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Rollup merge of #88375 - joshlf:patch-3, r=dtolnay

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Clarify that ManuallyDrop<T> has same layout as T

This PR implements the documentation change under discussion in https://github.com/rust-lang/unsafe-code-guidelines/issues/302. It should not be approved or merged until the discussion there is resolved.
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Dylan DPC 2022-03-28 04:12:09 +02:00 committed by GitHub
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11
library/core/src/mem/manually_drop.rs
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@ -4,11 +4,12 @@
/// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
/// This wrapper is 0-cost.
///
/// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`.
/// As a consequence, it has *no effect* on the assumptions that the compiler makes
/// about its contents. For example, initializing a `ManuallyDrop<&mut T>`
/// with [`mem::zeroed`] is undefined behavior.
/// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead.
/// `ManuallyDrop<T>` is guaranteed to have the same layout as `T`, and is subject
/// to the same layout optimizations as `T`. As a consequence, it has *no effect*
/// on the assumptions that the compiler makes about its contents. For example,
/// initializing a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined
/// behavior. If you need to handle uninitialized data, use [`MaybeUninit<T>`]
/// instead.
///
/// Note that accessing the value inside a `ManuallyDrop<T>` is safe.
/// This means that a `ManuallyDrop<T>` whose content has been dropped must not