Rollup merge of #110706 - scottmcm:transmute_unchecked, r=oli-obk
Add `intrinsics::transmute_unchecked` This takes a whole 3 lines in `compiler/` since it lowers to `CastKind::Transmute` in MIR *exactly* the same as the existing `intrinsics::transmute` does, it just doesn't have the fancy checking in `hir_typeck`. Added to enable experimenting with the request in <https://github.com/rust-lang/rust/pull/106281#issuecomment-1496648190> and because the portable-simd folks might be interested for dependently-sized array-vector conversions. It also simplifies a couple places in `core`. See also https://github.com/rust-lang/rust/pull/108442#issuecomment-1474777273, where `CastKind::Transmute` was added having exactly these semantics before the lang meeting (which I wasn't in) independently expressed interest.
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commit
3ecae2932c
7 changed files with 43 additions and 54 deletions
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@ -3,8 +3,9 @@
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use crate::num::NonZeroUsize;
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use crate::{
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fmt,
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intrinsics::transmute_unchecked,
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iter::{self, ExactSizeIterator, FusedIterator, TrustedLen},
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mem::{self, MaybeUninit},
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mem::MaybeUninit,
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ops::{IndexRange, Range},
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ptr,
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};
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@ -63,18 +64,11 @@ impl<T, const N: usize> IntoIterator for [T; N] {
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// an array of `T`.
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//
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// With that, this initialization satisfies the invariants.
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// FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
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// works with const generics:
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// `mem::transmute::<[T; N], [MaybeUninit<T>; N]>(array)`
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//
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// Until then, we can use `mem::transmute_copy` to create a bitwise copy
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// as a different type, then forget `array` so that it is not dropped.
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unsafe {
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let iter = IntoIter { data: mem::transmute_copy(&self), alive: IndexRange::zero_to(N) };
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mem::forget(self);
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iter
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}
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// FIXME: If normal `transmute` ever gets smart enough to allow this
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// directly, use it instead of `transmute_unchecked`.
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let data: [MaybeUninit<T>; N] = unsafe { transmute_unchecked(self) };
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IntoIter { data, alive: IndexRange::zero_to(N) }
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}
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}
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@ -1376,6 +1376,20 @@ extern "rust-intrinsic" {
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#[rustc_nounwind]
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pub fn transmute<Src, Dst>(src: Src) -> Dst;
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/// Like [`transmute`], but even less checked at compile-time: rather than
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/// giving an error for `size_of::<Src>() != size_of::<Dst>()`, it's
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/// **Undefined Behaviour** at runtime.
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///
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/// Prefer normal `transmute` where possible, for the extra checking, since
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/// both do exactly the same thing at runtime, if they both compile.
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///
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/// This is not expected to ever be exposed directly to users, rather it
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/// may eventually be exposed through some more-constrained API.
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#[cfg(not(bootstrap))]
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#[rustc_const_stable(feature = "const_transmute", since = "1.56.0")]
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#[rustc_nounwind]
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pub fn transmute_unchecked<Src, Dst>(src: Src) -> Dst;
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/// Returns `true` if the actual type given as `T` requires drop
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/// glue; returns `false` if the actual type provided for `T`
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/// implements `Copy`.
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@ -2798,3 +2812,11 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
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write_bytes(dst, val, count)
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}
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}
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/// Polyfill for bootstrap
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#[cfg(bootstrap)]
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pub const unsafe fn transmute_unchecked<Src, Dst>(src: Src) -> Dst {
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use crate::mem::*;
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// SAFETY: It's a transmute -- the caller promised it's fine.
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unsafe { transmute_copy(&ManuallyDrop::new(src)) }
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}
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@ -945,14 +945,10 @@ impl<T> MaybeUninit<T> {
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// * `MaybeUninit<T>` and T are guaranteed to have the same layout
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// * `MaybeUninit` does not drop, so there are no double-frees
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// And thus the conversion is safe
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let ret = unsafe {
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unsafe {
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intrinsics::assert_inhabited::<[T; N]>();
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(&array as *const _ as *const [T; N]).read()
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};
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// FIXME: required to avoid `~const Destruct` bound
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super::forget(array);
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ret
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intrinsics::transmute_unchecked(array)
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}
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}
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/// Assuming all the elements are initialized, get a slice to them.
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