Fails to detect use after drop (after transmute)

[gmorenz]

I was surprised when miri failed to catch this, I'm not sure if it might just be considered out of scope for now?

use std::mem::transmute;

pub struct Test {
    b: bool,
}

impl Drop for Test {
    fn drop(&mut self) {
        self.b = false;
        println!("Dropping");
    }
}

fn main() {
    let test = Test{  b: true };
    let r = unsafe{ transmute::<&'_ bool, &'_ bool>(&test.b) };
    drop(test);
    println!("{:?}", *r);
}

[RalfJung]

There's no use-after-free here since there is no free as part of the drop.

There is a use-after-drop, but I think the game is still open whether that is UB in itself or not. (Of course it is UB for Box, but that will already be detected.)

We had a related issue somewhere that I currently cannot find... about marking memory as uninitialized on move.

[RalfJung]

Ah, here is the related issue: rust-lang/unsafe-code-guidelines#188

So, given also that https://doc.rust-lang.org/reference/behavior-considered-undefined.html does not list "use-after-drop" or "use-after-move" as a form of UB in Rust, I think currently there is no bug in Miri here, so I am inclined to close this.

[gmorenz]

rust-lang/unsafe-code-guidelines#188 does seem exactly on point... so your inclination seems right to me.