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Change lint message to be stronger for &T -> &mut T transmute by 5225...

 2 years ago
source link: https://github.com/rust-lang/rust/pull/92704
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5225225 commented 20 days ago •

edited

The old message implied that it's only UB if you use the reference to mutate, which (as far as I know) is not true. As in, the following program has UB, and a &T -> &mut T transmute is effectively an unreachable_unchecked.

fn main() {
    #[allow(mutable_transmutes)]
    unsafe {
        let _ = std::mem::transmute::<&i32, &mut i32>(&0);
    }
}

In the future, it might be a good idea to use the edition system to make this a hard error, since I don't think it is ever defined behaviour? Unless we rule that &UnsafeCell<i32> -> &mut i32 is fine. (That, and you always could just use .get(), so you're not losing anything)


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