deny transmute &[T] to &[bool]

[KisaragiEffective]

What it does

consider following example:

let single_u64: &[u64] = &[0xDEAD_BEEF_DEAD_BEEF];
let bools: &[bool] = unsafe { std::mem::transmute(single_u64) };
assert_eq!(bools.len(), 64);

but this is not true, because transmute does not change slice length.
this contains UB: Every integral value except 0 or 1 cannot be bool.
this also contains error-prone: bool's alignment is 1-byte, which may be surprizing if user thought bool is 1-bit (formally size_of::<[T; N]>() == size_of::<[bool; N * size_of::<T>]>()).

Lint Name

transmute_slice_to_bool_slice

Category

correctness, suspicious

Advantage

• Removes potential UB.
• Prevents error-prone.

Drawbacks

No response

Example

let single_u64: &[u64] = &[0xDEAD_BEEF_DEAD_BEEF];
let bools: &[bool] = unsafe { std::mem::transmute(single_u64) };

may be written as:

    // under #![feature(strict_provenance)] to help miri
    let bools: Vec<bool> = {
        let element_size_in_bytes = std::mem::size_of::<u64>();
        let original_slice_len = u64.len();
        let mut ret = Vec::with_capacity(original_slice_len * element_size_in_bytes * 8);
        for i in 0..original_slice_len {
            let elem: u64 = u64[i];
            let head: *const u8 = std::ptr::from_exposed_addr((&elem as *const u64).addr());
            let u8_slice: &[u8] = unsafe { std::slice::from_raw_parts(head, element_size_in_bytes) };
            let bits = element_size_in_bytes * 8;
            for bit in 0..bits {
                let is_bit = u8_slice[bit] & (1 << bit % 8) == 1;
                ret[i * element_size_in_bytes * 8 + bit] = is_bit;
            }
        }
        
        ret
    };
    
    // assert_eq!(bools, [true, true, false, true, true, true, true, false, true, false, true, false, true, true, false, true, true, false, true, true, true, true, true, false, true, true, true, false, true, true, true, true, true, true, false, true, true, true, true, false, true, false, true, false, true, true, false, true, true, false, true, true, true, true, true, false, true, true, true, false, true, true, true, true]);

but miri reports this code as an UB.