Compile-time filtering tuple or hlist by implemented trait?

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1

This is probably still a bit of a long shot in Rust's developing type system, but is there a way to filter a tuple or hlist of references based on whether those types implement one or ideally multiple traits? I'm wondering if there's some unholy combination of frunk, hlists, facet, and/or feature(min_specialization) (or other relatively stable/safe nightly features) to do this.

Essentially a function where I feed in (&FooImplementsTraits, &BarDoesntImplement, &BazImplementsTraits) and get out (&FooImplementsTraits, &BazImplementsTraits) (or an HList equivalent). Is that possible in Rust's type system at this point?

I know there's this trick that uses the preference for inherent methods, but that wouldn't work at the generic level over tuples/hlists. For use case context, I'm trying to work on improvements to gecs to be less macro-dependent, but I'm stuck on being able to filter archetypes by what component types they hold.

2

I think we're a ways off from that. You need something like negative impls in coherence or exclusive traits in coherence AFAICT.

3

Any nightly features or, at the very least, RFCs to follow that might make this possible? Doesn't have to be pretty if there's a dirty trick to make it work.

4

You can follow the breadcrumbs from here, but there's not much activity or tracking. with_negative_coherence is a related scarcely/non?-documented feature gate. I have no idea how far the feature gate may get you. Sorry that I can only give a starting point which may be a dead end.

It builds on negative impls which at least have seen some more active interest.

5

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