Rollup merge of rust-lang#140463 - joshlf:patch-13, r=RalfJung

Document MaybeUninit bit validity

Partially addresses rust-lang/unsafe-code-guidelines#555 by clarifying that it is sound to write any byte values (initialized or uninitialized) to any `MaybeUninit<T>` regardless of `T`.

r? `@RalfJung`

Author: Zalathar

library/core/src/mem/maybe_uninit.rs

@@ -253,6 +253,89 @@ use crate::{fmt, intrinsics, ptr, slice};
 ///     std::process::exit(*code); // UB! Accessing uninitialized memory.
 /// }
 /// ```
+///
+/// # Validity
+///
+/// `MaybeUninit<T>` has no validity requirements –- any sequence of [bytes] of
+/// the appropriate length, initialized or uninitialized, are a valid
+/// representation.
+///
+/// Moving or copying a value of type `MaybeUninit<T>` (i.e., performing a
+/// "typed copy") will exactly preserve the contents, including the
+/// [provenance], of all non-padding bytes of type `T` in the value's
+/// representation.
+///
+/// Therefore `MaybeUninit` can be used to perform a round trip of a value from
+/// type `T` to type `MaybeUninit<U>` then back to type `T`, while preserving
+/// the original value, if two conditions are met. One, type `U` must have the
+/// same size as type `T`. Two, for all byte offsets where type `U` has padding,
+/// the corresponding bytes in the representation of the value must be
+/// uninitialized.
+///
+/// For example, due to the fact that the type `[u8; size_of::<T>]` has no
+/// padding, the following is sound for any type `T` and will return the
+/// original value:
+///
+/// ```rust,no_run
+/// # use core::mem::{MaybeUninit, transmute};
+/// # struct T;
+/// fn identity(t: T) -> T {
+///     unsafe {
+///         let u: MaybeUninit<[u8; size_of::<T>()]> = transmute(t);
+///         transmute(u) // OK.
+///     }
+/// }
+/// ```
+///
+/// Note: Copying a value that contains references may implicitly reborrow them
+/// causing the provenance of the returned value to differ from that of the
+/// original. This applies equally to the trivial identity function:
+///
+/// ```rust,no_run
+/// fn trivial_identity<T>(t: T) -> T { t }
+/// ```
+///
+/// Note: Moving or copying a value whose representation has initialized bytes
+/// at byte offsets where the type has padding may lose the value of those
+/// bytes, so while the original value will be preserved, the original
+/// *representation* of that value as bytes may not be. Again, this applies
+/// equally to `trivial_identity`.
+///
+/// Note: Performing this round trip when type `U` has padding at byte offsets
+/// where the representation of the original value has initialized bytes may
+/// produce undefined behavior or a different value. For example, the following
+/// is unsound since `T` requires all bytes to be initialized:
+///
+/// ```rust,no_run
+/// # use core::mem::{MaybeUninit, transmute};
+/// #[repr(C)] struct T([u8; 4]);
+/// #[repr(C)] struct U(u8, u16);
+/// fn unsound_identity(t: T) -> T {
+///     unsafe {
+///         let u: MaybeUninit<U> = transmute(t);
+///         transmute(u) // UB.
+///     }
+/// }
+/// ```
+///
+/// Conversely, the following is sound since `T` allows uninitialized bytes in
+/// the representation of a value, but the round trip may alter the value:
+///
+/// ```rust,no_run
+/// # use core::mem::{MaybeUninit, transmute};
+/// #[repr(C)] struct T(MaybeUninit<[u8; 4]>);
+/// #[repr(C)] struct U(u8, u16);
+/// fn non_identity(t: T) -> T {
+///     unsafe {
+///         // May lose an initialized byte.
+///         let u: MaybeUninit<U> = transmute(t);
+///         transmute(u)
+///     }
+/// }
+/// ```
+///
+/// [bytes]: ../../reference/memory-model.html#bytes
+/// [provenance]: crate::ptr#provenance
 #[stable(feature = "maybe_uninit", since = "1.36.0")]
 // Lang item so we can wrap other types in it. This is useful for coroutines.
 #[lang = "maybe_uninit"]