Miri not detecting aliased mutable reference

[Kestrer]

I think that this code contains UB:

fn use_mut(r: &mut i32) {
    *r += 1;
    let _ = &mut *r;
}

fn main() {
    let r = &mut 5;
    let p = r as *mut _;
    use_mut(r);
    use_mut(unsafe { &mut *p });
    use_mut(r);
}

But when running under Miri, no errors are produced.

[RalfJung]

Interesting example, thanks for the report!

This is caused by the fact that the r that is passed to use_mut is a two-phase borrow. Miri treats two-phase borrows like raw pointers -- with the current version of Stacked Borrows, we cannot really do much better. Hopefully a future version of Stacked Borrows can restrict two-phase borrows better.

If you use use_mut(&mut *r) instead of use_mut(r), "real" reborrows happen and you get the expected error.

[RalfJung]

Closing in favor of rust-lang/unsafe-code-guidelines#85.