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https://github.com/rust-lang/rust
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31d23c436a
Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0. The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same. This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not. This code is guarded by `transmute_const_generics` (#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard. Initially reported in #98104.
35 lines
918 B
Rust
35 lines
918 B
Rust
//@ run-pass
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// Transmuting to/from ZSTs that contain generics.
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#![feature(transmute_generic_consts)]
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// Verify non-generic ZST -> generic ZST transmute
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unsafe fn cast_zst0<T>(from: ()) -> [T; 0] {
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::std::mem::transmute::<(), [T; 0]>(from)
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}
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// Verify generic ZST -> non-generic ZST transmute
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unsafe fn cast_zst1<T>(from: [T; 0]) -> () {
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::std::mem::transmute::<[T; 0], ()>(from)
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}
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// Verify transmute with generic compound types
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unsafe fn cast_zst2<T>(from: ()) -> [(T, T); 0] {
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::std::mem::transmute::<(), [(T, T); 0]>(from)
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}
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// Verify transmute with ZST propagation through arrays
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unsafe fn cast_zst3<T>(from: ()) -> [[T; 0]; 8] {
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::std::mem::transmute::<(), [[T; 0]; 8]>(from)
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}
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pub fn main() {
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unsafe {
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let _: [u32; 0] = cast_zst0(());
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let _ = cast_zst1::<u32>([]);
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let _: [(u32, u32); 0] = cast_zst2(());
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let _: [[u32; 0]; 8] = cast_zst3(());
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};
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}
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