This commit is just covering the feature gate itself and the tests
that made direct use of `!` and thus need to opt back into the
feature.
A follow on commit brings back the other change that motivates the
revert: Namely, going back to the old rules for falling back to `()`.
Replace feature(never_type) with feature(exhaustive_patterns).
feature(exhaustive_patterns) only covers the pattern-exhaustives checks
that used to be covered by feature(never_type)
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.
