trait_solver: Add minimal coroutine binder assumptions - #162159
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Dnreikronos wants to merge 28 commits into
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trait_solver: Add minimal coroutine binder assumptions#162159Dnreikronos wants to merge 28 commits into
Dnreikronos wants to merge 28 commits into
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Keep type-outlives constraints intact while leaving a minimal-mode binder, then remove only leaves proven by that binder. Ordinary binders continue through the normal eager leak check.
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Implements rust-lang/project-assumptions-on-binders#33.
Based on #161306, so the diff is going to look noisy until that lands.
The full binder-assumptions mode does a lot when it leaves a binder, including rewriting placeholder constraints into lower universes. For coroutine witnesses we only need a much smaller bit of that. The main thing I wanted to avoid was making a "minimal" flag that quietly got most of the full behavior anyway.
min_coroutinesonly builds assumptions for coroutine-witness binders. When it leaves one, it drops a constraint if that binder directly proves it and keeps the rest for the root context. Ordinary HRTBs still take the normal eager leak-check path. Type-outlives constraints also stay whole until the root; pulling them apart earlier can make the mode accept more than intended.Fwiw I think this is a nicer first version because the rule is pretty easy to explain: witness binders get assumptions, other binders don't. It gets the async cases we're after without turning on the wider experiment. I added the existing async examples and a couple of focused tests for the edges that worried me, ordinary binders and unsatisfied constraints leaving the binder.