Confusing [cfg]s #689

#[cfg(not(target_has_atomic = "ptr"))]
impl AtomicUsize {
    const fn new(v: usize) -> AtomicUsize {
        AtomicUsize { v: Cell::new(v) }
    }

    fn load(&self, _order: Ordering) -> usize {
        self.v.get()
    }

    fn store(&self, val: usize, _order: Ordering) {
        self.v.set(val)
    }

    #[cfg(target_has_atomic = "ptr")]
    fn compare_exchange(
        &self,
        current: usize,
        new: usize,
        _success: Ordering,
        _failure: Ordering,
    ) -> Result<usize, usize> {
        let prev = self.v.get();
        if current == prev {
            self.v.set(new);
        }
        Ok(prev)
    }
}