Remove LintBuffer from Session

[Mark-Simulacrum]

This moves the LintBuffer from Session into the Resolver, where it is used until lowering is done and then consumed by early lint passes. This also happily removes the failure mode of buffering lints too late where it would have previously lead to ICEs; it is statically no longer possible to do so.

I suspect that with a bit more work a similar move could be done for the lint buffer inside ParseSess, but this PR doesn't touch it (in part to keep itself small).

The last commit is the "interesting" commit -- the ones before it don't work (though they compile) as they sort of prepare the various crates for the lint buffer to be passed in rather than accessed through Session.

[Mark-Simulacrum]

I plan to prepare some similar documentation to what I'm working on now in rust-lang/rustc-dev-guide#476 once this lands -- we don't have a lot that talks about lint buffering today, but I've gained a decent understanding of why we need it in working on this so can write that up.

[nikomatsakis]

Looks good to me! I left one small suggestion to add a helper method. r=me with that suggestion.

Fixed.

[Mark-Simulacrum]

@bors r=nikomatsakis

[bors]

📌 Commit a44126a has been approved by nikomatsakis

[Centril]

Possibly caused failure in #65854 (comment), @bors rollup=never

[bors]

☔ The latest upstream changes (presumably #65869) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

@bors r=nikomatsakis rollup=never

I can't reproduce the rollup failure locally so I suspect/hope it is #65830 that is responsible (that seems more likely than this PR, personally, given the small number of errors, and being limited to dead code, though that PR doesn't look immediately at fault either.

[bors]

📌 Commit bdcaedf1572a99a67919f686dd78c307918d9c48 has been approved by nikomatsakis

[bors]

⌛ Testing commit bdcaedf1572a99a67919f686dd78c307918d9c48 with merge d9d421ee53ed6e62dc900005b3db019ccbb4606d...

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-11-04T02:29:54.6616073Z Checking rustc_resolve v0.0.0 (/checkout/src/librustc_resolve)
2019-11-04T02:29:55.9373034Z error[E0599]: no method named `buffer_lint` found for type `&'b rustc::session::Session` in the current scope
2019-11-04T02:29:55.9376629Z --> src/librustc_resolve/resolve_imports.rs:982:36
2019-11-04T02:29:55.9378510Z |
2019-11-04T02:29:55.9379015Z 982 | self.r.session.buffer_lint(UNUSED_IMPORTS, directive.id, directive.span, msg);
2019-11-04T02:29:55.9379651Z | ^^^^^^^^^^^ method not found in `&'b rustc::session::Session`
2019-11-04T02:29:56.0025618Z error: aborting due to previous error
2019-11-04T02:29:56.0026642Z 2019-11-04T02:29:56.0035113Z For more information about this error, try `rustc --explain E0599`.
2019-11-04T02:29:56.0214828Z error: could not compile `rustc_resolve`.
---
2019-11-04T02:30:07.1722273Z local time: Mon Nov 4 02:30:07 UTC 2019
2019-11-04T02:30:07.3091555Z network time: Mon, 04 Nov 2019 02:30:07 GMT
2019-11-04T02:30:07.3094644Z == end clock drift check ==
2019-11-04T02:30:08.5654743Z 2019-11-04T02:30:08.5751144Z ##[error]Bash exited with code '1'.
2019-11-04T02:30:08.5781169Z ##[section]Starting: Checkout
2019-11-04T02:30:08.5783032Z ==============================================================================
2019-11-04T02:30:08.5783131Z Task : Get sources
2019-11-04T02:30:08.5783219Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[bors]

💔 Test failed - checks-azure

[Mark-Simulacrum]

@bors r=nikomatsakis

[bors]

📌 Commit c68df7c has been approved by nikomatsakis

[bors]

⌛ Testing commit c68df7c with merge ab6e478...

[bors]

☀️ Test successful - checks-azure
Approved by: nikomatsakis
Pushing ab6e478 to master...