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

Clarify that ManuallyDrop<T> has same layout as T

This PR implements the documentation change under discussion in rust-lang/unsafe-code-guidelines#302. It should not be approved or merged until the discussion there is resolved.

Author: Dylan-DPC

library/core/src/mem/manually_drop.rs

@@ -4,11 +4,12 @@ use crate::ptr;
 /// 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