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auto merge of #18176 : jkleint/rust/guide-borrow-wording, r=steveklabnik

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Explain the primary disadvantage of garbage collection is runtime
overhead and unpredictable pauses.  Elucidate where the name "race
condition" comes from.  Emphasize that Rust can guarantee your code is
free of race conditions and other memory errors, with no runtime
overhead.

cc @steveklabnik
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bors 2014-10-25 09:17:05 +00:00
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@ -3466,13 +3466,14 @@ for destroying that resource as well. Given that we're discussing pointers
right now, let's discuss this in the context of memory allocation, though
it applies to other resources as well.
When you allocate heap memory, you need a mechanism to free that memory. Many
languages let the programmer control the allocation, and then use a garbage
collector to handle the deallocation. This is a valid, time-tested strategy,
but it's not without its drawbacks. Because the programmer does not have to
think as much about deallocation, allocation becomes something commonplace,
because it's easy. And if you need precise control over when something is
deallocated, leaving it up to your runtime can make this difficult.
When you allocate heap memory, you need a mechanism to free that memory. Many
languages use a garbage collector to handle deallocation. This is a valid,
time-tested strategy, but it's not without its drawbacks: it adds overhead, and
can lead to unpredictable pauses in execution. Because the programmer does not
have to think as much about deallocation, allocation becomes something
commonplace, leading to more memory usage. And if you need precise control
over when something is deallocated, leaving it up to your runtime can make this
difficult.
Rust chooses a different path, and that path is called **ownership**. Any
binding that creates a resource is the **owner** of that resource.
@ -3498,17 +3499,19 @@ memory. The length of time that the borrower is borrowing the pointer
from you is called a **lifetime**.
If two distinct bindings share a pointer, and the memory that pointer points to
is immutable, then there are no problems. But if it's mutable, both pointers
can attempt to write to the memory at the same time, causing a **race
condition**. Therefore, if someone wants to mutate something that they've
borrowed from you, you must not have lent out that pointer to anyone else.
is immutable, then there are no problems. But if it's mutable, the result of
changing it can vary unpredictably depending on who happens to access it first,
which is called a **race condition**. To avoid this, if someone wants to mutate
something that they've borrowed from you, you must not have lent out that
pointer to anyone else.
Rust has a sophisticated system called the **borrow checker** to make sure that
everyone plays by these rules. At compile time, it verifies that none of these
rules are broken. If there's no problem, our program compiles successfully, and
there is no runtime overhead for any of this. The borrow checker works only at
compile time. If the borrow checker did find a problem, it will report a
**lifetime error**, and your program will refuse to compile.
rules are broken. If our program compiles successfully, Rust can guarantee it
is free of data races and other memory errors, and there is no runtime overhead
for any of this. The borrow checker works only at compile time. If the borrow
checker did find a problem, it will report a **lifetime error**, and your
program will refuse to compile.
That's a lot to take in. It's also one of the _most_ important concepts in
all of Rust. Let's see this syntax in action: