Allow calling interrupt::free with zero-arity closures. (0.7.x)

[reitermarkus]

0.8 will only accept zero-arity closures, this will make the transition a bit easier by being able to update calls before it is released.

[Dirbaio]

This is unsound, it allows any value for the 'cs lifetime. This code should be rejected but it's not.

    interrupt::free(|cs| {
        let cs: &'static CriticalSection = cs;
    });

[Dirbaio]

The sound way of writiing it would be this:

impl<F, R> InterruptFreeFn<CriticalSection, R> for F
where
    F: for<'cs> FnOnce(&'cs CriticalSection) -> R,
{
    #[inline]
    unsafe fn call(self) -> R {
        self(&CriticalSection::new())
    }
}

However this fails to correctly infer the closure type (playground):

interrupt::free(|cs| {})

Interestingly, these work.

interrupt::free(|cs: &CriticalSection| {})
interrupt::free(|cs: &_| {})

Closure type inference through "wrapper" traits like these is known to be flaky, see this issue for example rust-lang/rust#70263 . I'm not sure if what you're trying to do is possible in today's stable Rust.

[reitermarkus]

This is unsound

Thanks, good catch.

I'm not sure if what you're trying to do is possible in today's stable Rust.

Yeah, seems like it.