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When using std::ptr::copy and similar methods, does the memory need to be valid for T? #330

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rust-lang/rust
#99084
@5225225

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@5225225

Plenty of methods in std::ptr take a generic T and use *mut/const T, but does this mean that the following code is UB?

let src: u8 = 42;
let mut dest: u8 = 10;
std::ptr::copy(&src as *const u8 as *const bool, &mut dest as *mut u8 as *mut bool);

What about copying a u32 as a (u16, u8), or a (u16, u8, !), that doesn't uninit any padding, right?

Same question applies to copy_nonoverlapping, swap(_nonoverlapping), write_bytes. I don't see any other methods that this would apply to.

What about copying a pointer with an integer type, that preserves provenance?


I'd be inclined to say "no, it's not UB, the generic parameter is only used for its size and alignment, no other property of the type matters", but if that's the case, we should make that clear. It definitely seems to be implied.

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