we need something like const_eval_select for async-ness and probably for most other kinds of keyword generic

[programmerjake]

Motivation

Inspired by this Zulip thread

Some code needs to do completely different things for async vs. non-async versions (e.g. handling timeouts, calling OS functions, etc.), so we need some way of making those functions easy to use from callers that are generic over async-ness.

Similar reasoning applies for most other keyword-generics.

Details

I think we need something like const_eval_select for async-ness and probably for most other keyword-generic kinds.

maybe the API for async-ness could be like:

pub ~async fn asyncness_select<I, R, F: IntoFuture<Output = R>, A: FnOnce(I) -> F, S: FnOnce(I) -> R>(async_fn: A, sync_fn: S, input: I) -> R;

[yoshuawuyts]

This seems like a duplicate of #6. Closing in favor of that issue.