-Zmiri-tag-raw-pointers causes a simple copy_nonoverlapping to fail

[AaronKutch]

I was trying out -Zmiri-strict-provenance on my library https://github.com/AaronKutch/awint and it lead to tests failing. I looked at the other things strict provenance turns on, and the problems start with -Zmiri-tag-raw-pointers. I think I have minimized it to:

fn main() {
    let mut x: [u64; 4] = [0, 1, 2, 3];
    unsafe {
        ptr::copy_nonoverlapping(x.as_ptr(), x.as_mut_ptr().add(2), 2);
    }
    assert_eq!(x, [0, 1, 0, 1]);
}

which fails with

error: Undefined Behavior: attempting a read access using <1670> at alloc776[0x0], but that tag does not exist in the borrow stack for this location
    --> C:\Users\aaron\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\intrinsics.rs:2103:14
     |
2103 |     unsafe { copy_nonoverlapping(src, dst, count) }
     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |              |
     |              attempting a read access using <1670> at alloc776[0x0], but that tag does not exist in the borrow stack for this location
     |              this error occurs as part of an access at alloc776[0x0..0x10]
     |
     = 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

     = note: inside `std::intrinsics::copy_nonoverlapping::<u64>` at C:\Users\aaron\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\intrinsics.rs:2103:14

[RalfJung]

Yeah, when you call x.as_mut_ptr() that creates a mutable reference to x and thus invalidates all previously created pointers, including the raw pointer that was returned by x.as_ptr(). So, this is expected.

See rust-lang/unsafe-code-guidelines#133 for the underlying issue. I am going to close the Miri side of this.

The fix is to not use any references when you do raw ptr shenanigans:

let mut x: [u64; 4] = [0, 1, 2, 3];
let ptr = x.as_mut_ptr();
// Reference-free zone begins here.
ptr::copy_nonoverlapping(ptr, ptr.add(2), 2);

[AaronKutch]

Ok, kind of makes sense