Run rust-analyzer on the rustc test suit

[HKalbasi]

Rust-analyzer supports several diagnostics, running it against the rustc ui tests and checking the equivalency of them (we don't have equal error messages and spans, but even checking the count of errors will probably work) can help a lot in discovering rare edge case bugs in the analysis and diagnostics logic I think.

We can start having them in metrics (as it will probably have many fails at the start) and later moving them into the CI if there were near zero problems and it was fast to check.

[Veykril]

Tbh, I don't think that's worth it currently. We could do it for a select subset of diagnostics (ui tests), but given the general lack of diagnostics r-a has in regards to typeck and name resolution I don't think we gain a lot from this as it stands.

... and later moving them into the CI if there were near zero problems and it was fast to check.

That's never gonna be fast, in fact I would not be surprised if this doubled CI time in general. There are a lot of ui tests in rustc and afaik all of them depend on std (this might be wrong)?

[flodiebold]

We could at least already check that e.g. we have no error-level diagnostics on run-pass tests, couldn't we? I think it'd be useful even just to start having the infrastructure and have more code to smoke-test new diagnostics on. I agree it's probably not worth doing in CI yet.
As for performance, one option might be to do it all in one RA instance and just update the main program text, so we wouldn't have to reanalyze std for each one?