Clarify guarantees for `Box` allocation
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@ -34,6 +34,9 @@ extern "Rust" {
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/// This type implements the [`Alloc`] trait by forwarding calls
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/// to the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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///
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/// Note: while this type is unstable, the functionality it provides can be
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/// accessed through the [free functions in `alloc`](index.html#functions).
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#[unstable(feature = "allocator_api", issue = "32838")]
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#[derive(Copy, Clone, Default, Debug)]
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pub struct Global;
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@ -4,6 +4,16 @@
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//! heap allocation in Rust. Boxes provide ownership for this allocation, and
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//! drop their contents when they go out of scope.
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//!
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//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
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//! its allocation. It is valid to convert both ways between a [`Box`] and a
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//! raw pointer allocated with the [`Global`] allocator, given that the
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//! [`Layout`] used with the allocator is correct for the type. More precisely,
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//! a `value: *mut T` that has been allocated with the [`Global`] allocator
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//! with `Layout::for_value(&*value)` may be converted into a box using
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//! `Box::<T>::from_raw(value)`. Conversely, the memory backing a `value: *mut
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//! T` obtained from `Box::<T>::into_raw` may be deallocated using the
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//! [`Global`] allocator with `Layout::for_value(&*value)`.
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//!
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//! # Examples
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//!
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//! Move a value from the stack to the heap by creating a [`Box`]:
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