Span's From<&[T]> and From<&[T; N]> facilitate UB

[SFBdragon]

Dereferencing &x as *const _ as *mut _ is UB.

This means some of the API surface of Talc is genuinely problematic, particularly the From implementations on Span for &[T] and &[T; N]. Not UB in itself (and technically Talc's docs on unsafety cover this), but it makes UB needlessly easy to invoke.

Here, the .into() is doing the legwork in hiding the issue as per the above. MIRI indeed takes issue with it.

fn main() {
    let mut arena = [0u8; 10000];

    unsafe {
        let mut talc = talc::Talc::new(talc::ClaimOnOom::new(arena.as_ref().into()));
        let x = talc.malloc(std::alloc::Layout::new::<[u64; 3]>()).unwrap();
        *x.as_ptr().cast::<[u64; 3]>() = [1, 2, 3];
    }
}

These From implementations should be removed. This is a breaking change though and will necessitate a major version bump.