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Make must_cast by-value and by-shared-ref functions const (#261)

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* Make must_cast by-value and by-shared-ref functions const (but not by-mut-ref)

* Add comment for transmute! macro new arm, and update Cargo.toml comment for must_cast MSRV.
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zachs18 2024-08-11 07:59:06 +00:00 committed by GitHub
parent 243302d3ef
commit 5eecd33318
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3 changed files with 16 additions and 5 deletions
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2
Cargo.toml
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@ -32,7 +32,7 @@ min_const_generics = [] # MSRV 1.51: support arrays via min_const_generics
wasm_simd = [] # MSRV 1.54.0: support wasm simd types
aarch64_simd = [] # MSRV 1.59.0: support aarch64 simd types
must_cast = [] # MSRV 1.57.0: support the `must` module.
must_cast = [] # MSRV 1.64.0: support the `must` module.
const_zeroed = [] # MSRV 1.75.0: support const `zeroed()`

11
src/lib.rs
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@ -123,6 +123,17 @@ macro_rules! transmute {
($val:expr) => {
::core::mem::transmute_copy(&::core::mem::ManuallyDrop::new($val))
};
// This arm is for use in const contexts, where the borrow required to use transmute_copy poses an issue
// since the compiler hedges that the type being borrowed could have interior mutability.
($srcty:ty; $dstty:ty; $val:expr) => {
{
union Transmute<A, B> {
src: ::core::mem::ManuallyDrop<A>,
dst: ::core::mem::ManuallyDrop<B>,
}
::core::mem::ManuallyDrop::into_inner(Transmute::<$srcty, $dstty> { src: ::core::mem::ManuallyDrop::new($val) }.dst)
}
}
}
/// A macro to implement marker traits for various simd types.

8
src/must.rs
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@ -38,9 +38,9 @@ impl<A, B> Cast<A, B> {
/// let bytes : [u8; 3] = bytemuck::must_cast(12_u16);
/// ```
#[inline]
pub fn must_cast<A: NoUninit, B: AnyBitPattern>(a: A) -> B {
pub const fn must_cast<A: NoUninit, B: AnyBitPattern>(a: A) -> B {
let _ = Cast::<A, B>::ASSERT_SIZE_EQUAL;
unsafe { transmute!(a) }
unsafe { transmute!(A; B; a) }
}
/// Convert `&A` into `&B` if infalliable, or fail to compile.
@ -64,7 +64,7 @@ pub fn must_cast<A: NoUninit, B: AnyBitPattern>(a: A) -> B {
/// let bytes : &u16 = bytemuck::must_cast_ref(&[1u8, 2u8]);
/// ```
#[inline]
pub fn must_cast_ref<A: NoUninit, B: AnyBitPattern>(a: &A) -> &B {
pub const fn must_cast_ref<A: NoUninit, B: AnyBitPattern>(a: &A) -> &B {
let _ = Cast::<A, B>::ASSERT_SIZE_EQUAL;
let _ = Cast::<A, B>::ASSERT_ALIGN_GREATER_THAN_EQUAL;
unsafe { &*(a as *const A as *const B) }
@ -143,7 +143,7 @@ pub fn must_cast_mut<
/// let zsts: &[()] = bytemuck::must_cast_slice(bytes);
/// ```
#[inline]
pub fn must_cast_slice<A: NoUninit, B: AnyBitPattern>(a: &[A]) -> &[B] {
pub const fn must_cast_slice<A: NoUninit, B: AnyBitPattern>(a: &[A]) -> &[B] {
let _ = Cast::<A, B>::ASSERT_SIZE_MULTIPLE_OF_OR_INPUT_ZST;
let _ = Cast::<A, B>::ASSERT_ALIGN_GREATER_THAN_EQUAL;
let new_len = if size_of::<A>() == size_of::<B>() {