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Make std::mem::transmute_copy accept ?Sized inputs #112457

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Make std::mem::transmute_copy accept ?Sized inputs #112457

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f832add
Make `std::mem::transmute_copy` accept `?Sized` inputs
nvzqz Jun 9, 2023
c9575a6
Add `std::mem::transmute_copy` slice test
nvzqz Jun 9, 2023
3f1825a
Change `std::mem::transmute_copy` to use `debug_assert!`
nvzqz Jun 9, 2023
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6 changes: 3 additions & 3 deletions library/core/src/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1052,14 +1052,14 @@ pub const fn copy<T: Copy>(x: &T) -> T {
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_transmute_copy", issue = "83165")]
pub const unsafe fn transmute_copy<Src, Dst>(src: &Src) -> Dst {
pub const unsafe fn transmute_copy<Src: ?Sized, Dst>(src: &Src) -> Dst {
assert!(
size_of::<Src>() >= size_of::<Dst>(),
size_of_val(src) >= size_of::<Dst>(),
"cannot transmute_copy if Dst is larger than Src"
);
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@workingjubilee workingjubilee Jun 9, 2023 •
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This is no longer guaranteed to be compiled out in --release. I do not believe we want this assert in this case.

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@nvzqz nvzqz Jun 9, 2023 •
edited
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Changed this to debug_assert! in 3f1825a

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Why is this not guaranteed to be compiled out?

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@tbu- because size_of_val for dynamically-sized types requires a runtime check since the type's size is not known at compile-time

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Changing this to debug_assert! means this check no longer triggers for 99% of users who do not work with a debug build of the stdlib.


// If Dst has a higher alignment requirement, src might not be suitably aligned.
if align_of::<Dst>() > align_of::<Src>() {
if align_of::<Dst>() > align_of_val(src) {
// SAFETY: `src` is a reference which is guaranteed to be valid for reads.
// The caller must guarantee that the actual transmutation is safe.
unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }
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5 changes: 5 additions & 0 deletions library/core/tests/mem.rs
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Expand Up @@ -121,6 +121,11 @@ fn test_transmute_copy_shrink() {
assert_eq!(0_u8, unsafe { transmute_copy(&0_u64) });
}

#[test]
fn test_transmute_copy_slice() {
assert_eq!(1, unsafe { transmute_copy(&[1][..]) });
}

#[test]
fn test_transmute_copy_unaligned() {
#[repr(C)]
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