[PATCH 3/5] rust: sync: support using bool with READ_ONCE

[Alice Ryhl]

On Tue, Jan 06, 2026 at 01:43:26PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 31, 2025 at 12:22:27PM +0000, Alice Ryhl wrote:
> > +/// Read an integer as a boolean once.
> > +///
> > +/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`.
> > +///
> > +/// # Safety
> > +///
> > +/// It must be safe to `READ_ONCE` the `ptr` with type `u8`.
> > +#[inline(always)]
> > +#[track_caller]
> > +unsafe fn read_once_bool(ptr: *const bool) -> bool {
> > +    // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside
> > +    // of `read_once_1`.
> > +    //
> > +    // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`.
> > +    let byte = unsafe { read_once_1(ptr.cast::<u8>()) };
> > +    byte != 0u8
> > +}
> 
> Does this hardcode that sizeof(_Bool) == 1? There are ABIs where this is
> not the case.

Hm, it hardcodes that the Rust type called bool is sizeof(_) == 1.
Presumably bindgen will not translate _Bool to bool when it appears in C
types on such platforms. But I don't really know - I have not looked
into this case.

Alice