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Add an explanation for transmute_unchecked

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Maybe Waffle 2023-11-07 20:03:07 +00:00
parent b3d50255d9
commit eca9a1533f
1 changed files with 9 additions and 0 deletions
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9
compiler/rustc_middle/src/query/erase.rs
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@ -30,6 +30,13 @@ pub fn erase<T: EraseType>(src: T) -> Erase<T> {
}; };
Erased::<<T as EraseType>::Result> { Erased::<<T as EraseType>::Result> {
// `transmute_unchecked` is needed here because it does not have `transmute`'s size check
// (and thus allows to transmute between `T` and `MaybeUninit<T::Result>`) (we do the size
// check ourselves in the `const` block above).
//
// `transmute_copy` is also commonly used for this (and it would work here since
// `EraseType: Copy`), but `transmute_unchecked` better explains the intent.
//
// SAFETY: It is safe to transmute to MaybeUninit for types with the same sizes. // SAFETY: It is safe to transmute to MaybeUninit for types with the same sizes.
data: unsafe { transmute_unchecked::<T, MaybeUninit<T::Result>>(src) }, data: unsafe { transmute_unchecked::<T, MaybeUninit<T::Result>>(src) },
} }
@ -39,6 +46,8 @@ pub fn erase<T: EraseType>(src: T) -> Erase<T> {
#[inline(always)] #[inline(always)]
pub fn restore<T: EraseType>(value: Erase<T>) -> T { pub fn restore<T: EraseType>(value: Erase<T>) -> T {
let value: Erased<<T as EraseType>::Result> = value; let value: Erased<<T as EraseType>::Result> = value;
// See comment in `erase` for why we use `transmute_unchecked`.
//
// SAFETY: Due to the use of impl Trait in `Erase` the only way to safely create an instance // SAFETY: Due to the use of impl Trait in `Erase` the only way to safely create an instance
// of `Erase` is to call `erase`, so we know that `value.data` is a valid instance of `T` of // of `Erase` is to call `erase`, so we know that `value.data` is a valid instance of `T` of
// the right size. // the right size.