Unroll factor in `#[unroll]` attributes should accept any const expr, not just literals

[Kmeakin]

Related to the #![feature(loop_hints)] attributes: #156874

It would be nice if the N in #[unroll(N)] could take an arbitrary const expression, not just an integer literal:

fn foo<const UNROLL_FACTOR: usize>() {
    #[unroll(UNROLL_FACTOR)]
    loop {
        ...
    }
}

If that's not possible for technical reasons, another possibility would be a std::hint::unroll function which takes a const parameter and a closure:

fn foo<const UNROLL_FACTOR: usize>() {
    std::hint::unroll::<UNROLL_FACTOR>(|| loop {
        ...
    });
}

This would make it easier to experiment with different unroll factors without having to duplicate the same function for each unroll factor

[Kmeakin]

@rustbot label +F-loop-hints

[saethlin]

Both of these seem quite challenging to implement. Attributes are handled at macro expansion time, so we only have tokens to put into an attribute. I feel like the first suggestion is asking attribute parsing to somehow intersect with name resolution?

The second suggestion seems even harder because now you want the unrolling information would have to be pushed across a function boundary. I suspect the way you've written the code is fooling you into thinking this is simpler than it is, consider this:

let func = || loop { ... };
std::hint::unroll::<UNROLL_FACTOR>(func);
std::hint::unroll::<UNROLL_FACTOR / 2>(func);

Which makes it impossible to correctly lower func because we need to put a single value on its loop metadata node. And what if there are multiple loops inside the closure? Of course we could just make arbitrary choices in these cases or try to issue a diagnostic but my point is that it is just so much simpler to have an attribute that demands it be put on a specific expression.