capture diagnostic output in queries

[nikomatsakis]

Right now, queries produce results, but as a side-effect they may issue diagnostics. For incremental compilation, if we are going to (e.g.) skip typeck, but still produce the same output, we really need to capture this output so that we can replay it.

Right now, to issue a diagnostic, you basically directly invoke methods on the session, such as span_err and friends. When emit() is called, these just directly dump out to stderr.

To fix this issue, we would want to record those diagnostics. There is already a diagnostics object that is created (and thus can in principle be saved) -- but we'd have to find the right place to save it (in the query tables, presumably).

We also have to figure out how to recover the tcx. The easiest way to handle this would be to check the TLS for the current tcx (using ty::tls::with()) and record it that way. We generally discourage using TLS to get the tcx except in specific scenarios, but this may fall under that category.

I'm tagging this for mentoring without instructions. There is still some amount of pending design work here that will require experimentation. Plus, once the work is done, we'll have to figure out (separately) how to actually save the errors to disk and replay them etc.

[nikomatsakis]

cc @eddyb @michaelwoerister

[eddyb]

We also have to figure out how to recover the tcx.

No need! Everything already passes tcx.sess, just make it work on tcx instead.
(If that is too cumbersome, Session could also do the tracking for the query engine)

[nikomatsakis]

@eddyb well, we issue diagnostics directly on the session sometimes, when the tcx is not available yet, right? I was nervous about that. At minimum, we'd have to move the methods to some new place, so that we can be sure that all existing code uses the tcx methods when it should.

[eddyb]

I am aware of nothing that has &Session but no TyCtxt when the former came from the latter.

[nikomatsakis]

@eddyb the point is that if you have the methods available on sess, then people might (by accident) do tcx.sess.span_err(), so we don't want that to "skip past" the recording of side-effects.

[eddyb]

Then we can have the query engine mess with the Session and manually save/restore diagnostic records.

[nikomatsakis]

@eddyb yes that would be fine