Surprising impact of unused &mut

[jrmuizel]

The following code fails with miri

use std::ptr::NonNull;
use std::cell::Cell;

struct F {
x: Cell<u32>,
       y: u32
}

impl F {
    fn new() -> F {
        F { x: Cell::new(5), y: 9 }
    }
}

#[test]
fn it_works() {
    let mut b = Box::new(F::new());
    let p: NonNull<F> = (&*b).into();

    fn f(rr: &F, mut p: NonNull<F>) {
        let r = unsafe { p.as_ref() };
        r.x.set(9);
    }

    {
        let m = &mut *b;
    }
    f(&b, p);
    b.y = 5;
}

error[E0080]: Miri evaluation error: not granting access to tag <untagged> because incompatible item is protected: [SharedReadWrite for <79372> (call 36446)]
   --> /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/cell.rs:430:31
    |
430 |         mem::replace(unsafe { &mut *self.value.get() }, val)
    |                               ^^^^^^^^^^^^^^^^^^^^^^ Miri evaluation error: not granting access to tag <untagged> because incompatible item is protected: [SharedReadWrite for <79372> (call 36446)]
    |
    = note: inside call to `std::cell::Cell::<u32>::replace` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/cell.rs:387:19
note: inside call to `std::cell::Cell::<u32>::set` at src/lib.rs:23:9
   --> src/lib.rs:23:9
    |
23  |         r.x.set(9);
    |         ^^^^^^^^^^
note: inside call to `it_works::f` at src/lib.rs:29:5
   --> src/lib.rs:29:5
    |
29  |     f(&b, p);
    |     ^^^^^^^^
note: inside call to `it_works` at src/lib.rs:17:1
   --> src/lib.rs:17:1
    |
17  | / fn it_works() {
18  | |     let mut b = Box::new(F::new());
19  | |     let p: NonNull<F> = (&*b).into();
20  | |
...   |
30  | |     b.y = 5;
31  | | }
    | |_^

Commenting out let m = &mut *b; causes miri to succeed. I found this surprising as I wouldn't have expected let m = &mut *b; to have an effect.

[RalfJung]

At first glance, this seems like a duplicate of rust-lang/unsafe-code-guidelines#133 -- could you verify?

[jrmuizel]

That seems possible. I'll do some experimentation.

[RalfJung]

Closing as duplicate for now, feel free to reopen if you disagree.

[jrmuizel]

Is there a way to get miri to give a backtrace for the let m = &mut *b; operation?

[RalfJung]

@saethlin is our 'making backtraces nicer' wizard so they might have an idea

[saethlin]

Yes, but also no. I spent a few hours hacking on this and I can't figure out how to stitch together enough state to figure out what the tag of that &mut is so that I can locate it and print its creation span. This is the best thing I've come up with (on top of my draft diagnostics in #2030):

test it_works ... error: Undefined Behavior: not granting access to tag <untagged> because incompatible item is protected: [SharedReadWrite for <157304> (call 47369)]
   --> /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/cell.rs:404:31
    |
404 |         mem::replace(unsafe { &mut *self.value.get() }, val)
    |                               ^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <untagged> because incompatible item is protected: [SharedReadWrite for <157304> (call 47369)]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: tag was most recently created at offsets [0x0..0x4]
   --> src/main.rs:22:9
    |
22  |         r.x.set(9);
    |         ^^^^^^^^^^
help: this tag was also created here at offsets [0x0..0x8]
   --> src/main.rs:18:25
    |
18  |     let p: NonNull<F> = (&*b).into();
    |                         ^^^^^^^^^^^^
help: <157290> was protected due to a tag which was created here
   --> src/main.rs:28:7
    |
28  |     f(&b, p);
    |       ^^
help: this protector is live for this call
   --> src/main.rs:20:5
    |
20  | /     fn f(rr: &F, mut p: NonNull<F>) {
21  | |         let r = unsafe { p.as_ref() };
22  | |         r.x.set(9);
23  | |     }
    | |_____^