False positives from invalid_reference_casting

[khuey]

use std::ptr;

fn apply_mask(array: &mut [u8], offset: usize, mask: u64) {
    assert!(offset + 8 <= array.len());
    let a1 = &mut array[offset];
    let a2 = a1 as *mut u8;
    let a3 = a2 as *mut u64;
    unsafe {
        ptr::write_unaligned(a3, ptr::read_unaligned(a3) | mask);
    };
}

Starting with rustc 1.78.0 (because of #118983) this produces the following

error: casting references to a bigger memory layout than the backing allocation is undefined behavior, even if the reference is unused
 --> src/lib.rs:9:9
  |
5 |     let a1 = &mut array[offset];
  |                   ------------- backing allocation comes from here
6 |     let a2 = a1 as *mut u8;
7 |     let a3 = a2 as *mut u64;
  |              -------------- casting happend here
8 |     unsafe {
9 |         ptr::write_unaligned(a3, ptr::read_unaligned(a3) | mask);
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: casting from `u8` (1 bytes) to `u64` (8 bytes)
  = note: `#[deny(invalid_reference_casting)]` on by default

Perhaps I'm missing something but this is not undefined behavior as far as I can tell. The lint is wrong about the backing allocation and I suspect the false warning is downstream from that?

[asquared31415]

Under the memory model checked by Miri by default, a &mut u8 is not permitted to access more than the one byte it points to, even if you got it from a larger object. Reborrowing to a reference type shrinks the allowed access range. Use array[offset..(offset + 8)].as_mut_ptr() instead to obtain a pointer that is valid to write to the correct region. array[offset..].as_mut_ptr() would also be valid, as that region contains the correct bytes, but it's less precise about its intent.

@rustbot label -regression-from-stable-to-stable -C-bug -I-prioritize +C-discussion

[asquared31415]

Miri also agrees that the code as written contains undefined behavior, in the top right of the page select "Tools > Miri" to run and you'll see that it says that there's undefined behavior.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e2d5ba429bbd2fb8401599c04d046213

The important bit from the error message is that the code is trying to access alloc1270[0x0..0x8] (the intended 8 bytes), but the pointer does not have permission for that range, only [0x0..0x1] (the first byte).

[asquared31415]

I think that the invalid_reference_casting lint's wording should be changed though, "backing allocation" is not precise and does not correctly describe what it's detecting. cc #118983

@rustbot label +A-diagnostics +D-papercut