Rollup merge of #61310 - RalfJung:mem, r=Centril
split libcore::mem into multiple files r? @Centril
This commit is contained in:
Oliver Scherer
GitHub
commit
18967566c7
1406
src/libcore/mem.rs
1406
src/libcore/mem.rs
File diff suppressed because it is too large
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use crate::ptr;
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use crate::ops::{Deref, DerefMut};
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/// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
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///
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/// This wrapper is 0-cost.
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///
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/// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`.
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/// As a consequence, it has *no effect* on the assumptions that the compiler makes
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/// about all values being initialized at their type. In particular, initializing
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/// a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined behavior.
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/// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead.
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///
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/// # Examples
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///
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/// This wrapper helps with explicitly documenting the drop order dependencies between fields of
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/// the type:
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///
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/// ```rust
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/// use std::mem::ManuallyDrop;
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/// struct Peach;
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/// struct Banana;
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/// struct Melon;
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/// struct FruitBox {
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/// // Immediately clear there’s something non-trivial going on with these fields.
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/// peach: ManuallyDrop<Peach>,
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/// melon: Melon, // Field that’s independent of the other two.
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/// banana: ManuallyDrop<Banana>,
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/// }
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///
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/// impl Drop for FruitBox {
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/// fn drop(&mut self) {
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/// unsafe {
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/// // Explicit ordering in which field destructors are run specified in the intuitive
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/// // location – the destructor of the structure containing the fields.
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/// // Moreover, one can now reorder fields within the struct however much they want.
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/// ManuallyDrop::drop(&mut self.peach);
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/// ManuallyDrop::drop(&mut self.banana);
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/// }
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/// // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
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/// // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
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/// }
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/// }
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/// ```
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///
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/// [`mem::zeroed`]: fn.zeroed.html
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/// [`MaybeUninit<T>`]: union.MaybeUninit.html
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#[stable(feature = "manually_drop", since = "1.20.0")]
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#[lang = "manually_drop"]
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#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
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#[repr(transparent)]
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pub struct ManuallyDrop<T: ?Sized> {
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value: T,
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}
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impl<T> ManuallyDrop<T> {
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/// Wrap a value to be manually dropped.
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///
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/// # Examples
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///
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/// ```rust
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/// use std::mem::ManuallyDrop;
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/// ManuallyDrop::new(Box::new(()));
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/// ```
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#[stable(feature = "manually_drop", since = "1.20.0")]
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#[inline(always)]
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pub const fn new(value: T) -> ManuallyDrop<T> {
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ManuallyDrop { value }
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}
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/// Extracts the value from the `ManuallyDrop` container.
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///
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/// This allows the value to be dropped again.
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///
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/// # Examples
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///
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/// ```rust
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/// use std::mem::ManuallyDrop;
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/// let x = ManuallyDrop::new(Box::new(()));
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/// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
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/// ```
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#[stable(feature = "manually_drop", since = "1.20.0")]
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#[inline(always)]
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pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
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slot.value
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}
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/// Takes the contained value out.
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///
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/// This method is primarily intended for moving out values in drop.
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/// Instead of using [`ManuallyDrop::drop`] to manually drop the value,
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/// you can use this method to take the value and use it however desired.
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/// `Drop` will be invoked on the returned value following normal end-of-scope rules.
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///
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/// If you have ownership of the container, you can use [`ManuallyDrop::into_inner`] instead.
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///
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/// # Safety
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///
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/// This function semantically moves out the contained value without preventing further usage.
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/// It is up to the user of this method to ensure that this container is not used again.
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///
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/// [`ManuallyDrop::drop`]: #method.drop
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/// [`ManuallyDrop::into_inner`]: #method.into_inner
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#[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
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#[unstable(feature = "manually_drop_take", issue = "55422")]
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#[inline]
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pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
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ManuallyDrop::into_inner(ptr::read(slot))
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}
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}
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impl<T: ?Sized> ManuallyDrop<T> {
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/// Manually drops the contained value.
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///
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/// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
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///
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/// # Safety
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///
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/// This function runs the destructor of the contained value and thus the wrapped value
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/// now represents uninitialized data. It is up to the user of this method to ensure the
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/// uninitialized data is not actually used.
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///
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/// [`ManuallyDrop::into_inner`]: #method.into_inner
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#[stable(feature = "manually_drop", since = "1.20.0")]
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#[inline]
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pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
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ptr::drop_in_place(&mut slot.value)
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}
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}
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#[stable(feature = "manually_drop", since = "1.20.0")]
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impl<T: ?Sized> Deref for ManuallyDrop<T> {
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type Target = T;
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#[inline(always)]
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fn deref(&self) -> &T {
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&self.value
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}
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}
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#[stable(feature = "manually_drop", since = "1.20.0")]
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impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
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#[inline(always)]
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fn deref_mut(&mut self) -> &mut T {
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&mut self.value
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}
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}
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@ -0,0 +1,519 @@
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use crate::intrinsics;
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use crate::mem::ManuallyDrop;
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/// A wrapper type to construct uninitialized instances of `T`.
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///
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/// # Initialization invariant
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///
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/// The compiler, in general, assumes that variables are properly initialized
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/// at their respective type. For example, a variable of reference type must
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/// be aligned and non-NULL. This is an invariant that must *always* be upheld,
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/// even in unsafe code. As a consequence, zero-initializing a variable of reference
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/// type causes instantaneous [undefined behavior][ub], no matter whether that reference
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/// ever gets used to access memory:
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///
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/// ```rust,no_run
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/// use std::mem::{self, MaybeUninit};
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///
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/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<&i32>`:
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/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior!
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/// ```
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///
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/// This is exploited by the compiler for various optimizations, such as eliding
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/// run-time checks and optimizing `enum` layout.
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///
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/// Similarly, entirely uninitialized memory may have any content, while a `bool` must
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/// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
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///
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/// ```rust,no_run
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/// use std::mem::{self, MaybeUninit};
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///
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/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<bool>`:
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/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
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/// ```
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///
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/// Moreover, uninitialized memory is special in that the compiler knows that
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/// it does not have a fixed value. This makes it undefined behavior to have
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/// uninitialized data in a variable even if that variable has an integer type,
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/// which otherwise can hold any *fixed* bit pattern:
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///
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/// ```rust,no_run
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/// use std::mem::{self, MaybeUninit};
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///
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/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<i32>`:
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/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
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/// ```
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/// (Notice that the rules around uninitialized integers are not finalized yet, but
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/// until they are, it is advisable to avoid them.)
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///
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/// On top of that, remember that most types have additional invariants beyond merely
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/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
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/// is considered initialized because the only requirement the compiler knows about it
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/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
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/// *immediate* undefined behavior, but will cause undefined behavior with most
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/// safe operations (including dropping it).
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///
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/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
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///
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/// # Examples
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///
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/// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
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/// It is a signal to the compiler indicating that the data here might *not*
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/// be initialized:
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// // Create an explicitly uninitialized reference. The compiler knows that data inside
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/// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
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/// let mut x = MaybeUninit::<&i32>::uninit();
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/// // Set it to a valid value.
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/// unsafe { x.as_mut_ptr().write(&0); }
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/// // Extract the initialized data -- this is only allowed *after* properly
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/// // initializing `x`!
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/// let x = unsafe { x.assume_init() };
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/// ```
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///
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/// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
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///
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/// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
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/// any of the run-time tracking and without any of the safety checks.
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///
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/// ## out-pointers
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///
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/// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
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/// from a function, pass it a pointer to some (uninitialized) memory to put the
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/// result into. This can be useful when it is important for the caller to control
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/// how the memory the result is stored in gets allocated, and you want to avoid
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/// unnecessary moves.
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///
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/// ```
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/// use std::mem::MaybeUninit;
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///
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/// unsafe fn make_vec(out: *mut Vec<i32>) {
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/// // `write` does not drop the old contents, which is important.
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/// out.write(vec![1, 2, 3]);
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/// }
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///
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/// let mut v = MaybeUninit::uninit();
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/// unsafe { make_vec(v.as_mut_ptr()); }
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/// // Now we know `v` is initialized! This also makes sure the vector gets
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/// // properly dropped.
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/// let v = unsafe { v.assume_init() };
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/// assert_eq!(&v, &[1, 2, 3]);
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/// ```
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///
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/// ## Initializing an array element-by-element
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///
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/// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
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///
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/// ```
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/// use std::mem::{self, MaybeUninit};
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/// use std::ptr;
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///
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/// let data = {
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/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
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/// // safe because the type we are claiming to have initialized here is a
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/// // bunch of `MaybeUninit`s, which do not require initialization.
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/// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
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/// MaybeUninit::uninit().assume_init()
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/// };
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///
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/// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop,
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/// // we have a memory leak, but there is no memory safety issue.
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/// for elem in &mut data[..] {
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/// unsafe { ptr::write(elem.as_mut_ptr(), vec![42]); }
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/// }
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///
|
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/// // Everything is initialized. Transmute the array to the
|
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/// // initialized type.
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/// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
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/// };
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///
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/// assert_eq!(&data[0], &[42]);
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/// ```
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///
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/// You can also work with partially initialized arrays, which could
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/// be found in low-level datastructures.
|
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///
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/// ```
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/// use std::mem::MaybeUninit;
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/// use std::ptr;
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///
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/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
|
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/// // safe because the type we are claiming to have initialized here is a
|
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/// // bunch of `MaybeUninit`s, which do not require initialization.
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/// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
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/// // Count the number of elements we have assigned.
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/// let mut data_len: usize = 0;
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///
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/// for elem in &mut data[0..500] {
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/// unsafe { ptr::write(elem.as_mut_ptr(), String::from("hello")); }
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/// data_len += 1;
|
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/// }
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///
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/// // For each item in the array, drop if we allocated it.
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/// for elem in &mut data[0..data_len] {
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/// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
|
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/// }
|
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/// ```
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///
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/// ## Initializing a struct field-by-field
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///
|
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/// There is currently no supported way to create a raw pointer or reference
|
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/// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible
|
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/// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
|
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/// to its fields.
|
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///
|
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/// [ub]: ../../reference/behavior-considered-undefined.html
|
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///
|
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/// # Layout
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||||
///
|
||||
/// `MaybeUninit<T>` is guaranteed to have the same size and alignment as `T`:
|
||||
///
|
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/// ```rust
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/// use std::mem::{MaybeUninit, size_of, align_of};
|
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/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
|
||||
/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
|
||||
/// ```
|
||||
///
|
||||
/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
|
||||
/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
|
||||
/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
|
||||
/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
|
||||
/// optimizations, potentially resulting in a larger size:
|
||||
///
|
||||
/// ```rust
|
||||
/// # use std::mem::{MaybeUninit, size_of};
|
||||
/// assert_eq!(size_of::<Option<bool>>(), 1);
|
||||
/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
|
||||
/// ```
|
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#[allow(missing_debug_implementations)]
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[derive(Copy)]
|
||||
pub union MaybeUninit<T> {
|
||||
uninit: (),
|
||||
value: ManuallyDrop<T>,
|
||||
}
|
||||
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
impl<T: Copy> Clone for MaybeUninit<T> {
|
||||
#[inline(always)]
|
||||
fn clone(&self) -> Self {
|
||||
// Not calling `T::clone()`, we cannot know if we are initialized enough for that.
|
||||
*self
|
||||
}
|
||||
}
|
||||
|
||||
impl<T> MaybeUninit<T> {
|
||||
/// Creates a new `MaybeUninit<T>` initialized with the given value.
|
||||
/// It is safe to call [`assume_init`] on the return value of this function.
|
||||
///
|
||||
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
|
||||
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
|
||||
///
|
||||
/// [`assume_init`]: #method.assume_init
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline(always)]
|
||||
pub const fn new(val: T) -> MaybeUninit<T> {
|
||||
MaybeUninit { value: ManuallyDrop::new(val) }
|
||||
}
|
||||
|
||||
/// Creates a new `MaybeUninit<T>` in an uninitialized state.
|
||||
///
|
||||
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
|
||||
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
|
||||
///
|
||||
/// See the [type-level documentation][type] for some examples.
|
||||
///
|
||||
/// [type]: union.MaybeUninit.html
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline(always)]
|
||||
pub const fn uninit() -> MaybeUninit<T> {
|
||||
MaybeUninit { uninit: () }
|
||||
}
|
||||
|
||||
/// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
|
||||
/// filled with `0` bytes. It depends on `T` whether that already makes for
|
||||
/// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
|
||||
/// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
|
||||
/// be null.
|
||||
///
|
||||
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
|
||||
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
|
||||
///
|
||||
/// # Example
|
||||
///
|
||||
/// Correct usage of this function: initializing a struct with zero, where all
|
||||
/// fields of the struct can hold the bit-pattern 0 as a valid value.
|
||||
///
|
||||
/// ```rust
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let x = MaybeUninit::<(u8, bool)>::zeroed();
|
||||
/// let x = unsafe { x.assume_init() };
|
||||
/// assert_eq!(x, (0, false));
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this function: initializing a struct with zero, where some fields
|
||||
/// cannot hold 0 as a valid value.
|
||||
///
|
||||
/// ```rust,no_run
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// enum NotZero { One = 1, Two = 2 };
|
||||
///
|
||||
/// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
|
||||
/// let x = unsafe { x.assume_init() };
|
||||
/// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
|
||||
/// // This is undefined behavior.
|
||||
/// ```
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline]
|
||||
pub fn zeroed() -> MaybeUninit<T> {
|
||||
let mut u = MaybeUninit::<T>::uninit();
|
||||
unsafe {
|
||||
u.as_mut_ptr().write_bytes(0u8, 1);
|
||||
}
|
||||
u
|
||||
}
|
||||
|
||||
/// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value
|
||||
/// without dropping it, so be careful not to use this twice unless you want to
|
||||
/// skip running the destructor. For your convenience, this also returns a mutable
|
||||
/// reference to the (now safely initialized) contents of `self`.
|
||||
#[unstable(feature = "maybe_uninit_extra", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub fn write(&mut self, val: T) -> &mut T {
|
||||
unsafe {
|
||||
self.value = ManuallyDrop::new(val);
|
||||
self.get_mut()
|
||||
}
|
||||
}
|
||||
|
||||
/// Gets a pointer to the contained value. Reading from this pointer or turning it
|
||||
/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
|
||||
/// Writing to memory that this pointer (non-transitively) points to is undefined behavior
|
||||
/// (except inside an `UnsafeCell<T>`).
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Correct usage of this method:
|
||||
///
|
||||
/// ```rust
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
|
||||
/// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
|
||||
/// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
|
||||
/// let x_vec = unsafe { &*x.as_ptr() };
|
||||
/// assert_eq!(x_vec.len(), 3);
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this method:
|
||||
///
|
||||
/// ```rust,no_run
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let x = MaybeUninit::<Vec<u32>>::uninit();
|
||||
/// let x_vec = unsafe { &*x.as_ptr() };
|
||||
/// // We have created a reference to an uninitialized vector! This is undefined behavior.
|
||||
/// ```
|
||||
///
|
||||
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
|
||||
/// until they are, it is advisable to avoid them.)
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline(always)]
|
||||
pub fn as_ptr(&self) -> *const T {
|
||||
unsafe { &*self.value as *const T }
|
||||
}
|
||||
|
||||
/// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
|
||||
/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Correct usage of this method:
|
||||
///
|
||||
/// ```rust
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
|
||||
/// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
|
||||
/// // Create a reference into the `MaybeUninit<Vec<u32>>`.
|
||||
/// // This is okay because we initialized it.
|
||||
/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
|
||||
/// x_vec.push(3);
|
||||
/// assert_eq!(x_vec.len(), 4);
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this method:
|
||||
///
|
||||
/// ```rust,no_run
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
|
||||
/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
|
||||
/// // We have created a reference to an uninitialized vector! This is undefined behavior.
|
||||
/// ```
|
||||
///
|
||||
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
|
||||
/// until they are, it is advisable to avoid them.)
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline(always)]
|
||||
pub fn as_mut_ptr(&mut self) -> *mut T {
|
||||
unsafe { &mut *self.value as *mut T }
|
||||
}
|
||||
|
||||
/// Extracts the value from the `MaybeUninit<T>` container. This is a great way
|
||||
/// to ensure that the data will get dropped, because the resulting `T` is
|
||||
/// subject to the usual drop handling.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
|
||||
/// state. Calling this when the content is not yet fully initialized causes immediate undefined
|
||||
/// behavior. The [type-level documentation][inv] contains more information about
|
||||
/// this initialization invariant.
|
||||
///
|
||||
/// [inv]: #initialization-invariant
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Correct usage of this method:
|
||||
///
|
||||
/// ```rust
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<bool>::uninit();
|
||||
/// unsafe { x.as_mut_ptr().write(true); }
|
||||
/// let x_init = unsafe { x.assume_init() };
|
||||
/// assert_eq!(x_init, true);
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this method:
|
||||
///
|
||||
/// ```rust,no_run
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let x = MaybeUninit::<Vec<u32>>::uninit();
|
||||
/// let x_init = unsafe { x.assume_init() };
|
||||
/// // `x` had not been initialized yet, so this last line caused undefined behavior.
|
||||
/// ```
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
#[inline(always)]
|
||||
pub unsafe fn assume_init(self) -> T {
|
||||
intrinsics::panic_if_uninhabited::<T>();
|
||||
ManuallyDrop::into_inner(self.value)
|
||||
}
|
||||
|
||||
/// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
|
||||
/// to the usual drop handling.
|
||||
///
|
||||
/// Whenever possible, it is preferrable to use [`assume_init`] instead, which
|
||||
/// prevents duplicating the content of the `MaybeUninit<T>`.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
|
||||
/// state. Calling this when the content is not yet fully initialized causes undefined
|
||||
/// behavior. The [type-level documentation][inv] contains more information about
|
||||
/// this initialization invariant.
|
||||
///
|
||||
/// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using
|
||||
/// multiple copies of the data (by calling `read` multiple times, or first
|
||||
/// calling `read` and then [`assume_init`]), it is your responsibility
|
||||
/// to ensure that that data may indeed be duplicated.
|
||||
///
|
||||
/// [inv]: #initialization-invariant
|
||||
/// [`assume_init`]: #method.assume_init
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Correct usage of this method:
|
||||
///
|
||||
/// ```rust
|
||||
/// #![feature(maybe_uninit_extra)]
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<u32>::uninit();
|
||||
/// x.write(13);
|
||||
/// let x1 = unsafe { x.read() };
|
||||
/// // `u32` is `Copy`, so we may read multiple times.
|
||||
/// let x2 = unsafe { x.read() };
|
||||
/// assert_eq!(x1, x2);
|
||||
///
|
||||
/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
|
||||
/// x.write(None);
|
||||
/// let x1 = unsafe { x.read() };
|
||||
/// // Duplicating a `None` value is okay, so we may read multiple times.
|
||||
/// let x2 = unsafe { x.read() };
|
||||
/// assert_eq!(x1, x2);
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this method:
|
||||
///
|
||||
/// ```rust,no_run
|
||||
/// #![feature(maybe_uninit_extra)]
|
||||
/// use std::mem::MaybeUninit;
|
||||
///
|
||||
/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
|
||||
/// x.write(Some(vec![0,1,2]));
|
||||
/// let x1 = unsafe { x.read() };
|
||||
/// let x2 = unsafe { x.read() };
|
||||
/// // We now created two copies of the same vector, leading to a double-free when
|
||||
/// // they both get dropped!
|
||||
/// ```
|
||||
#[unstable(feature = "maybe_uninit_extra", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub unsafe fn read(&self) -> T {
|
||||
intrinsics::panic_if_uninhabited::<T>();
|
||||
self.as_ptr().read()
|
||||
}
|
||||
|
||||
/// Gets a reference to the contained value.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
|
||||
/// state. Calling this when the content is not yet fully initialized causes undefined
|
||||
/// behavior.
|
||||
#[unstable(feature = "maybe_uninit_ref", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub unsafe fn get_ref(&self) -> &T {
|
||||
&*self.value
|
||||
}
|
||||
|
||||
/// Gets a mutable reference to the contained value.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
|
||||
/// state. Calling this when the content is not yet fully initialized causes undefined
|
||||
/// behavior.
|
||||
// FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references
|
||||
// to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make
|
||||
// a final decision about the rules before stabilization.
|
||||
#[unstable(feature = "maybe_uninit_ref", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub unsafe fn get_mut(&mut self) -> &mut T {
|
||||
&mut *self.value
|
||||
}
|
||||
|
||||
/// Gets a pointer to the first element of the array.
|
||||
#[unstable(feature = "maybe_uninit_slice", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T {
|
||||
this as *const [MaybeUninit<T>] as *const T
|
||||
}
|
||||
|
||||
/// Gets a mutable pointer to the first element of the array.
|
||||
#[unstable(feature = "maybe_uninit_slice", issue = "53491")]
|
||||
#[inline(always)]
|
||||
pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T {
|
||||
this as *mut [MaybeUninit<T>] as *mut T
|
||||
}
|
||||
}
|
||||
|
|
@ -0,0 +1,752 @@
|
|||
//! Basic functions for dealing with memory.
|
||||
//!
|
||||
//! This module contains functions for querying the size and alignment of
|
||||
//! types, initializing and manipulating memory.
|
||||
|
||||
#![stable(feature = "rust1", since = "1.0.0")]
|
||||
|
||||
use crate::clone;
|
||||
use crate::cmp;
|
||||
use crate::fmt;
|
||||
use crate::hash;
|
||||
use crate::intrinsics;
|
||||
use crate::marker::{Copy, PhantomData, Sized};
|
||||
use crate::ptr;
|
||||
|
||||
mod manually_drop;
|
||||
#[stable(feature = "manually_drop", since = "1.20.0")]
|
||||
pub use manually_drop::ManuallyDrop;
|
||||
|
||||
mod maybe_uninit;
|
||||
#[stable(feature = "maybe_uninit", since = "1.36.0")]
|
||||
pub use maybe_uninit::MaybeUninit;
|
||||
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
#[doc(inline)]
|
||||
pub use crate::intrinsics::transmute;
|
||||
|
||||
/// Takes ownership and "forgets" about the value **without running its destructor**.
|
||||
///
|
||||
/// Any resources the value manages, such as heap memory or a file handle, will linger
|
||||
/// forever in an unreachable state. However, it does not guarantee that pointers
|
||||
/// to this memory will remain valid.
|
||||
///
|
||||
/// * If you want to leak memory, see [`Box::leak`][leak].
|
||||
/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
|
||||
/// * If you want to dispose of a value properly, running its destructor, see
|
||||
/// [`mem::drop`][drop].
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// `forget` is not marked as `unsafe`, because Rust's safety guarantees
|
||||
/// do not include a guarantee that destructors will always run. For example,
|
||||
/// a program can create a reference cycle using [`Rc`][rc], or call
|
||||
/// [`process::exit`][exit] to exit without running destructors. Thus, allowing
|
||||
/// `mem::forget` from safe code does not fundamentally change Rust's safety
|
||||
/// guarantees.
|
||||
///
|
||||
/// That said, leaking resources such as memory or I/O objects is usually undesirable,
|
||||
/// so `forget` is only recommended for specialized use cases like those shown below.
|
||||
///
|
||||
/// Because forgetting a value is allowed, any `unsafe` code you write must
|
||||
/// allow for this possibility. You cannot return a value and expect that the
|
||||
/// caller will necessarily run the value's destructor.
|
||||
///
|
||||
/// [rc]: ../../std/rc/struct.Rc.html
|
||||
/// [exit]: ../../std/process/fn.exit.html
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Leak an I/O object, never closing the file:
|
||||
///
|
||||
/// ```no_run
|
||||
/// use std::mem;
|
||||
/// use std::fs::File;
|
||||
///
|
||||
/// let file = File::open("foo.txt").unwrap();
|
||||
/// mem::forget(file);
|
||||
/// ```
|
||||
///
|
||||
/// The practical use cases for `forget` are rather specialized and mainly come
|
||||
/// up in unsafe or FFI code.
|
||||
///
|
||||
/// [drop]: fn.drop.html
|
||||
/// [uninit]: fn.uninitialized.html
|
||||
/// [clone]: ../clone/trait.Clone.html
|
||||
/// [swap]: fn.swap.html
|
||||
/// [box]: ../../std/boxed/struct.Box.html
|
||||
/// [leak]: ../../std/boxed/struct.Box.html#method.leak
|
||||
/// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
|
||||
/// [ub]: ../../reference/behavior-considered-undefined.html
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn forget<T>(t: T) {
|
||||
ManuallyDrop::new(t);
|
||||
}
|
||||
|
||||
/// Like [`forget`], but also accepts unsized values.
|
||||
///
|
||||
/// This function is just a shim intended to be removed when the `unsized_locals` feature gets
|
||||
/// stabilized.
|
||||
///
|
||||
/// [`forget`]: fn.forget.html
|
||||
#[inline]
|
||||
#[unstable(feature = "forget_unsized", issue = "0")]
|
||||
pub fn forget_unsized<T: ?Sized>(t: T) {
|
||||
unsafe { intrinsics::forget(t) }
|
||||
}
|
||||
|
||||
/// Returns the size of a type in bytes.
|
||||
///
|
||||
/// More specifically, this is the offset in bytes between successive elements
|
||||
/// in an array with that item type including alignment padding. Thus, for any
|
||||
/// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
|
||||
///
|
||||
/// In general, the size of a type is not stable across compilations, but
|
||||
/// specific types such as primitives are.
|
||||
///
|
||||
/// The following table gives the size for primitives.
|
||||
///
|
||||
/// Type | size_of::\<Type>()
|
||||
/// ---- | ---------------
|
||||
/// () | 0
|
||||
/// bool | 1
|
||||
/// u8 | 1
|
||||
/// u16 | 2
|
||||
/// u32 | 4
|
||||
/// u64 | 8
|
||||
/// u128 | 16
|
||||
/// i8 | 1
|
||||
/// i16 | 2
|
||||
/// i32 | 4
|
||||
/// i64 | 8
|
||||
/// i128 | 16
|
||||
/// f32 | 4
|
||||
/// f64 | 8
|
||||
/// char | 4
|
||||
///
|
||||
/// Furthermore, `usize` and `isize` have the same size.
|
||||
///
|
||||
/// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
|
||||
/// the same size. If `T` is Sized, all of those types have the same size as `usize`.
|
||||
///
|
||||
/// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
|
||||
/// have the same size. Likewise for `*const T` and `*mut T`.
|
||||
///
|
||||
/// # Size of `#[repr(C)]` items
|
||||
///
|
||||
/// The `C` representation for items has a defined layout. With this layout,
|
||||
/// the size of items is also stable as long as all fields have a stable size.
|
||||
///
|
||||
/// ## Size of Structs
|
||||
///
|
||||
/// For `structs`, the size is determined by the following algorithm.
|
||||
///
|
||||
/// For each field in the struct ordered by declaration order:
|
||||
///
|
||||
/// 1. Add the size of the field.
|
||||
/// 2. Round up the current size to the nearest multiple of the next field's [alignment].
|
||||
///
|
||||
/// Finally, round the size of the struct to the nearest multiple of its [alignment].
|
||||
/// The alignment of the struct is usually the largest alignment of all its
|
||||
/// fields; this can be changed with the use of `repr(align(N))`.
|
||||
///
|
||||
/// Unlike `C`, zero sized structs are not rounded up to one byte in size.
|
||||
///
|
||||
/// ## Size of Enums
|
||||
///
|
||||
/// Enums that carry no data other than the discriminant have the same size as C enums
|
||||
/// on the platform they are compiled for.
|
||||
///
|
||||
/// ## Size of Unions
|
||||
///
|
||||
/// The size of a union is the size of its largest field.
|
||||
///
|
||||
/// Unlike `C`, zero sized unions are not rounded up to one byte in size.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// // Some primitives
|
||||
/// assert_eq!(4, mem::size_of::<i32>());
|
||||
/// assert_eq!(8, mem::size_of::<f64>());
|
||||
/// assert_eq!(0, mem::size_of::<()>());
|
||||
///
|
||||
/// // Some arrays
|
||||
/// assert_eq!(8, mem::size_of::<[i32; 2]>());
|
||||
/// assert_eq!(12, mem::size_of::<[i32; 3]>());
|
||||
/// assert_eq!(0, mem::size_of::<[i32; 0]>());
|
||||
///
|
||||
///
|
||||
/// // Pointer size equality
|
||||
/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
|
||||
/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
|
||||
/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
|
||||
/// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
|
||||
/// ```
|
||||
///
|
||||
/// Using `#[repr(C)]`.
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// #[repr(C)]
|
||||
/// struct FieldStruct {
|
||||
/// first: u8,
|
||||
/// second: u16,
|
||||
/// third: u8
|
||||
/// }
|
||||
///
|
||||
/// // The size of the first field is 1, so add 1 to the size. Size is 1.
|
||||
/// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
|
||||
/// // The size of the second field is 2, so add 2 to the size. Size is 4.
|
||||
/// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
|
||||
/// // The size of the third field is 1, so add 1 to the size. Size is 5.
|
||||
/// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
|
||||
/// // fields is 2), so add 1 to the size for padding. Size is 6.
|
||||
/// assert_eq!(6, mem::size_of::<FieldStruct>());
|
||||
///
|
||||
/// #[repr(C)]
|
||||
/// struct TupleStruct(u8, u16, u8);
|
||||
///
|
||||
/// // Tuple structs follow the same rules.
|
||||
/// assert_eq!(6, mem::size_of::<TupleStruct>());
|
||||
///
|
||||
/// // Note that reordering the fields can lower the size. We can remove both padding bytes
|
||||
/// // by putting `third` before `second`.
|
||||
/// #[repr(C)]
|
||||
/// struct FieldStructOptimized {
|
||||
/// first: u8,
|
||||
/// third: u8,
|
||||
/// second: u16
|
||||
/// }
|
||||
///
|
||||
/// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
|
||||
///
|
||||
/// // Union size is the size of the largest field.
|
||||
/// #[repr(C)]
|
||||
/// union ExampleUnion {
|
||||
/// smaller: u8,
|
||||
/// larger: u16
|
||||
/// }
|
||||
///
|
||||
/// assert_eq!(2, mem::size_of::<ExampleUnion>());
|
||||
/// ```
|
||||
///
|
||||
/// [alignment]: ./fn.align_of.html
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
#[rustc_promotable]
|
||||
pub const fn size_of<T>() -> usize {
|
||||
intrinsics::size_of::<T>()
|
||||
}
|
||||
|
||||
/// Returns the size of the pointed-to value in bytes.
|
||||
///
|
||||
/// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
|
||||
/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
|
||||
/// then `size_of_val` can be used to get the dynamically-known size.
|
||||
///
|
||||
/// [slice]: ../../std/primitive.slice.html
|
||||
/// [trait object]: ../../book/ch17-02-trait-objects.html
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// assert_eq!(4, mem::size_of_val(&5i32));
|
||||
///
|
||||
/// let x: [u8; 13] = [0; 13];
|
||||
/// let y: &[u8] = &x;
|
||||
/// assert_eq!(13, mem::size_of_val(y));
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
|
||||
unsafe { intrinsics::size_of_val(val) }
|
||||
}
|
||||
|
||||
/// Returns the [ABI]-required minimum alignment of a type.
|
||||
///
|
||||
/// Every reference to a value of the type `T` must be a multiple of this number.
|
||||
///
|
||||
/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
|
||||
///
|
||||
/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// # #![allow(deprecated)]
|
||||
/// use std::mem;
|
||||
///
|
||||
/// assert_eq!(4, mem::min_align_of::<i32>());
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
#[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
|
||||
pub fn min_align_of<T>() -> usize {
|
||||
intrinsics::min_align_of::<T>()
|
||||
}
|
||||
|
||||
/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
|
||||
///
|
||||
/// Every reference to a value of the type `T` must be a multiple of this number.
|
||||
///
|
||||
/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// # #![allow(deprecated)]
|
||||
/// use std::mem;
|
||||
///
|
||||
/// assert_eq!(4, mem::min_align_of_val(&5i32));
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
#[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
|
||||
pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
|
||||
unsafe { intrinsics::min_align_of_val(val) }
|
||||
}
|
||||
|
||||
/// Returns the [ABI]-required minimum alignment of a type.
|
||||
///
|
||||
/// Every reference to a value of the type `T` must be a multiple of this number.
|
||||
///
|
||||
/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
|
||||
///
|
||||
/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// assert_eq!(4, mem::align_of::<i32>());
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
#[rustc_promotable]
|
||||
pub const fn align_of<T>() -> usize {
|
||||
intrinsics::min_align_of::<T>()
|
||||
}
|
||||
|
||||
/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
|
||||
///
|
||||
/// Every reference to a value of the type `T` must be a multiple of this number.
|
||||
///
|
||||
/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// assert_eq!(4, mem::align_of_val(&5i32));
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
|
||||
unsafe { intrinsics::min_align_of_val(val) }
|
||||
}
|
||||
|
||||
/// Returns `true` if dropping values of type `T` matters.
|
||||
///
|
||||
/// This is purely an optimization hint, and may be implemented conservatively:
|
||||
/// it may return `true` for types that don't actually need to be dropped.
|
||||
/// As such always returning `true` would be a valid implementation of
|
||||
/// this function. However if this function actually returns `false`, then you
|
||||
/// can be certain dropping `T` has no side effect.
|
||||
///
|
||||
/// Low level implementations of things like collections, which need to manually
|
||||
/// drop their data, should use this function to avoid unnecessarily
|
||||
/// trying to drop all their contents when they are destroyed. This might not
|
||||
/// make a difference in release builds (where a loop that has no side-effects
|
||||
/// is easily detected and eliminated), but is often a big win for debug builds.
|
||||
///
|
||||
/// Note that `ptr::drop_in_place` already performs this check, so if your workload
|
||||
/// can be reduced to some small number of drop_in_place calls, using this is
|
||||
/// unnecessary. In particular note that you can drop_in_place a slice, and that
|
||||
/// will do a single needs_drop check for all the values.
|
||||
///
|
||||
/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
|
||||
/// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
|
||||
/// values one at a time and should use this API.
|
||||
///
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Here's an example of how a collection might make use of needs_drop:
|
||||
///
|
||||
/// ```
|
||||
/// use std::{mem, ptr};
|
||||
///
|
||||
/// pub struct MyCollection<T> {
|
||||
/// # data: [T; 1],
|
||||
/// /* ... */
|
||||
/// }
|
||||
/// # impl<T> MyCollection<T> {
|
||||
/// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
|
||||
/// # fn free_buffer(&mut self) {}
|
||||
/// # }
|
||||
///
|
||||
/// impl<T> Drop for MyCollection<T> {
|
||||
/// fn drop(&mut self) {
|
||||
/// unsafe {
|
||||
/// // drop the data
|
||||
/// if mem::needs_drop::<T>() {
|
||||
/// for x in self.iter_mut() {
|
||||
/// ptr::drop_in_place(x);
|
||||
/// }
|
||||
/// }
|
||||
/// self.free_buffer();
|
||||
/// }
|
||||
/// }
|
||||
/// }
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "needs_drop", since = "1.21.0")]
|
||||
pub const fn needs_drop<T>() -> bool {
|
||||
intrinsics::needs_drop::<T>()
|
||||
}
|
||||
|
||||
/// Creates a value whose bytes are all zero.
|
||||
///
|
||||
/// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
|
||||
/// It is useful for FFI sometimes, but should generally be avoided.
|
||||
///
|
||||
/// There is no guarantee that an all-zero byte-pattern represents a valid value of
|
||||
/// some type `T`. For example, the all-zero byte-pattern is not a valid value
|
||||
/// for reference types (`&T` and `&mut T`). Using `zeroed` on such types
|
||||
/// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv]
|
||||
/// that there always is a valid value in a variable it considers initialized.
|
||||
///
|
||||
/// [zeroed]: union.MaybeUninit.html#method.zeroed
|
||||
/// [ub]: ../../reference/behavior-considered-undefined.html
|
||||
/// [inv]: union.MaybeUninit.html#initialization-invariant
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Correct usage of this function: initializing an integer with zero.
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// let x: i32 = unsafe { mem::zeroed() };
|
||||
/// assert_eq!(0, x);
|
||||
/// ```
|
||||
///
|
||||
/// *Incorrect* usage of this function: initializing a reference with zero.
|
||||
///
|
||||
/// ```no_run
|
||||
/// use std::mem;
|
||||
///
|
||||
/// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub unsafe fn zeroed<T>() -> T {
|
||||
intrinsics::panic_if_uninhabited::<T>();
|
||||
intrinsics::init()
|
||||
}
|
||||
|
||||
/// Bypasses Rust's normal memory-initialization checks by pretending to
|
||||
/// produce a value of type `T`, while doing nothing at all.
|
||||
///
|
||||
/// **This functon is deprecated.** Use [`MaybeUninit<T>`] instead.
|
||||
///
|
||||
/// The reason for deprecation is that the function basically cannot be used
|
||||
/// correctly: [the Rust compiler assumes][inv] that values are properly initialized.
|
||||
/// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
|
||||
/// undefined behavior for returning a `bool` that is not definitely either `true`
|
||||
/// or `false`. Worse, truly uninitialized memory like what gets returned here
|
||||
/// is special in that the compiler knows that it does not have a fixed value.
|
||||
/// This makes it undefined behavior to have uninitialized data in a variable even
|
||||
/// if that variable has an integer type.
|
||||
/// (Notice that the rules around uninitialized integers are not finalized yet, but
|
||||
/// until they are, it is advisable to avoid them.)
|
||||
///
|
||||
/// [`MaybeUninit<T>`]: union.MaybeUninit.html
|
||||
/// [inv]: union.MaybeUninit.html#initialization-invariant
|
||||
#[inline]
|
||||
#[rustc_deprecated(since = "1.38.0", reason = "use `mem::MaybeUninit` instead")]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub unsafe fn uninitialized<T>() -> T {
|
||||
intrinsics::panic_if_uninhabited::<T>();
|
||||
intrinsics::uninit()
|
||||
}
|
||||
|
||||
/// Swaps the values at two mutable locations, without deinitializing either one.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// let mut x = 5;
|
||||
/// let mut y = 42;
|
||||
///
|
||||
/// mem::swap(&mut x, &mut y);
|
||||
///
|
||||
/// assert_eq!(42, x);
|
||||
/// assert_eq!(5, y);
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn swap<T>(x: &mut T, y: &mut T) {
|
||||
unsafe {
|
||||
ptr::swap_nonoverlapping_one(x, y);
|
||||
}
|
||||
}
|
||||
|
||||
/// Moves `src` into the referenced `dest`, returning the previous `dest` value.
|
||||
///
|
||||
/// Neither value is dropped.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// A simple example:
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// let mut v: Vec<i32> = vec![1, 2];
|
||||
///
|
||||
/// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
|
||||
/// assert_eq!(vec![1, 2], old_v);
|
||||
/// assert_eq!(vec![3, 4, 5], v);
|
||||
/// ```
|
||||
///
|
||||
/// `replace` allows consumption of a struct field by replacing it with another value.
|
||||
/// Without `replace` you can run into issues like these:
|
||||
///
|
||||
/// ```compile_fail,E0507
|
||||
/// struct Buffer<T> { buf: Vec<T> }
|
||||
///
|
||||
/// impl<T> Buffer<T> {
|
||||
/// fn get_and_reset(&mut self) -> Vec<T> {
|
||||
/// // error: cannot move out of dereference of `&mut`-pointer
|
||||
/// let buf = self.buf;
|
||||
/// self.buf = Vec::new();
|
||||
/// buf
|
||||
/// }
|
||||
/// }
|
||||
/// ```
|
||||
///
|
||||
/// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
|
||||
/// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
|
||||
/// `self`, allowing it to be returned:
|
||||
///
|
||||
/// ```
|
||||
/// # #![allow(dead_code)]
|
||||
/// use std::mem;
|
||||
///
|
||||
/// # struct Buffer<T> { buf: Vec<T> }
|
||||
/// impl<T> Buffer<T> {
|
||||
/// fn get_and_reset(&mut self) -> Vec<T> {
|
||||
/// mem::replace(&mut self.buf, Vec::new())
|
||||
/// }
|
||||
/// }
|
||||
/// ```
|
||||
///
|
||||
/// [`Clone`]: ../../std/clone/trait.Clone.html
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn replace<T>(dest: &mut T, mut src: T) -> T {
|
||||
swap(dest, &mut src);
|
||||
src
|
||||
}
|
||||
|
||||
/// Disposes of a value.
|
||||
///
|
||||
/// This does call the argument's implementation of [`Drop`][drop].
|
||||
///
|
||||
/// This effectively does nothing for types which implement `Copy`, e.g.
|
||||
/// integers. Such values are copied and _then_ moved into the function, so the
|
||||
/// value persists after this function call.
|
||||
///
|
||||
/// This function is not magic; it is literally defined as
|
||||
///
|
||||
/// ```
|
||||
/// pub fn drop<T>(_x: T) { }
|
||||
/// ```
|
||||
///
|
||||
/// Because `_x` is moved into the function, it is automatically dropped before
|
||||
/// the function returns.
|
||||
///
|
||||
/// [drop]: ../ops/trait.Drop.html
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// Basic usage:
|
||||
///
|
||||
/// ```
|
||||
/// let v = vec![1, 2, 3];
|
||||
///
|
||||
/// drop(v); // explicitly drop the vector
|
||||
/// ```
|
||||
///
|
||||
/// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
|
||||
/// release a [`RefCell`] borrow:
|
||||
///
|
||||
/// ```
|
||||
/// use std::cell::RefCell;
|
||||
///
|
||||
/// let x = RefCell::new(1);
|
||||
///
|
||||
/// let mut mutable_borrow = x.borrow_mut();
|
||||
/// *mutable_borrow = 1;
|
||||
///
|
||||
/// drop(mutable_borrow); // relinquish the mutable borrow on this slot
|
||||
///
|
||||
/// let borrow = x.borrow();
|
||||
/// println!("{}", *borrow);
|
||||
/// ```
|
||||
///
|
||||
/// Integers and other types implementing [`Copy`] are unaffected by `drop`.
|
||||
///
|
||||
/// ```
|
||||
/// #[derive(Copy, Clone)]
|
||||
/// struct Foo(u8);
|
||||
///
|
||||
/// let x = 1;
|
||||
/// let y = Foo(2);
|
||||
/// drop(x); // a copy of `x` is moved and dropped
|
||||
/// drop(y); // a copy of `y` is moved and dropped
|
||||
///
|
||||
/// println!("x: {}, y: {}", x, y.0); // still available
|
||||
/// ```
|
||||
///
|
||||
/// [`RefCell`]: ../../std/cell/struct.RefCell.html
|
||||
/// [`Copy`]: ../../std/marker/trait.Copy.html
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub fn drop<T>(_x: T) { }
|
||||
|
||||
/// Interprets `src` as having type `&U`, and then reads `src` without moving
|
||||
/// the contained value.
|
||||
///
|
||||
/// This function will unsafely assume the pointer `src` is valid for
|
||||
/// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
|
||||
/// the `&U`. It will also unsafely create a copy of the contained value instead of
|
||||
/// moving out of `src`.
|
||||
///
|
||||
/// It is not a compile-time error if `T` and `U` have different sizes, but it
|
||||
/// is highly encouraged to only invoke this function where `T` and `U` have the
|
||||
/// same size. This function triggers [undefined behavior][ub] if `U` is larger than
|
||||
/// `T`.
|
||||
///
|
||||
/// [ub]: ../../reference/behavior-considered-undefined.html
|
||||
/// [size_of]: fn.size_of.html
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// #[repr(packed)]
|
||||
/// struct Foo {
|
||||
/// bar: u8,
|
||||
/// }
|
||||
///
|
||||
/// let foo_slice = [10u8];
|
||||
///
|
||||
/// unsafe {
|
||||
/// // Copy the data from 'foo_slice' and treat it as a 'Foo'
|
||||
/// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
|
||||
/// assert_eq!(foo_struct.bar, 10);
|
||||
///
|
||||
/// // Modify the copied data
|
||||
/// foo_struct.bar = 20;
|
||||
/// assert_eq!(foo_struct.bar, 20);
|
||||
/// }
|
||||
///
|
||||
/// // The contents of 'foo_slice' should not have changed
|
||||
/// assert_eq!(foo_slice, [10]);
|
||||
/// ```
|
||||
#[inline]
|
||||
#[stable(feature = "rust1", since = "1.0.0")]
|
||||
pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
|
||||
ptr::read_unaligned(src as *const T as *const U)
|
||||
}
|
||||
|
||||
/// Opaque type representing the discriminant of an enum.
|
||||
///
|
||||
/// See the [`discriminant`] function in this module for more information.
|
||||
///
|
||||
/// [`discriminant`]: fn.discriminant.html
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
|
||||
|
||||
// N.B. These trait implementations cannot be derived because we don't want any bounds on T.
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> Copy for Discriminant<T> {}
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> clone::Clone for Discriminant<T> {
|
||||
fn clone(&self) -> Self {
|
||||
*self
|
||||
}
|
||||
}
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> cmp::PartialEq for Discriminant<T> {
|
||||
fn eq(&self, rhs: &Self) -> bool {
|
||||
self.0 == rhs.0
|
||||
}
|
||||
}
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> cmp::Eq for Discriminant<T> {}
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> hash::Hash for Discriminant<T> {
|
||||
fn hash<H: hash::Hasher>(&self, state: &mut H) {
|
||||
self.0.hash(state);
|
||||
}
|
||||
}
|
||||
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
impl<T> fmt::Debug for Discriminant<T> {
|
||||
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
|
||||
fmt.debug_tuple("Discriminant")
|
||||
.field(&self.0)
|
||||
.finish()
|
||||
}
|
||||
}
|
||||
|
||||
/// Returns a value uniquely identifying the enum variant in `v`.
|
||||
///
|
||||
/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
|
||||
/// return value is unspecified.
|
||||
///
|
||||
/// # Stability
|
||||
///
|
||||
/// The discriminant of an enum variant may change if the enum definition changes. A discriminant
|
||||
/// of some variant will not change between compilations with the same compiler.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// This can be used to compare enums that carry data, while disregarding
|
||||
/// the actual data:
|
||||
///
|
||||
/// ```
|
||||
/// use std::mem;
|
||||
///
|
||||
/// enum Foo { A(&'static str), B(i32), C(i32) }
|
||||
///
|
||||
/// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
|
||||
/// assert!(mem::discriminant(&Foo::B(1)) == mem::discriminant(&Foo::B(2)));
|
||||
/// assert!(mem::discriminant(&Foo::B(3)) != mem::discriminant(&Foo::C(3)));
|
||||
/// ```
|
||||
#[stable(feature = "discriminant_value", since = "1.21.0")]
|
||||
pub fn discriminant<T>(v: &T) -> Discriminant<T> {
|
||||
unsafe {
|
||||
Discriminant(intrinsics::discriminant_value(v), PhantomData)
|
||||
}
|
||||
}
|
||||
|
|
@ -5,7 +5,7 @@ LL | bytes: [u8; std::mem::size_of::<Foo>()]
|
|||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
|
||||
note: ...which requires const-evaluating `Foo::bytes::{{constant}}#0`...
|
||||
--> $SRC_DIR/libcore/mem.rs:LL:COL
|
||||
--> $SRC_DIR/libcore/mem/mod.rs:LL:COL
|
||||
|
|
||||
LL | intrinsics::size_of::<T>()
|
||||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
|
|
|||
|
|
@ -1,5 +1,5 @@
|
|||
error: reached the type-length limit while instantiating `std::mem::drop::<std::option::Op... G), (G, G, G), (G, G, G))))))>>`
|
||||
--> $SRC_DIR/libcore/mem.rs:LL:COL
|
||||
--> $SRC_DIR/libcore/mem/mod.rs:LL:COL
|
||||
|
|
||||
LL | pub fn drop<T>(_x: T) { }
|
||||
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
|
|
|||
Loading…
Reference in New Issue