two values of the same type, and overwrites one with the other. If `&mut T` was
|
||||
variant over T, then `&mut &'static str` would be a subtype of `&mut &'a str`,
|
||||
since `&'static str` is a subtype of `&'a str`. Therefore the lifetime of
|
||||
`forever_str` would successfully be "shrunk" down to the shorter lifetime of
|
||||
`string`, and `overwrite` would be called successfully. `string` would
|
||||
subsequently be dropped, and `forever_str` would point to freed memory when we
|
||||
print it! Therefore `&mut` should be invariant.
|
||||
|
||||
This is the general theme of variance vs invariance: if variance would allow you
|
||||
to store a short-lived value into a longer-lived slot, then you must be
|
||||
invariant.
|
||||
|
||||
However it *is* sound for `&'a mut T` to be variant over `'a`. The key difference
|
||||
between `'a` and T is that `'a` is a property of the reference itself,
|
||||
while T is something the reference is borrowing. If you change T's type, then
|
||||
the source still remembers the original type. However if you change the
|
||||
lifetime's type, no one but the reference knows this information, so it's fine.
|
||||
Put another way: `&'a mut T` owns `'a`, but only *borrows* T.
|
||||
|
||||
`Box` and `Vec` are interesting cases because they're variant, but you can
|
||||
definitely store values in them! This is where Rust gets really clever: it's
|
||||
fine for them to be variant because you can only store values
|
||||
in them *via a mutable reference*! The mutable reference makes the whole type
|
||||
invariant, and therefore prevents you from smuggling a short-lived type into
|
||||
them.
|
||||
|
||||
Being variant allows `Box` and `Vec` to be weakened when shared
|
||||
immutably. So you can pass a `&Box<&'static str>` where a `&Box<&'a str>` is
|
||||
expected.
|
||||
|
||||
However what should happen when passing *by-value* is less obvious. It turns out
|
||||
that, yes, you can use subtyping when passing by-value. That is, this works:
|
||||
|
||||
```rust
|
||||
fn get_box<'a>(str: &'a str) -> Box<&'a str> {
|
||||
// string literals are `&'static str`s
|
||||
Box::new("hello")
|
||||
}
|
||||
```
|
||||
|
||||
Weakening when you pass by-value is fine because there's no one else who
|
||||
"remembers" the old lifetime in the Box. The reason a variant `&mut` was
|
||||
trouble was because there's always someone else who remembers the original
|
||||
subtype: the actual owner.
|
||||
|
||||
The invariance of the cell types can be seen as follows: `&` is like an `&mut`
|
||||
for a cell, because you can still store values in them through an `&`. Therefore
|
||||
cells must be invariant to avoid lifetime smuggling.
|
||||
|
||||
`Fn` is the most subtle case because it has mixed variance. To see why
|
||||
`Fn(T) -> U` should be invariant over T, consider the following function
|
||||
signature:
|
||||
|
||||
```rust,ignore
|
||||
// 'a is derived from some parent scope
|
||||
fn foo(&'a str) -> usize;
|
||||
```
|
||||
|
||||
This signature claims that it can handle any `&str` that lives at least as
|
||||
long as `'a`. Now if this signature was variant over `&'a str`, that
|
||||
would mean
|
||||
|
||||
```rust,ignore
|
||||
fn foo(&'static str) -> usize;
|
||||
```
|
||||
|
||||
could be provided in its place, as it would be a subtype. However this function
|
||||
has a stronger requirement: it says that it can only handle `&'static str`s,
|
||||
and nothing else. Giving `&'a str`s to it would be unsound, as it's free to
|
||||
assume that what it's given lives forever. Therefore functions are not variant
|
||||
over their arguments.
|
||||
|
||||
To see why `Fn(T) -> U` should be variant over U, consider the following
|
||||
function signature:
|
||||
|
||||
```rust,ignore
|
||||
// 'a is derived from some parent scope
|
||||
fn foo(usize) -> &'a str;
|
||||
```
|
||||
|
||||
This signature claims that it will return something that outlives `'a`. It is
|
||||
therefore completely reasonable to provide
|
||||
|
||||
```rust,ignore
|
||||
fn foo(usize) -> &'static str;
|
||||
```
|
||||
|
||||
in its place. Therefore functions are variant over their return type.
|
||||
|
||||
`*const` has the exact same semantics as `&`, so variance follows. `*mut` on the
|
||||
other hand can dereference to an `&mut` whether shared or not, so it is marked
|
||||
as invariant just like cells.
|
||||
|
||||
This is all well and good for the types the standard library provides, but
|
||||
how is variance determined for type that *you* define? A struct, informally
|
||||
speaking, inherits the variance of its fields. If a struct `Foo`
|
||||
has a generic argument `A` that is used in a field `a`, then Foo's variance
|
||||
over `A` is exactly `a`'s variance. However this is complicated if `A` is used
|
||||
in multiple fields.
|
||||
|
||||
* If all uses of A are variant, then Foo is variant over A
|
||||
* Otherwise, Foo is invariant over A
|
||||
|
||||
```rust
|
||||
use std::cell::Cell;
|
||||
|
||||
struct Foo<'a, 'b, A: 'a, B: 'b, C, D, E, F, G, H> {
|
||||
a: &'a A, // variant over 'a and A
|
||||
b: &'b mut B, // variant over 'b and invariant over B
|
||||
c: *const C, // variant over C
|
||||
d: *mut D, // invariant over D
|
||||
e: Vec<E>, // variant over E
|
||||
f: Cell<F>, // invariant over F
|
||||
g: G, // variant over G
|
||||
h1: H, // would also be variant over H except...
|
||||
h2: Cell<H>, // invariant over H, because invariance wins
|
||||
}
|
||||
```
|
|
@ -1,35 +0,0 @@
|
|||
# Transmutes
|
||||
|
||||
Get out of our way type system! We're going to reinterpret these bits or die
|
||||
trying! Even though this book is all about doing things that are unsafe, I
|
||||
really can't emphasize that you should deeply think about finding Another Way
|
||||
than the operations covered in this section. This is really, truly, the most
|
||||
horribly unsafe thing you can do in Rust. The railguards here are dental floss.
|
||||
|
||||
`mem::transmute<T, U>` takes a value of type `T` and reinterprets it to have
|
||||
type `U`. The only restriction is that the `T` and `U` are verified to have the
|
||||
same size. The ways to cause Undefined Behavior with this are mind boggling.
|
||||
|
||||
* First and foremost, creating an instance of *any* type with an invalid state
|
||||
is going to cause arbitrary chaos that can't really be predicted.
|
||||
* Transmute has an overloaded return type. If you do not specify the return type
|
||||
it may produce a surprising type to satisfy inference.
|
||||
* Making a primitive with an invalid value is UB
|
||||
* Transmuting between non-repr(C) types is UB
|
||||
* Transmuting an & to &mut is UB
|
||||
* Transmuting an & to &mut is *always* UB
|
||||
* No you can't do it
|
||||
* No you're not special
|
||||
* Transmuting to a reference without an explicitly provided lifetime
|
||||
produces an [unbounded lifetime]
|
||||
|
||||
`mem::transmute_copy<T, U>` somehow manages to be *even more* wildly unsafe than
|
||||
this. It copies `size_of<U>` bytes out of an `&T` and interprets them as a `U`.
|
||||
The size check that `mem::transmute` has is gone (as it may be valid to copy
|
||||
out a prefix), though it is Undefined Behavior for `U` to be larger than `T`.
|
||||
|
||||
Also of course you can get most of the functionality of these functions using
|
||||
pointer casts.
|
||||
|
||||
|
||||
[unbounded lifetime]: unbounded-lifetimes.html
|
|
@ -1,36 +0,0 @@
|
|||
# Unbounded Lifetimes
|
||||
|
||||
Unsafe code can often end up producing references or lifetimes out of thin air.
|
||||
Such lifetimes come into the world as *unbounded*. The most common source of this
|
||||
is dereferencing a raw pointer, which produces a reference with an unbounded lifetime.
|
||||
Such a lifetime becomes as big as context demands. This is in fact more powerful
|
||||
than simply becoming `'static`, because for instance `&'static &'a T`
|
||||
will fail to typecheck, but the unbound lifetime will perfectly mold into
|
||||
`&'a &'a T` as needed. However for most intents and purposes, such an unbounded
|
||||
lifetime can be regarded as `'static`.
|
||||
|
||||
Almost no reference is `'static`, so this is probably wrong. `transmute` and
|
||||
`transmute_copy` are the two other primary offenders. One should endeavor to
|
||||
bound an unbounded lifetime as quickly as possible, especially across function
|
||||
boundaries.
|
||||
|
||||
Given a function, any output lifetimes that don't derive from inputs are
|
||||
unbounded. For instance:
|
||||
|
||||
```rust,ignore
|
||||
fn get_str<'a>() -> &'a str;
|
||||
```
|
||||
|
||||
will produce an `&str` with an unbounded lifetime. The easiest way to avoid
|
||||
unbounded lifetimes is to use lifetime elision at the function boundary.
|
||||
If an output lifetime is elided, then it *must* be bounded by an input lifetime.
|
||||
Of course it might be bounded by the *wrong* lifetime, but this will usually
|
||||
just cause a compiler error, rather than allow memory safety to be trivially
|
||||
violated.
|
||||
|
||||
Within a function, bounding lifetimes is more error-prone. The safest and easiest
|
||||
way to bound a lifetime is to return it from a function with a bound lifetime.
|
||||
However if this is unacceptable, the reference can be placed in a location with
|
||||
a specific lifetime. Unfortunately it's impossible to name all lifetimes involved
|
||||
in a function.
|
||||
|
|
@ -1,85 +0,0 @@
|
|||
# Unchecked Uninitialized Memory
|
||||
|
||||
One interesting exception to this rule is working with arrays. Safe Rust doesn't
|
||||
permit you to partially initialize an array. When you initialize an array, you
|
||||
can either set every value to the same thing with `let x = [val; N]`, or you can
|
||||
specify each member individually with `let x = [val1, val2, val3]`.
|
||||
Unfortunately this is pretty rigid, especially if you need to initialize your
|
||||
array in a more incremental or dynamic way.
|
||||
|
||||
Unsafe Rust gives us a powerful tool to handle this problem:
|
||||
`mem::uninitialized`. This function pretends to return a value when really
|
||||
it does nothing at all. Using it, we can convince Rust that we have initialized
|
||||
a variable, allowing us to do trickier things with conditional and incremental
|
||||
initialization.
|
||||
|
||||
Unfortunately, this opens us up to all kinds of problems. Assignment has a
|
||||
different meaning to Rust based on whether it believes that a variable is
|
||||
initialized or not. If it's believed uninitialized, then Rust will semantically
|
||||
just memcopy the bits over the uninitialized ones, and do nothing else. However
|
||||
if Rust believes a value to be initialized, it will try to `Drop` the old value!
|
||||
Since we've tricked Rust into believing that the value is initialized, we can no
|
||||
longer safely use normal assignment.
|
||||
|
||||
This is also a problem if you're working with a raw system allocator, which
|
||||
returns a pointer to uninitialized memory.
|
||||
|
||||
To handle this, we must use the `ptr` module. In particular, it provides
|
||||
three functions that allow us to assign bytes to a location in memory without
|
||||
dropping the old value: `write`, `copy`, and `copy_nonoverlapping`.
|
||||
|
||||
* `ptr::write(ptr, val)` takes a `val` and moves it into the address pointed
|
||||
to by `ptr`.
|
||||
* `ptr::copy(src, dest, count)` copies the bits that `count` T's would occupy
|
||||
from src to dest. (this is equivalent to memmove -- note that the argument
|
||||
order is reversed!)
|
||||
* `ptr::copy_nonoverlapping(src, dest, count)` does what `copy` does, but a
|
||||
little faster on the assumption that the two ranges of memory don't overlap.
|
||||
(this is equivalent to memcpy -- note that the argument order is reversed!)
|
||||
|
||||
It should go without saying that these functions, if misused, will cause serious
|
||||
havoc or just straight up Undefined Behavior. The only things that these
|
||||
functions *themselves* require is that the locations you want to read and write
|
||||
are allocated. However the ways writing arbitrary bits to arbitrary
|
||||
locations of memory can break things are basically uncountable!
|
||||
|
||||
Putting this all together, we get the following:
|
||||
|
||||
```rust
|
||||
use std::mem;
|
||||
use std::ptr;
|
||||
|
||||
// size of the array is hard-coded but easy to change. This means we can't
|
||||
// use [a, b, c] syntax to initialize the array, though!
|
||||
const SIZE: usize = 10;
|
||||
|
||||
let mut x: [Box<u32>; SIZE];
|
||||
|
||||
unsafe {
|
||||
// convince Rust that x is Totally Initialized
|
||||
x = mem::uninitialized();
|
||||
for i in 0..SIZE {
|
||||
// very carefully overwrite each index without reading it
|
||||
// NOTE: exception safety is not a concern; Box can't panic
|
||||
ptr::write(&mut x[i], Box::new(i as u32));
|
||||
}
|
||||
}
|
||||
|
||||
println!("{:?}", x);
|
||||
```
|
||||
|
||||
It's worth noting that you don't need to worry about `ptr::write`-style
|
||||
shenanigans with types which don't implement `Drop` or contain `Drop` types,
|
||||
because Rust knows not to try to drop them. Similarly you should be able to
|
||||
assign to fields of partially initialized structs directly if those fields don't
|
||||
contain any `Drop` types.
|
||||
|
||||
However when working with uninitialized memory you need to be ever-vigilant for
|
||||
Rust trying to drop values you make like this before they're fully initialized.
|
||||
Every control path through that variable's scope must initialize the value
|
||||
before it ends, if it has a destructor.
|
||||
*[This includes code panicking](unwinding.html)*.
|
||||
|
||||
And that's about it for working with uninitialized memory! Basically nothing
|
||||
anywhere expects to be handed uninitialized memory, so if you're going to pass
|
||||
it around at all, be sure to be *really* careful.
|
|
@ -1,10 +0,0 @@
|
|||
# Working With Uninitialized Memory
|
||||
|
||||
All runtime-allocated memory in a Rust program begins its life as
|
||||
*uninitialized*. In this state the value of the memory is an indeterminate pile
|
||||
of bits that may or may not even reflect a valid state for the type that is
|
||||
supposed to inhabit that location of memory. Attempting to interpret this memory
|
||||
as a value of *any* type will cause Undefined Behavior. Do Not Do This.
|
||||
|
||||
Rust provides mechanisms to work with uninitialized memory in checked (safe) and
|
||||
unchecked (unsafe) ways.
|
|
@ -1,49 +0,0 @@
|
|||
# Unwinding
|
||||
|
||||
Rust has a *tiered* error-handling scheme:
|
||||
|
||||
* If something might reasonably be absent, Option is used.
|
||||
* If something goes wrong and can reasonably be handled, Result is used.
|
||||
* If something goes wrong and cannot reasonably be handled, the thread panics.
|
||||
* If something catastrophic happens, the program aborts.
|
||||
|
||||
Option and Result are overwhelmingly preferred in most situations, especially
|
||||
since they can be promoted into a panic or abort at the API user's discretion.
|
||||
Panics cause the thread to halt normal execution and unwind its stack, calling
|
||||
destructors as if every function instantly returned.
|
||||
|
||||
As of 1.0, Rust is of two minds when it comes to panics. In the long-long-ago,
|
||||
Rust was much more like Erlang. Like Erlang, Rust had lightweight tasks,
|
||||
and tasks were intended to kill themselves with a panic when they reached an
|
||||
untenable state. Unlike an exception in Java or C++, a panic could not be
|
||||
caught at any time. Panics could only be caught by the owner of the task, at which
|
||||
point they had to be handled or *that* task would itself panic.
|
||||
|
||||
Unwinding was important to this story because if a task's
|
||||
destructors weren't called, it would cause memory and other system resources to
|
||||
leak. Since tasks were expected to die during normal execution, this would make
|
||||
Rust very poor for long-running systems!
|
||||
|
||||
As the Rust we know today came to be, this style of programming grew out of
|
||||
fashion in the push for less-and-less abstraction. Light-weight tasks were
|
||||
killed in the name of heavy-weight OS threads. Still, on stable Rust as of 1.0
|
||||
panics can only be caught by the parent thread. This means catching a panic
|
||||
requires spinning up an entire OS thread! This unfortunately stands in conflict
|
||||
to Rust's philosophy of zero-cost abstractions.
|
||||
|
||||
There is an unstable API called `catch_panic` that enables catching a panic
|
||||
without spawning a thread. Still, we would encourage you to only do this
|
||||
sparingly. In particular, Rust's current unwinding implementation is heavily
|
||||
optimized for the "doesn't unwind" case. If a program doesn't unwind, there
|
||||
should be no runtime cost for the program being *ready* to unwind. As a
|
||||
consequence, actually unwinding will be more expensive than in e.g. Java.
|
||||
Don't build your programs to unwind under normal circumstances. Ideally, you
|
||||
should only panic for programming errors or *extreme* problems.
|
||||
|
||||
Rust's unwinding strategy is not specified to be fundamentally compatible
|
||||
with any other language's unwinding. As such, unwinding into Rust from another
|
||||
language, or unwinding into another language from Rust is Undefined Behavior.
|
||||
You must *absolutely* catch any panics at the FFI boundary! What you do at that
|
||||
point is up to you, but *something* must be done. If you fail to do this,
|
||||
at best, your application will crash and burn. At worst, your application *won't*
|
||||
crash and burn, and will proceed with completely clobbered state.
|
|
@ -1,223 +0,0 @@
|
|||
# Allocating Memory
|
||||
|
||||
Using Unique throws a wrench in an important feature of Vec (and indeed all of
|
||||
the std collections): an empty Vec doesn't actually allocate at all. So if we
|
||||
can't allocate, but also can't put a null pointer in `ptr`, what do we do in
|
||||
`Vec::new`? Well, we just put some other garbage in there!
|
||||
|
||||
This is perfectly fine because we already have `cap == 0` as our sentinel for no
|
||||
allocation. We don't even need to handle it specially in almost any code because
|
||||
we usually need to check if `cap > len` or `len > 0` anyway. The traditional
|
||||
Rust value to put here is `0x01`. The standard library actually exposes this
|
||||
as `alloc::heap::EMPTY`. There are quite a few places where we'll
|
||||
want to use `heap::EMPTY` because there's no real allocation to talk about but
|
||||
`null` would make the compiler do bad things.
|
||||
|
||||
All of the `heap` API is totally unstable under the `heap_api` feature, though.
|
||||
We could trivially define `heap::EMPTY` ourselves, but we'll want the rest of
|
||||
the `heap` API anyway, so let's just get that dependency over with.
|
||||
|
||||
So:
|
||||
|
||||
```rust,ignore
|
||||
#![feature(alloc, heap_api)]
|
||||
|
||||
use std::mem;
|
||||
|
||||
use alloc::heap::EMPTY;
|
||||
|
||||
impl<T> Vec<T> {
|
||||
fn new() -> Self {
|
||||
assert!(mem::size_of::<T>() != 0, "We're not ready to handle ZSTs");
|
||||
unsafe {
|
||||
// need to cast EMPTY to the actual ptr type we want, let
|
||||
// inference handle it.
|
||||
Vec { ptr: Unique::new(heap::EMPTY as *mut _), len: 0, cap: 0 }
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
I slipped in that assert there because zero-sized types will require some
|
||||
special handling throughout our code, and I want to defer the issue for now.
|
||||
Without this assert, some of our early drafts will do some Very Bad Things.
|
||||
|
||||
Next we need to figure out what to actually do when we *do* want space. For
|
||||
that, we'll need to use the rest of the heap APIs. These basically allow us to
|
||||
talk directly to Rust's allocator (jemalloc by default).
|
||||
|
||||
We'll also need a way to handle out-of-memory (OOM) conditions. The standard
|
||||
library calls the `abort` intrinsic, which just calls an illegal instruction to
|
||||
crash the whole program. The reason we abort and don't panic is because
|
||||
unwinding can cause allocations to happen, and that seems like a bad thing to do
|
||||
when your allocator just came back with "hey I don't have any more memory".
|
||||
|
||||
Of course, this is a bit silly since most platforms don't actually run out of
|
||||
memory in a conventional way. Your operating system will probably kill the
|
||||
application by another means if you legitimately start using up all the memory.
|
||||
The most likely way we'll trigger OOM is by just asking for ludicrous quantities
|
||||
of memory at once (e.g. half the theoretical address space). As such it's
|
||||
*probably* fine to panic and nothing bad will happen. Still, we're trying to be
|
||||
like the standard library as much as possible, so we'll just kill the whole
|
||||
program.
|
||||
|
||||
We said we don't want to use intrinsics, so doing exactly what `std` does is
|
||||
out. Instead, we'll call `std::process::exit` with some random number.
|
||||
|
||||
```rust
|
||||
fn oom() {
|
||||
::std::process::exit(-9999);
|
||||
}
|
||||
```
|
||||
|
||||
Okay, now we can write growing. Roughly, we want to have this logic:
|
||||
|
||||
```text
|
||||
if cap == 0:
|
||||
allocate()
|
||||
cap = 1
|
||||
else:
|
||||
reallocate()
|
||||
cap *= 2
|
||||
```
|
||||
|
||||
But Rust's only supported allocator API is so low level that we'll need to do a
|
||||
fair bit of extra work. We also need to guard against some special
|
||||
conditions that can occur with really large allocations or empty allocations.
|
||||
|
||||
In particular, `ptr::offset` will cause us a lot of trouble, because it has
|
||||
the semantics of LLVM's GEP inbounds instruction. If you're fortunate enough to
|
||||
not have dealt with this instruction, here's the basic story with GEP: alias
|
||||
analysis, alias analysis, alias analysis. It's super important to an optimizing
|
||||
compiler to be able to reason about data dependencies and aliasing.
|
||||
|
||||
As a simple example, consider the following fragment of code:
|
||||
|
||||
```rust
|
||||
# let x = &mut 0;
|
||||
# let y = &mut 0;
|
||||
*x *= 7;
|
||||
*y *= 3;
|
||||
```
|
||||
|
||||
If the compiler can prove that `x` and `y` point to different locations in
|
||||
memory, the two operations can in theory be executed in parallel (by e.g.
|
||||
loading them into different registers and working on them independently).
|
||||
However the compiler can't do this in general because if x and y point to
|
||||
the same location in memory, the operations need to be done to the same value,
|
||||
and they can't just be merged afterwards.
|
||||
|
||||
When you use GEP inbounds, you are specifically telling LLVM that the offsets
|
||||
you're about to do are within the bounds of a single "allocated" entity. The
|
||||
ultimate payoff being that LLVM can assume that if two pointers are known to
|
||||
point to two disjoint objects, all the offsets of those pointers are *also*
|
||||
known to not alias (because you won't just end up in some random place in
|
||||
memory). LLVM is heavily optimized to work with GEP offsets, and inbounds
|
||||
offsets are the best of all, so it's important that we use them as much as
|
||||
possible.
|
||||
|
||||
So that's what GEP's about, how can it cause us trouble?
|
||||
|
||||
The first problem is that we index into arrays with unsigned integers, but
|
||||
GEP (and as a consequence `ptr::offset`) takes a signed integer. This means
|
||||
that half of the seemingly valid indices into an array will overflow GEP and
|
||||
actually go in the wrong direction! As such we must limit all allocations to
|
||||
`isize::MAX` elements. This actually means we only need to worry about
|
||||
byte-sized objects, because e.g. `> isize::MAX` `u16`s will truly exhaust all of
|
||||
the system's memory. However in order to avoid subtle corner cases where someone
|
||||
reinterprets some array of `< isize::MAX` objects as bytes, std limits all
|
||||
allocations to `isize::MAX` bytes.
|
||||
|
||||
On all 64-bit targets that Rust currently supports we're artificially limited
|
||||
to significantly less than all 64 bits of the address space (modern x64
|
||||
platforms only expose 48-bit addressing), so we can rely on just running out of
|
||||
memory first. However on 32-bit targets, particularly those with extensions to
|
||||
use more of the address space (PAE x86 or x32), it's theoretically possible to
|
||||
successfully allocate more than `isize::MAX` bytes of memory.
|
||||
|
||||
However since this is a tutorial, we're not going to be particularly optimal
|
||||
here, and just unconditionally check, rather than use clever platform-specific
|
||||
`cfg`s.
|
||||
|
||||
The other corner-case we need to worry about is empty allocations. There will
|
||||
be two kinds of empty allocations we need to worry about: `cap = 0` for all T,
|
||||
and `cap > 0` for zero-sized types.
|
||||
|
||||
These cases are tricky because they come
|
||||
down to what LLVM means by "allocated". LLVM's notion of an
|
||||
allocation is significantly more abstract than how we usually use it. Because
|
||||
LLVM needs to work with different languages' semantics and custom allocators,
|
||||
it can't really intimately understand allocation. Instead, the main idea behind
|
||||
allocation is "doesn't overlap with other stuff". That is, heap allocations,
|
||||
stack allocations, and globals don't randomly overlap. Yep, it's about alias
|
||||
analysis. As such, Rust can technically play a bit fast and loose with the notion of
|
||||
an allocation as long as it's *consistent*.
|
||||
|
||||
Getting back to the empty allocation case, there are a couple of places where
|
||||
we want to offset by 0 as a consequence of generic code. The question is then:
|
||||
is it consistent to do so? For zero-sized types, we have concluded that it is
|
||||
indeed consistent to do a GEP inbounds offset by an arbitrary number of
|
||||
elements. This is a runtime no-op because every element takes up no space,
|
||||
and it's fine to pretend that there's infinite zero-sized types allocated
|
||||
at `0x01`. No allocator will ever allocate that address, because they won't
|
||||
allocate `0x00` and they generally allocate to some minimal alignment higher
|
||||
than a byte. Also generally the whole first page of memory is
|
||||
protected from being allocated anyway (a whole 4k, on many platforms).
|
||||
|
||||
However what about for positive-sized types? That one's a bit trickier. In
|
||||
principle, you can argue that offsetting by 0 gives LLVM no information: either
|
||||
there's an element before the address or after it, but it can't know which.
|
||||
However we've chosen to conservatively assume that it may do bad things. As
|
||||
such we will guard against this case explicitly.
|
||||
|
||||
*Phew*
|
||||
|
||||
Ok with all the nonsense out of the way, let's actually allocate some memory:
|
||||
|
||||
```rust,ignore
|
||||
fn grow(&mut self) {
|
||||
// this is all pretty delicate, so let's say it's all unsafe
|
||||
unsafe {
|
||||
// current API requires us to specify size and alignment manually.
|
||||
let align = mem::align_of::<T>();
|
||||
let elem_size = mem::size_of::<T>();
|
||||
|
||||
let (new_cap, ptr) = if self.cap == 0 {
|
||||
let ptr = heap::allocate(elem_size, align);
|
||||
(1, ptr)
|
||||
} else {
|
||||
// as an invariant, we can assume that `self.cap < isize::MAX`,
|
||||
// so this doesn't need to be checked.
|
||||
let new_cap = self.cap * 2;
|
||||
// Similarly this can't overflow due to previously allocating this
|
||||
let old_num_bytes = self.cap * elem_size;
|
||||
|
||||
// check that the new allocation doesn't exceed `isize::MAX` at all
|
||||
// regardless of the actual size of the capacity. This combines the
|
||||
// `new_cap <= isize::MAX` and `new_num_bytes <= usize::MAX` checks
|
||||
// we need to make. We lose the ability to allocate e.g. 2/3rds of
|
||||
// the address space with a single Vec of i16's on 32-bit though.
|
||||
// Alas, poor Yorick -- I knew him, Horatio.
|
||||
assert!(old_num_bytes <= (::std::isize::MAX as usize) / 2,
|
||||
"capacity overflow");
|
||||
|
||||
let new_num_bytes = old_num_bytes * 2;
|
||||
let ptr = heap::reallocate(*self.ptr as *mut _,
|
||||
old_num_bytes,
|
||||
new_num_bytes,
|
||||
align);
|
||||
(new_cap, ptr)
|
||||
};
|
||||
|
||||
// If allocate or reallocate fail, we'll get `null` back
|
||||
if ptr.is_null() { oom(); }
|
||||
|
||||
self.ptr = Unique::new(ptr as *mut _);
|
||||
self.cap = new_cap;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Nothing particularly tricky here. Just computing sizes and alignments and doing
|
||||
some careful multiplication checks.
|
||||
|
|
@ -1,29 +0,0 @@
|
|||
# Deallocating
|
||||
|
||||
Next we should implement Drop so that we don't massively leak tons of resources.
|
||||
The easiest way is to just call `pop` until it yields None, and then deallocate
|
||||
our buffer. Note that calling `pop` is unneeded if `T: !Drop`. In theory we can
|
||||
ask Rust if `T` `needs_drop` and omit the calls to `pop`. However in practice
|
||||
LLVM is *really* good at removing simple side-effect free code like this, so I
|
||||
wouldn't bother unless you notice it's not being stripped (in this case it is).
|
||||
|
||||
We must not call `heap::deallocate` when `self.cap == 0`, as in this case we
|
||||
haven't actually allocated any memory.
|
||||
|
||||
|
||||
```rust,ignore
|
||||
impl<T> Drop for Vec<T> {
|
||||
fn drop(&mut self) {
|
||||
if self.cap != 0 {
|
||||
while let Some(_) = self.pop() { }
|
||||
|
||||
let align = mem::align_of::<T>();
|
||||
let elem_size = mem::size_of::<T>();
|
||||
let num_bytes = elem_size * self.cap;
|
||||
unsafe {
|
||||
heap::deallocate(*self.ptr as *mut _, num_bytes, align);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,42 +0,0 @@
|
|||
# Deref
|
||||
|
||||
Alright! We've got a decent minimal stack implemented. We can push, we can
|
||||
pop, and we can clean up after ourselves. However there's a whole mess of
|
||||
functionality we'd reasonably want. In particular, we have a proper array, but
|
||||
none of the slice functionality. That's actually pretty easy to solve: we can
|
||||
implement `Deref<Target=[T]>`. This will magically make our Vec coerce to, and
|
||||
behave like, a slice in all sorts of conditions.
|
||||
|
||||
All we need is `slice::from_raw_parts`. It will correctly handle empty slices
|
||||
for us. Later once we set up zero-sized type support it will also Just Work
|
||||
for those too.
|
||||
|
||||
```rust,ignore
|
||||
use std::ops::Deref;
|
||||
|
||||
impl<T> Deref for Vec<T> {
|
||||
type Target = [T];
|
||||
fn deref(&self) -> &[T] {
|
||||
unsafe {
|
||||
::std::slice::from_raw_parts(*self.ptr, self.len)
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And let's do DerefMut too:
|
||||
|
||||
```rust,ignore
|
||||
use std::ops::DerefMut;
|
||||
|
||||
impl<T> DerefMut for Vec<T> {
|
||||
fn deref_mut(&mut self) -> &mut [T] {
|
||||
unsafe {
|
||||
::std::slice::from_raw_parts_mut(*self.ptr, self.len)
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Now we have `len`, `first`, `last`, indexing, slicing, sorting, `iter`,
|
||||
`iter_mut`, and all other sorts of bells and whistles provided by slice. Sweet!
|
|
@ -1,152 +0,0 @@
|
|||
# Drain
|
||||
|
||||
Let's move on to Drain. Drain is largely the same as IntoIter, except that
|
||||
instead of consuming the Vec, it borrows the Vec and leaves its allocation
|
||||
untouched. For now we'll only implement the "basic" full-range version.
|
||||
|
||||
```rust,ignore
|
||||
use std::marker::PhantomData;
|
||||
|
||||
struct Drain<'a, T: 'a> {
|
||||
// Need to bound the lifetime here, so we do it with `&'a mut Vec<T>`
|
||||
// because that's semantically what we contain. We're "just" calling
|
||||
// `pop()` and `remove(0)`.
|
||||
vec: PhantomData<&'a mut Vec<T>>
|
||||
start: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
|
||||
impl<'a, T> Iterator for Drain<'a, T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
```
|
||||
|
||||
-- wait, this is seeming familiar. Let's do some more compression. Both
|
||||
IntoIter and Drain have the exact same structure, let's just factor it out.
|
||||
|
||||
```rust
|
||||
struct RawValIter<T> {
|
||||
start: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
|
||||
impl<T> RawValIter<T> {
|
||||
// unsafe to construct because it has no associated lifetimes.
|
||||
// This is necessary to store a RawValIter in the same struct as
|
||||
// its actual allocation. OK since it's a private implementation
|
||||
// detail.
|
||||
unsafe fn new(slice: &[T]) -> Self {
|
||||
RawValIter {
|
||||
start: slice.as_ptr(),
|
||||
end: if slice.len() == 0 {
|
||||
// if `len = 0`, then this is not actually allocated memory.
|
||||
// Need to avoid offsetting because that will give wrong
|
||||
// information to LLVM via GEP.
|
||||
slice.as_ptr()
|
||||
} else {
|
||||
slice.as_ptr().offset(slice.len() as isize)
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
// Iterator and DoubleEndedIterator impls identical to IntoIter.
|
||||
```
|
||||
|
||||
And IntoIter becomes the following:
|
||||
|
||||
```rust,ignore
|
||||
pub struct IntoIter<T> {
|
||||
_buf: RawVec<T>, // we don't actually care about this. Just need it to live.
|
||||
iter: RawValIter<T>,
|
||||
}
|
||||
|
||||
impl<T> Iterator for IntoIter<T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> { self.iter.next() }
|
||||
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
|
||||
}
|
||||
|
||||
impl<T> DoubleEndedIterator for IntoIter<T> {
|
||||
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
|
||||
}
|
||||
|
||||
impl<T> Drop for IntoIter<T> {
|
||||
fn drop(&mut self) {
|
||||
for _ in &mut self.iter {}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Vec<T> {
|
||||
pub fn into_iter(self) -> IntoIter<T> {
|
||||
unsafe {
|
||||
let iter = RawValIter::new(&self);
|
||||
|
||||
let buf = ptr::read(&self.buf);
|
||||
mem::forget(self);
|
||||
|
||||
IntoIter {
|
||||
iter: iter,
|
||||
_buf: buf,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Note that I've left a few quirks in this design to make upgrading Drain to work
|
||||
with arbitrary subranges a bit easier. In particular we *could* have RawValIter
|
||||
drain itself on drop, but that won't work right for a more complex Drain.
|
||||
We also take a slice to simplify Drain initialization.
|
||||
|
||||
Alright, now Drain is really easy:
|
||||
|
||||
```rust,ignore
|
||||
use std::marker::PhantomData;
|
||||
|
||||
pub struct Drain<'a, T: 'a> {
|
||||
vec: PhantomData<&'a mut Vec<T>>,
|
||||
iter: RawValIter<T>,
|
||||
}
|
||||
|
||||
impl<'a, T> Iterator for Drain<'a, T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> { self.iter.next() }
|
||||
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
|
||||
}
|
||||
|
||||
impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
|
||||
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
|
||||
}
|
||||
|
||||
impl<'a, T> Drop for Drain<'a, T> {
|
||||
fn drop(&mut self) {
|
||||
for _ in &mut self.iter {}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Vec<T> {
|
||||
pub fn drain(&mut self) -> Drain<T> {
|
||||
unsafe {
|
||||
let iter = RawValIter::new(&self);
|
||||
|
||||
// this is a mem::forget safety thing. If Drain is forgotten, we just
|
||||
// leak the whole Vec's contents. Also we need to do this *eventually*
|
||||
// anyway, so why not do it now?
|
||||
self.len = 0;
|
||||
|
||||
Drain {
|
||||
iter: iter,
|
||||
vec: PhantomData,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
For more details on the `mem::forget` problem, see the
|
||||
[section on leaks][leaks].
|
||||
|
||||
[leaks]: leaking.html
|
|
@ -1,321 +0,0 @@
|
|||
# The Final Code
|
||||
|
||||
```rust
|
||||
#![feature(unique)]
|
||||
#![feature(alloc, heap_api)]
|
||||
|
||||
extern crate alloc;
|
||||
|
||||
use std::ptr::{Unique, self};
|
||||
use std::mem;
|
||||
use std::ops::{Deref, DerefMut};
|
||||
use std::marker::PhantomData;
|
||||
|
||||
use alloc::heap;
|
||||
|
||||
struct RawVec<T> {
|
||||
ptr: Unique<T>,
|
||||
cap: usize,
|
||||
}
|
||||
|
||||
impl<T> RawVec<T> {
|
||||
fn new() -> Self {
|
||||
unsafe {
|
||||
// !0 is usize::MAX. This branch should be stripped at compile time.
|
||||
let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
|
||||
|
||||
// heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
|
||||
RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
|
||||
}
|
||||
}
|
||||
|
||||
fn grow(&mut self) {
|
||||
unsafe {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
|
||||
// since we set the capacity to usize::MAX when elem_size is
|
||||
// 0, getting to here necessarily means the Vec is overfull.
|
||||
assert!(elem_size != 0, "capacity overflow");
|
||||
|
||||
let align = mem::align_of::<T>();
|
||||
|
||||
let (new_cap, ptr) = if self.cap == 0 {
|
||||
let ptr = heap::allocate(elem_size, align);
|
||||
(1, ptr)
|
||||
} else {
|
||||
let new_cap = 2 * self.cap;
|
||||
let ptr = heap::reallocate(*self.ptr as *mut _,
|
||||
self.cap * elem_size,
|
||||
new_cap * elem_size,
|
||||
align);
|
||||
(new_cap, ptr)
|
||||
};
|
||||
|
||||
// If allocate or reallocate fail, we'll get `null` back
|
||||
if ptr.is_null() { oom() }
|
||||
|
||||
self.ptr = Unique::new(ptr as *mut _);
|
||||
self.cap = new_cap;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Drop for RawVec<T> {
|
||||
fn drop(&mut self) {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
if self.cap != 0 && elem_size != 0 {
|
||||
let align = mem::align_of::<T>();
|
||||
|
||||
let num_bytes = elem_size * self.cap;
|
||||
unsafe {
|
||||
heap::deallocate(*self.ptr as *mut _, num_bytes, align);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
pub struct Vec<T> {
|
||||
buf: RawVec<T>,
|
||||
len: usize,
|
||||
}
|
||||
|
||||
impl<T> Vec<T> {
|
||||
fn ptr(&self) -> *mut T { *self.buf.ptr }
|
||||
|
||||
fn cap(&self) -> usize { self.buf.cap }
|
||||
|
||||
pub fn new() -> Self {
|
||||
Vec { buf: RawVec::new(), len: 0 }
|
||||
}
|
||||
pub fn push(&mut self, elem: T) {
|
||||
if self.len == self.cap() { self.buf.grow(); }
|
||||
|
||||
unsafe {
|
||||
ptr::write(self.ptr().offset(self.len as isize), elem);
|
||||
}
|
||||
|
||||
// Can't fail, we'll OOM first.
|
||||
self.len += 1;
|
||||
}
|
||||
|
||||
pub fn pop(&mut self) -> Option<T> {
|
||||
if self.len == 0 {
|
||||
None
|
||||
} else {
|
||||
self.len -= 1;
|
||||
unsafe {
|
||||
Some(ptr::read(self.ptr().offset(self.len as isize)))
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
pub fn insert(&mut self, index: usize, elem: T) {
|
||||
assert!(index <= self.len, "index out of bounds");
|
||||
if self.cap() == self.len { self.buf.grow(); }
|
||||
|
||||
unsafe {
|
||||
if index < self.len {
|
||||
ptr::copy(self.ptr().offset(index as isize),
|
||||
self.ptr().offset(index as isize + 1),
|
||||
self.len - index);
|
||||
}
|
||||
ptr::write(self.ptr().offset(index as isize), elem);
|
||||
self.len += 1;
|
||||
}
|
||||
}
|
||||
|
||||
pub fn remove(&mut self, index: usize) -> T {
|
||||
assert!(index < self.len, "index out of bounds");
|
||||
unsafe {
|
||||
self.len -= 1;
|
||||
let result = ptr::read(self.ptr().offset(index as isize));
|
||||
ptr::copy(self.ptr().offset(index as isize + 1),
|
||||
self.ptr().offset(index as isize),
|
||||
self.len - index);
|
||||
result
|
||||
}
|
||||
}
|
||||
|
||||
pub fn into_iter(self) -> IntoIter<T> {
|
||||
unsafe {
|
||||
let iter = RawValIter::new(&self);
|
||||
let buf = ptr::read(&self.buf);
|
||||
mem::forget(self);
|
||||
|
||||
IntoIter {
|
||||
iter: iter,
|
||||
_buf: buf,
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
pub fn drain(&mut self) -> Drain<T> {
|
||||
unsafe {
|
||||
let iter = RawValIter::new(&self);
|
||||
|
||||
// this is a mem::forget safety thing. If Drain is forgotten, we just
|
||||
// leak the whole Vec's contents. Also we need to do this *eventually*
|
||||
// anyway, so why not do it now?
|
||||
self.len = 0;
|
||||
|
||||
Drain {
|
||||
iter: iter,
|
||||
vec: PhantomData,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Drop for Vec<T> {
|
||||
fn drop(&mut self) {
|
||||
while let Some(_) = self.pop() {}
|
||||
// allocation is handled by RawVec
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Deref for Vec<T> {
|
||||
type Target = [T];
|
||||
fn deref(&self) -> &[T] {
|
||||
unsafe {
|
||||
::std::slice::from_raw_parts(self.ptr(), self.len)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> DerefMut for Vec<T> {
|
||||
fn deref_mut(&mut self) -> &mut [T] {
|
||||
unsafe {
|
||||
::std::slice::from_raw_parts_mut(self.ptr(), self.len)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
struct RawValIter<T> {
|
||||
start: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
|
||||
impl<T> RawValIter<T> {
|
||||
unsafe fn new(slice: &[T]) -> Self {
|
||||
RawValIter {
|
||||
start: slice.as_ptr(),
|
||||
end: if mem::size_of::<T>() == 0 {
|
||||
((slice.as_ptr() as usize) + slice.len()) as *const _
|
||||
} else if slice.len() == 0 {
|
||||
slice.as_ptr()
|
||||
} else {
|
||||
slice.as_ptr().offset(slice.len() as isize)
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Iterator for RawValIter<T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
let result = ptr::read(self.start);
|
||||
self.start = if mem::size_of::<T>() == 0 {
|
||||
(self.start as usize + 1) as *const _
|
||||
} else {
|
||||
self.start.offset(1)
|
||||
};
|
||||
Some(result)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
fn size_hint(&self) -> (usize, Option<usize>) {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
let len = (self.end as usize - self.start as usize)
|
||||
/ if elem_size == 0 { 1 } else { elem_size };
|
||||
(len, Some(len))
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> DoubleEndedIterator for RawValIter<T> {
|
||||
fn next_back(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
self.end = if mem::size_of::<T>() == 0 {
|
||||
(self.end as usize - 1) as *const _
|
||||
} else {
|
||||
self.end.offset(-1)
|
||||
};
|
||||
Some(ptr::read(self.end))
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
|
||||
|
||||
pub struct IntoIter<T> {
|
||||
_buf: RawVec<T>, // we don't actually care about this. Just need it to live.
|
||||
iter: RawValIter<T>,
|
||||
}
|
||||
|
||||
impl<T> Iterator for IntoIter<T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> { self.iter.next() }
|
||||
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
|
||||
}
|
||||
|
||||
impl<T> DoubleEndedIterator for IntoIter<T> {
|
||||
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
|
||||
}
|
||||
|
||||
impl<T> Drop for IntoIter<T> {
|
||||
fn drop(&mut self) {
|
||||
for _ in &mut *self {}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
|
||||
|
||||
pub struct Drain<'a, T: 'a> {
|
||||
vec: PhantomData<&'a mut Vec<T>>,
|
||||
iter: RawValIter<T>,
|
||||
}
|
||||
|
||||
impl<'a, T> Iterator for Drain<'a, T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> { self.iter.next_back() }
|
||||
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
|
||||
}
|
||||
|
||||
impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
|
||||
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
|
||||
}
|
||||
|
||||
impl<'a, T> Drop for Drain<'a, T> {
|
||||
fn drop(&mut self) {
|
||||
// pre-drain the iter
|
||||
for _ in &mut self.iter {}
|
||||
}
|
||||
}
|
||||
|
||||
/// Abort the process, we're out of memory!
|
||||
///
|
||||
/// In practice this is probably dead code on most OSes
|
||||
fn oom() {
|
||||
::std::process::exit(-9999);
|
||||
}
|
||||
|
||||
# fn main() {}
|
||||
```
|
|
@ -1,51 +0,0 @@
|
|||
# Insert and Remove
|
||||
|
||||
Something *not* provided by slice is `insert` and `remove`, so let's do those
|
||||
next.
|
||||
|
||||
Insert needs to shift all the elements at the target index to the right by one.
|
||||
To do this we need to use `ptr::copy`, which is our version of C's `memmove`.
|
||||
This copies some chunk of memory from one location to another, correctly
|
||||
handling the case where the source and destination overlap (which will
|
||||
definitely happen here).
|
||||
|
||||
If we insert at index `i`, we want to shift the `[i .. len]` to `[i+1 .. len+1]`
|
||||
using the old len.
|
||||
|
||||
```rust,ignore
|
||||
pub fn insert(&mut self, index: usize, elem: T) {
|
||||
// Note: `<=` because it's valid to insert after everything
|
||||
// which would be equivalent to push.
|
||||
assert!(index <= self.len, "index out of bounds");
|
||||
if self.cap == self.len { self.grow(); }
|
||||
|
||||
unsafe {
|
||||
if index < self.len {
|
||||
// ptr::copy(src, dest, len): "copy from source to dest len elems"
|
||||
ptr::copy(self.ptr.offset(index as isize),
|
||||
self.ptr.offset(index as isize + 1),
|
||||
self.len - index);
|
||||
}
|
||||
ptr::write(self.ptr.offset(index as isize), elem);
|
||||
self.len += 1;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Remove behaves in the opposite manner. We need to shift all the elements from
|
||||
`[i+1 .. len + 1]` to `[i .. len]` using the *new* len.
|
||||
|
||||
```rust,ignore
|
||||
pub fn remove(&mut self, index: usize) -> T {
|
||||
// Note: `<` because it's *not* valid to remove after everything
|
||||
assert!(index < self.len, "index out of bounds");
|
||||
unsafe {
|
||||
self.len -= 1;
|
||||
let result = ptr::read(self.ptr.offset(index as isize));
|
||||
ptr::copy(self.ptr.offset(index as isize + 1),
|
||||
self.ptr.offset(index as isize),
|
||||
self.len - index);
|
||||
result
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,147 +0,0 @@
|
|||
# IntoIter
|
||||
|
||||
Let's move on to writing iterators. `iter` and `iter_mut` have already been
|
||||
written for us thanks to The Magic of Deref. However there's two interesting
|
||||
iterators that Vec provides that slices can't: `into_iter` and `drain`.
|
||||
|
||||
IntoIter consumes the Vec by-value, and can consequently yield its elements
|
||||
by-value. In order to enable this, IntoIter needs to take control of Vec's
|
||||
allocation.
|
||||
|
||||
IntoIter needs to be DoubleEnded as well, to enable reading from both ends.
|
||||
Reading from the back could just be implemented as calling `pop`, but reading
|
||||
from the front is harder. We could call `remove(0)` but that would be insanely
|
||||
expensive. Instead we're going to just use ptr::read to copy values out of
|
||||
either end of the Vec without mutating the buffer at all.
|
||||
|
||||
To do this we're going to use a very common C idiom for array iteration. We'll
|
||||
make two pointers; one that points to the start of the array, and one that
|
||||
points to one-element past the end. When we want an element from one end, we'll
|
||||
read out the value pointed to at that end and move the pointer over by one. When
|
||||
the two pointers are equal, we know we're done.
|
||||
|
||||
Note that the order of read and offset are reversed for `next` and `next_back`
|
||||
For `next_back` the pointer is always after the element it wants to read next,
|
||||
while for `next` the pointer is always at the element it wants to read next.
|
||||
To see why this is, consider the case where every element but one has been
|
||||
yielded.
|
||||
|
||||
The array looks like this:
|
||||
|
||||
```text
|
||||
S E
|
||||
[X, X, X, O, X, X, X]
|
||||
```
|
||||
|
||||
If E pointed directly at the element it wanted to yield next, it would be
|
||||
indistinguishable from the case where there are no more elements to yield.
|
||||
|
||||
Although we don't actually care about it during iteration, we also need to hold
|
||||
onto the Vec's allocation information in order to free it once IntoIter is
|
||||
dropped.
|
||||
|
||||
So we're going to use the following struct:
|
||||
|
||||
```rust,ignore
|
||||
struct IntoIter<T> {
|
||||
buf: Unique<T>,
|
||||
cap: usize,
|
||||
start: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
```
|
||||
|
||||
And this is what we end up with for initialization:
|
||||
|
||||
```rust,ignore
|
||||
impl<T> Vec<T> {
|
||||
fn into_iter(self) -> IntoIter<T> {
|
||||
// Can't destructure Vec since it's Drop
|
||||
let ptr = self.ptr;
|
||||
let cap = self.cap;
|
||||
let len = self.len;
|
||||
|
||||
// Make sure not to drop Vec since that will free the buffer
|
||||
mem::forget(self);
|
||||
|
||||
unsafe {
|
||||
IntoIter {
|
||||
buf: ptr,
|
||||
cap: cap,
|
||||
start: *ptr,
|
||||
end: if cap == 0 {
|
||||
// can't offset off this pointer, it's not allocated!
|
||||
*ptr
|
||||
} else {
|
||||
ptr.offset(len as isize)
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Here's iterating forward:
|
||||
|
||||
```rust,ignore
|
||||
impl<T> Iterator for IntoIter<T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
let result = ptr::read(self.start);
|
||||
self.start = self.start.offset(1);
|
||||
Some(result)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
fn size_hint(&self) -> (usize, Option<usize>) {
|
||||
let len = (self.end as usize - self.start as usize)
|
||||
/ mem::size_of::<T>();
|
||||
(len, Some(len))
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And here's iterating backwards.
|
||||
|
||||
```rust,ignore
|
||||
impl<T> DoubleEndedIterator for IntoIter<T> {
|
||||
fn next_back(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
self.end = self.end.offset(-1);
|
||||
Some(ptr::read(self.end))
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Because IntoIter takes ownership of its allocation, it needs to implement Drop
|
||||
to free it. However it also wants to implement Drop to drop any elements it
|
||||
contains that weren't yielded.
|
||||
|
||||
|
||||
```rust,ignore
|
||||
impl<T> Drop for IntoIter<T> {
|
||||
fn drop(&mut self) {
|
||||
if self.cap != 0 {
|
||||
// drop any remaining elements
|
||||
for _ in &mut *self {}
|
||||
|
||||
let align = mem::align_of::<T>();
|
||||
let elem_size = mem::size_of::<T>();
|
||||
let num_bytes = elem_size * self.cap;
|
||||
unsafe {
|
||||
heap::deallocate(*self.buf as *mut _, num_bytes, align);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,100 +0,0 @@
|
|||
# Layout
|
||||
|
||||
First off, we need to come up with the struct layout. A Vec has three parts:
|
||||
a pointer to the allocation, the size of the allocation, and the number of
|
||||
elements that have been initialized.
|
||||
|
||||
Naively, this means we just want this design:
|
||||
|
||||
```rust
|
||||
pub struct Vec<T> {
|
||||
ptr: *mut T,
|
||||
cap: usize,
|
||||
len: usize,
|
||||
}
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
And indeed this would compile. Unfortunately, it would be incorrect. First, the
|
||||
compiler will give us too strict variance. So a `&Vec<&'static str>`
|
||||
couldn't be used where an `&Vec<&'a str>` was expected. More importantly, it
|
||||
will give incorrect ownership information to the drop checker, as it will
|
||||
conservatively assume we don't own any values of type `T`. See [the chapter
|
||||
on ownership and lifetimes][ownership] for all the details on variance and
|
||||
drop check.
|
||||
|
||||
As we saw in the ownership chapter, we should use `Unique<T>` in place of
|
||||
`*mut T` when we have a raw pointer to an allocation we own. Unique is unstable,
|
||||
so we'd like to not use it if possible, though.
|
||||
|
||||
As a recap, Unique is a wrapper around a raw pointer that declares that:
|
||||
|
||||
* We are variant over `T`
|
||||
* We may own a value of type `T` (for drop check)
|
||||
* We are Send/Sync if `T` is Send/Sync
|
||||
* We deref to `*mut T` (so it largely acts like a `*mut` in our code)
|
||||
* Our pointer is never null (so `Option<Vec<T>>` is null-pointer-optimized)
|
||||
|
||||
We can implement all of the above requirements except for the last
|
||||
one in stable Rust:
|
||||
|
||||
```rust
|
||||
use std::marker::PhantomData;
|
||||
use std::ops::Deref;
|
||||
use std::mem;
|
||||
|
||||
struct Unique<T> {
|
||||
ptr: *const T, // *const for variance
|
||||
_marker: PhantomData<T>, // For the drop checker
|
||||
}
|
||||
|
||||
// Deriving Send and Sync is safe because we are the Unique owners
|
||||
// of this data. It's like Unique<T> is "just" T.
|
||||
unsafe impl<T: Send> Send for Unique<T> {}
|
||||
unsafe impl<T: Sync> Sync for Unique<T> {}
|
||||
|
||||
impl<T> Unique<T> {
|
||||
pub fn new(ptr: *mut T) -> Self {
|
||||
Unique { ptr: ptr, _marker: PhantomData }
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Deref for Unique<T> {
|
||||
type Target = *mut T;
|
||||
fn deref(&self) -> &*mut T {
|
||||
// There's no way to cast the *const to a *mut
|
||||
// while also taking a reference. So we just
|
||||
// transmute it since it's all "just pointers".
|
||||
unsafe { mem::transmute(&self.ptr) }
|
||||
}
|
||||
}
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
Unfortunately the mechanism for stating that your value is non-zero is
|
||||
unstable and unlikely to be stabilized soon. As such we're just going to
|
||||
take the hit and use std's Unique:
|
||||
|
||||
|
||||
```rust
|
||||
#![feature(unique)]
|
||||
|
||||
use std::ptr::{Unique, self};
|
||||
|
||||
pub struct Vec<T> {
|
||||
ptr: Unique<T>,
|
||||
cap: usize,
|
||||
len: usize,
|
||||
}
|
||||
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
If you don't care about the null-pointer optimization, then you can use the
|
||||
stable code. However we will be designing the rest of the code around enabling
|
||||
the optimization. In particular, `Unique::new` is unsafe to call, because
|
||||
putting `null` inside of it is Undefined Behavior. Our stable Unique doesn't
|
||||
need `new` to be unsafe because it doesn't make any interesting guarantees about
|
||||
its contents.
|
||||
|
||||
[ownership]: ownership.html
|
|
@ -1,55 +0,0 @@
|
|||
# Push and Pop
|
||||
|
||||
Alright. We can initialize. We can allocate. Let's actually implement some
|
||||
functionality! Let's start with `push`. All it needs to do is check if we're
|
||||
full to grow, unconditionally write to the next index, and then increment our
|
||||
length.
|
||||
|
||||
To do the write we have to be careful not to evaluate the memory we want to write
|
||||
to. At worst, it's truly uninitialized memory from the allocator. At best it's the
|
||||
bits of some old value we popped off. Either way, we can't just index to the memory
|
||||
and dereference it, because that will evaluate the memory as a valid instance of
|
||||
T. Worse, `foo[idx] = x` will try to call `drop` on the old value of `foo[idx]`!
|
||||
|
||||
The correct way to do this is with `ptr::write`, which just blindly overwrites the
|
||||
target address with the bits of the value we provide. No evaluation involved.
|
||||
|
||||
For `push`, if the old len (before push was called) is 0, then we want to write
|
||||
to the 0th index. So we should offset by the old len.
|
||||
|
||||
```rust,ignore
|
||||
pub fn push(&mut self, elem: T) {
|
||||
if self.len == self.cap { self.grow(); }
|
||||
|
||||
unsafe {
|
||||
ptr::write(self.ptr.offset(self.len as isize), elem);
|
||||
}
|
||||
|
||||
// Can't fail, we'll OOM first.
|
||||
self.len += 1;
|
||||
}
|
||||
```
|
||||
|
||||
Easy! How about `pop`? Although this time the index we want to access is
|
||||
initialized, Rust won't just let us dereference the location of memory to move
|
||||
the value out, because that would leave the memory uninitialized! For this we
|
||||
need `ptr::read`, which just copies out the bits from the target address and
|
||||
interprets it as a value of type T. This will leave the memory at this address
|
||||
logically uninitialized, even though there is in fact a perfectly good instance
|
||||
of T there.
|
||||
|
||||
For `pop`, if the old len is 1, we want to read out of the 0th index. So we
|
||||
should offset by the new len.
|
||||
|
||||
```rust,ignore
|
||||
pub fn pop(&mut self) -> Option<T> {
|
||||
if self.len == 0 {
|
||||
None
|
||||
} else {
|
||||
self.len -= 1;
|
||||
unsafe {
|
||||
Some(ptr::read(self.ptr.offset(self.len as isize)))
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
|
@ -1,136 +0,0 @@
|
|||
# RawVec
|
||||
|
||||
We've actually reached an interesting situation here: we've duplicated the logic
|
||||
for specifying a buffer and freeing its memory in Vec and IntoIter. Now that
|
||||
we've implemented it and identified *actual* logic duplication, this is a good
|
||||
time to perform some logic compression.
|
||||
|
||||
We're going to abstract out the `(ptr, cap)` pair and give them the logic for
|
||||
allocating, growing, and freeing:
|
||||
|
||||
```rust,ignore
|
||||
struct RawVec<T> {
|
||||
ptr: Unique<T>,
|
||||
cap: usize,
|
||||
}
|
||||
|
||||
impl<T> RawVec<T> {
|
||||
fn new() -> Self {
|
||||
assert!(mem::size_of::<T>() != 0, "TODO: implement ZST support");
|
||||
unsafe {
|
||||
RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: 0 }
|
||||
}
|
||||
}
|
||||
|
||||
// unchanged from Vec
|
||||
fn grow(&mut self) {
|
||||
unsafe {
|
||||
let align = mem::align_of::<T>();
|
||||
let elem_size = mem::size_of::<T>();
|
||||
|
||||
let (new_cap, ptr) = if self.cap == 0 {
|
||||
let ptr = heap::allocate(elem_size, align);
|
||||
(1, ptr)
|
||||
} else {
|
||||
let new_cap = 2 * self.cap;
|
||||
let ptr = heap::reallocate(*self.ptr as *mut _,
|
||||
self.cap * elem_size,
|
||||
new_cap * elem_size,
|
||||
align);
|
||||
(new_cap, ptr)
|
||||
};
|
||||
|
||||
// If allocate or reallocate fail, we'll get `null` back
|
||||
if ptr.is_null() { oom() }
|
||||
|
||||
self.ptr = Unique::new(ptr as *mut _);
|
||||
self.cap = new_cap;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
impl<T> Drop for RawVec<T> {
|
||||
fn drop(&mut self) {
|
||||
if self.cap != 0 {
|
||||
let align = mem::align_of::<T>();
|
||||
let elem_size = mem::size_of::<T>();
|
||||
let num_bytes = elem_size * self.cap;
|
||||
unsafe {
|
||||
heap::deallocate(*self.ptr as *mut _, num_bytes, align);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And change Vec as follows:
|
||||
|
||||
```rust,ignore
|
||||
pub struct Vec<T> {
|
||||
buf: RawVec<T>,
|
||||
len: usize,
|
||||
}
|
||||
|
||||
impl<T> Vec<T> {
|
||||
fn ptr(&self) -> *mut T { *self.buf.ptr }
|
||||
|
||||
fn cap(&self) -> usize { self.buf.cap }
|
||||
|
||||
pub fn new() -> Self {
|
||||
Vec { buf: RawVec::new(), len: 0 }
|
||||
}
|
||||
|
||||
// push/pop/insert/remove largely unchanged:
|
||||
// * `self.ptr -> self.ptr()`
|
||||
// * `self.cap -> self.cap()`
|
||||
// * `self.grow -> self.buf.grow()`
|
||||
}
|
||||
|
||||
impl<T> Drop for Vec<T> {
|
||||
fn drop(&mut self) {
|
||||
while let Some(_) = self.pop() {}
|
||||
// deallocation is handled by RawVec
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And finally we can really simplify IntoIter:
|
||||
|
||||
```rust,ignore
|
||||
struct IntoIter<T> {
|
||||
_buf: RawVec<T>, // we don't actually care about this. Just need it to live.
|
||||
start: *const T,
|
||||
end: *const T,
|
||||
}
|
||||
|
||||
// next and next_back literally unchanged since they never referred to the buf
|
||||
|
||||
impl<T> Drop for IntoIter<T> {
|
||||
fn drop(&mut self) {
|
||||
// only need to ensure all our elements are read;
|
||||
// buffer will clean itself up afterwards.
|
||||
for _ in &mut *self {}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Vec<T> {
|
||||
pub fn into_iter(self) -> IntoIter<T> {
|
||||
unsafe {
|
||||
// need to use ptr::read to unsafely move the buf out since it's
|
||||
// not Copy, and Vec implements Drop (so we can't destructure it).
|
||||
let buf = ptr::read(&self.buf);
|
||||
let len = self.len;
|
||||
mem::forget(self);
|
||||
|
||||
IntoIter {
|
||||
start: *buf.ptr,
|
||||
end: buf.ptr.offset(len as isize),
|
||||
_buf: buf,
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Much better.
|
|
@ -1,176 +0,0 @@
|
|||
# Handling Zero-Sized Types
|
||||
|
||||
It's time. We're going to fight the specter that is zero-sized types. Safe Rust
|
||||
*never* needs to care about this, but Vec is very intensive on raw pointers and
|
||||
raw allocations, which are exactly the two things that care about
|
||||
zero-sized types. We need to be careful of two things:
|
||||
|
||||
* The raw allocator API has undefined behavior if you pass in 0 for an
|
||||
allocation size.
|
||||
* raw pointer offsets are no-ops for zero-sized types, which will break our
|
||||
C-style pointer iterator.
|
||||
|
||||
Thankfully we abstracted out pointer-iterators and allocating handling into
|
||||
RawValIter and RawVec respectively. How mysteriously convenient.
|
||||
|
||||
|
||||
|
||||
|
||||
## Allocating Zero-Sized Types
|
||||
|
||||
So if the allocator API doesn't support zero-sized allocations, what on earth
|
||||
do we store as our allocation? Why, `heap::EMPTY` of course! Almost every operation
|
||||
with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
|
||||
to be considered to store or load them. This actually extends to `ptr::read` and
|
||||
`ptr::write`: they won't actually look at the pointer at all. As such we never need
|
||||
to change the pointer.
|
||||
|
||||
Note however that our previous reliance on running out of memory before overflow is
|
||||
no longer valid with zero-sized types. We must explicitly guard against capacity
|
||||
overflow for zero-sized types.
|
||||
|
||||
Due to our current architecture, all this means is writing 3 guards, one in each
|
||||
method of RawVec.
|
||||
|
||||
```rust,ignore
|
||||
impl<T> RawVec<T> {
|
||||
fn new() -> Self {
|
||||
unsafe {
|
||||
// !0 is usize::MAX. This branch should be stripped at compile time.
|
||||
let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
|
||||
|
||||
// heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
|
||||
RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
|
||||
}
|
||||
}
|
||||
|
||||
fn grow(&mut self) {
|
||||
unsafe {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
|
||||
// since we set the capacity to usize::MAX when elem_size is
|
||||
// 0, getting to here necessarily means the Vec is overfull.
|
||||
assert!(elem_size != 0, "capacity overflow");
|
||||
|
||||
let align = mem::align_of::<T>();
|
||||
|
||||
let (new_cap, ptr) = if self.cap == 0 {
|
||||
let ptr = heap::allocate(elem_size, align);
|
||||
(1, ptr)
|
||||
} else {
|
||||
let new_cap = 2 * self.cap;
|
||||
let ptr = heap::reallocate(*self.ptr as *mut _,
|
||||
self.cap * elem_size,
|
||||
new_cap * elem_size,
|
||||
align);
|
||||
(new_cap, ptr)
|
||||
};
|
||||
|
||||
// If allocate or reallocate fail, we'll get `null` back
|
||||
if ptr.is_null() { oom() }
|
||||
|
||||
self.ptr = Unique::new(ptr as *mut _);
|
||||
self.cap = new_cap;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> Drop for RawVec<T> {
|
||||
fn drop(&mut self) {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
|
||||
// don't free zero-sized allocations, as they were never allocated.
|
||||
if self.cap != 0 && elem_size != 0 {
|
||||
let align = mem::align_of::<T>();
|
||||
|
||||
let num_bytes = elem_size * self.cap;
|
||||
unsafe {
|
||||
heap::deallocate(*self.ptr as *mut _, num_bytes, align);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
That's it. We support pushing and popping zero-sized types now. Our iterators
|
||||
(that aren't provided by slice Deref) are still busted, though.
|
||||
|
||||
|
||||
|
||||
|
||||
## Iterating Zero-Sized Types
|
||||
|
||||
Zero-sized offsets are no-ops. This means that our current design will always
|
||||
initialize `start` and `end` as the same value, and our iterators will yield
|
||||
nothing. The current solution to this is to cast the pointers to integers,
|
||||
increment, and then cast them back:
|
||||
|
||||
```rust,ignore
|
||||
impl<T> RawValIter<T> {
|
||||
unsafe fn new(slice: &[T]) -> Self {
|
||||
RawValIter {
|
||||
start: slice.as_ptr(),
|
||||
end: if mem::size_of::<T>() == 0 {
|
||||
((slice.as_ptr() as usize) + slice.len()) as *const _
|
||||
} else if slice.len() == 0 {
|
||||
slice.as_ptr()
|
||||
} else {
|
||||
slice.as_ptr().offset(slice.len() as isize)
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Now we have a different bug. Instead of our iterators not running at all, our
|
||||
iterators now run *forever*. We need to do the same trick in our iterator impls.
|
||||
Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
|
||||
basically be treating the two pointers as if they point to bytes, we'll just
|
||||
map size 0 to divide by 1.
|
||||
|
||||
```rust,ignore
|
||||
impl<T> Iterator for RawValIter<T> {
|
||||
type Item = T;
|
||||
fn next(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
let result = ptr::read(self.start);
|
||||
self.start = if mem::size_of::<T>() == 0 {
|
||||
(self.start as usize + 1) as *const _
|
||||
} else {
|
||||
self.start.offset(1)
|
||||
};
|
||||
Some(result)
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
fn size_hint(&self) -> (usize, Option<usize>) {
|
||||
let elem_size = mem::size_of::<T>();
|
||||
let len = (self.end as usize - self.start as usize)
|
||||
/ if elem_size == 0 { 1 } else { elem_size };
|
||||
(len, Some(len))
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> DoubleEndedIterator for RawValIter<T> {
|
||||
fn next_back(&mut self) -> Option<T> {
|
||||
if self.start == self.end {
|
||||
None
|
||||
} else {
|
||||
unsafe {
|
||||
self.end = if mem::size_of::<T>() == 0 {
|
||||
(self.end as usize - 1) as *const _
|
||||
} else {
|
||||
self.end.offset(-1)
|
||||
};
|
||||
Some(ptr::read(self.end))
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
And that's it. Iteration works!
|
|
@ -1,20 +0,0 @@
|
|||
# Example: Implementing Vec
|
||||
|
||||
To bring everything together, we're going to write `std::Vec` from scratch.
|
||||
Because all the best tools for writing unsafe code are unstable, this
|
||||
project will only work on nightly (as of Rust 1.9.0). With the exception of the
|
||||
allocator API, much of the unstable code we'll use is expected to be stabilized
|
||||
in a similar form as it is today.
|
||||
|
||||
However we will generally try to avoid unstable code where possible. In
|
||||
particular we won't use any intrinsics that could make a code a little
|
||||
bit nicer or efficient because intrinsics are permanently unstable. Although
|
||||
many intrinsics *do* become stabilized elsewhere (`std::ptr` and `str::mem`
|
||||
consist of many intrinsics).
|
||||
|
||||
Ultimately this means our implementation may not take advantage of all
|
||||
possible optimizations, though it will be by no means *naive*. We will
|
||||
definitely get into the weeds over nitty-gritty details, even
|
||||
when the problem doesn't *really* merit it.
|
||||
|
||||
You wanted advanced. We're gonna go advanced.
|
|
@ -1,119 +0,0 @@
|
|||
# Working with Unsafe
|
||||
|
||||
Rust generally only gives us the tools to talk about Unsafe Rust in a scoped and
|
||||
binary manner. Unfortunately, reality is significantly more complicated than
|
||||
that. For instance, consider the following toy function:
|
||||
|
||||
```rust
|
||||
fn index(idx: usize, arr: &[u8]) -> Option<u8> {
|
||||
if idx < arr.len() {
|
||||
unsafe {
|
||||
Some(*arr.get_unchecked(idx))
|
||||
}
|
||||
} else {
|
||||
None
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Clearly, this function is safe. We check that the index is in bounds, and if it
|
||||
is, index into the array in an unchecked manner. But even in such a trivial
|
||||
function, the scope of the unsafe block is questionable. Consider changing the
|
||||
`<` to a `<=`:
|
||||
|
||||
```rust
|
||||
fn index(idx: usize, arr: &[u8]) -> Option<u8> {
|
||||
if idx <= arr.len() {
|
||||
unsafe {
|
||||
Some(*arr.get_unchecked(idx))
|
||||
}
|
||||
} else {
|
||||
None
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
This program is now unsound, and yet *we only modified safe code*. This is the
|
||||
fundamental problem of safety: it's non-local. The soundness of our unsafe
|
||||
operations necessarily depends on the state established by otherwise
|
||||
"safe" operations.
|
||||
|
||||
Safety is modular in the sense that opting into unsafety doesn't require you
|
||||
to consider arbitrary other kinds of badness. For instance, doing an unchecked
|
||||
index into a slice doesn't mean you suddenly need to worry about the slice being
|
||||
null or containing uninitialized memory. Nothing fundamentally changes. However
|
||||
safety *isn't* modular in the sense that programs are inherently stateful and
|
||||
your unsafe operations may depend on arbitrary other state.
|
||||
|
||||
Trickier than that is when we get into actual statefulness. Consider a simple
|
||||
implementation of `Vec`:
|
||||
|
||||
```rust
|
||||
use std::ptr;
|
||||
|
||||
// Note this definition is insufficient. See the section on implementing Vec.
|
||||
pub struct Vec<T> {
|
||||
ptr: *mut T,
|
||||
len: usize,
|
||||
cap: usize,
|
||||
}
|
||||
|
||||
// Note this implementation does not correctly handle zero-sized types.
|
||||
// We currently live in a nice imaginary world of only positive fixed-size
|
||||
// types.
|
||||
impl<T> Vec<T> {
|
||||
pub fn push(&mut self, elem: T) {
|
||||
if self.len == self.cap {
|
||||
// not important for this example
|
||||
self.reallocate();
|
||||
}
|
||||
unsafe {
|
||||
ptr::write(self.ptr.offset(self.len as isize), elem);
|
||||
self.len += 1;
|
||||
}
|
||||
}
|
||||
|
||||
# fn reallocate(&mut self) { }
|
||||
}
|
||||
|
||||
# fn main() {}
|
||||
```
|
||||
|
||||
This code is simple enough to reasonably audit and verify. Now consider
|
||||
adding the following method:
|
||||
|
||||
```rust,ignore
|
||||
fn make_room(&mut self) {
|
||||
// grow the capacity
|
||||
self.cap += 1;
|
||||
}
|
||||
```
|
||||
|
||||
This code is 100% Safe Rust but it is also completely unsound. Changing the
|
||||
capacity violates the invariants of Vec (that `cap` reflects the allocated space
|
||||
in the Vec). This is not something the rest of Vec can guard against. It *has*
|
||||
to trust the capacity field because there's no way to verify it.
|
||||
|
||||
`unsafe` does more than pollute a whole function: it pollutes a whole *module*.
|
||||
Generally, the only bullet-proof way to limit the scope of unsafe code is at the
|
||||
module boundary with privacy.
|
||||
|
||||
However this works *perfectly*. The existence of `make_room` is *not* a
|
||||
problem for the soundness of Vec because we didn't mark it as public. Only the
|
||||
module that defines this function can call it. Also, `make_room` directly
|
||||
accesses the private fields of Vec, so it can only be written in the same module
|
||||
as Vec.
|
||||
|
||||
It is therefore possible for us to write a completely safe abstraction that
|
||||
relies on complex invariants. This is *critical* to the relationship between
|
||||
Safe Rust and Unsafe Rust. We have already seen that Unsafe code must trust
|
||||
*some* Safe code, but can't trust *generic* Safe code. It can't trust an
|
||||
arbitrary implementor of a trait or any function that was passed to it to be
|
||||
well-behaved in a way that safe code doesn't care about.
|
||||
|
||||
However if unsafe code couldn't prevent client safe code from messing with its
|
||||
state in arbitrary ways, safety would be a lost cause. Thankfully, it *can*
|
||||
prevent arbitrary code from messing with critical state due to privacy.
|
||||
|
||||
Safety lives!
|
||||
|
Loading…
Reference in New Issue