Plumb inference obligations through librustc/middle and librustc_typeck, take 2

[soltanmm]

Next time I'm just going to ask what @nikomatsakis intends to do for a task; it seems to end up cleaner every time. :-)

I'm unsure whether some places should or should not be propagating obligations (namely locations in infer/mod.rs that return in this PR UnitResult as opposed to InferResult).

r? @nikomatsakis

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

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[jroesch]

I like this approach a lot better as well, looks much cleaner.

[soltanmm]

@jroesch Woo! Let's hope you still think that after seeing e6a9f7e.

@nikomatsakis You probably want to take a look as well at e6a9f7e. It propagates obligations through select.rs and cascades into librustc_trans, a lint, and an absurdist joke.

Up to y'all whether it should be on this PR or if a firewall should be set where the PR currently is.

[bors]

☔ The latest upstream changes (presumably #31979) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

@soltanmm I've been playing with this branch (and also making a few changes here and there). Feeling very good about this direction right now.

[soltanmm]

Closing in favor of #32542...