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?