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Rollup merge of #77836 - RalfJung:transmute_copy, r=Mark-Simulacrum

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transmute_copy: explain that alignment is handled correctly

The doc comment currently is somewhat misleading because if it actually transmuted `&T` to `&U`, a higher-aligned `U` would be problematic.
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Dylan DPC 2020-10-26 03:08:58 +01:00 committed by GitHub
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library/core/src/mem/mod.rs
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@ -884,10 +884,10 @@ 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`.
/// 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` (except that this is done in a way
/// that is correct even when `&U` makes stricter alignment requirements than `&T`). 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