Re: [PATCH v4 1/3] rust: Add `OnceLite` for executing code once

[jens.korinth@tuta.io]

> What is the use-case for this function?

`DO_ONCE_LITE` has very few uses in C; frankly, the only other use I could think
of was initialization. But since `OnceLite` cannot block or guarantee visibility
of the side-effects of the `call_once` expression in other threads, it can't be 
used for this case, either. _Unless_ there is some mechanism to wait
voluntarily when this is required, hence `is_completed`. (And it also exists in
`std::sync::Once`.)

`DO_ONCE_LITE_IF` has more uses in C, but this is a bit harder to do well with
`OnceLite`: A `do_once_lite_if` Rust macro can't short-circuit the condition to
avoid the evaluation if the atomic load already shows that it has been done / is
being done rn. Maybe a
`pub fn call_once<C: FnOnce() -> bool, F: FnOnce()>(cond: C, f: F)` could be
used to simulate the effect. Thoughts?

> Why not just have one atomic?

Do you also have an `AtomicU32` state var in mind, as Daniel suggested?

Jens