Commits on Aug 9, 2017

1. rustc: Rearchitect lints to be emitted more eagerly …

   In preparation for incremental compilation this commit refactors the lint
   handling infrastructure in the compiler to be more "eager" and overall more
   incremental-friendly. Many passes of the compiler can emit lints at various
   points but before this commit all lints were buffered in a table to be emitted
   at the very end of compilation. This commit changes these lints to be emitted
   immediately during compilation using pre-calculated lint level-related data
   structures.
   
   Linting today is split into two phases, one set of "early" lints run on the
   `syntax::ast` and a "late" set of lints run on the HIR. This commit moves the
   "early" lints to running as late as possible in compilation, just before HIR
   lowering. This notably means that we're catching resolve-related lints just
   before HIR lowering. The early linting remains a pass very similar to how it was
   before, maintaining context of the current lint level as it walks the tree.
   
   Post-HIR, however, linting is structured as a method on the `TyCtxt` which
   transitively executes a query to calculate lint levels. Each request to lint on
   a `TyCtxt` will query the entire crate's 'lint level data structure' and then go
   from there about whether the lint should be emitted or not.
   
   The query depends on the entire HIR crate but should be very quick to calculate
   (just a quick walk of the HIR) and the red-green system should notice that the
   lint level data structure rarely changes, and should hopefully preserve
   incrementality.
   
   Overall this resulted in a pretty big change to the test suite now that lints
   are emitted much earlier in compilation (on-demand vs only at the end). This in
   turn necessitated the addition of many `#![allow(warnings)]` directives
   throughout the compile-fail test suite and a number of updates to the UI test
   suite.

   

   alexcrichton committed Aug 9, 2017
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