Namely: labels, type parameters, bindings in patterns, parameter names in functions without body.
All of these do not need hygiene after lowering to HIR, only span locations.

Changing the `each_binding` utility method to take the `HirId` of a
binding pattern rather than its `NodeId` seems like a modest first step
in support of the `HirId`ification initiative #50928. (The inspiration
for choosing this in particular came from the present author's previous
work on diagnostics issued during liveness analysis, which is the most
greatly affected module in this change.)

`Liveness::users` is a vector that is occasionally enormous. For
example, doing a "clean incremental" check build of `inflate`, there is
one instance that represents 5,499 live nodes and 1087 vars, which
requires 5,977,413 entries. At 24 bytes per entry, that is 143MB.

This patch changes LiveNode from a usize to a u32. On 64-bit machines
that halves the size of these entries, significantly reducing peak
memory usage and memory traffic, and speeding up "clean incremental"
builds of `inflate` by about 10%.

Consider this a down payment on #50723. To recap, an `Applicability`
enum was recently (#50204) added, to convey to Rustfix and other tools
whether we think it's OK for them to blindly apply the suggestion, or
whether to prompt a human for guidance (because the suggestion might
contain placeholders that we can't infer, or because we think it has a
sufficiently high probability of being wrong even though it's—
presumably—right often enough to be worth emitting in the first place).

When a suggestion is marked as `MaybeIncorrect`, we try to use comments
to indicate precisely why (although there are a few places where we just
say `// speculative` because the present author's subjective judgement
balked at the idea that the suggestion has no false positives).

The `run-rustfix` directive is opporunistically set on some relevant UI
tests (and a couple tests that were in the `test/ui/suggestions`
directory, even if the suggestions didn't originate in librustc or
libsyntax). This is less trivial than it sounds, because a surprising
number of test files aren't equipped to be tested as fixed even when
they contain successfully fixable errors, because, e.g., there are more,
not-directly-related errors after fixing. Some test files need an
attribute or underscore to avoid unused warnings tripping up the "fixed
code is still producing diagnostics" check despite the fixes being
correct; this is an interesting contrast-to/inconsistency-with the
behavior of UI tests (which secretly pass `-A unused`), a behavior which
we probably ought to resolve one way or the other (filed issue #50926).

A few suggestion labels are reworded (e.g., to avoid phrasing it as a
question, which which is discouraged by the style guidelines listed in
`.span_suggestion`'s doc-comment).

In e4b1a797 (#47922), we corrected erroneous suggestions for unused
shorthand field pattern bindings, suggesting `field: _` where the
previous suggestion of `_field` wouldn't even have compiled
(#47390). Soon, it was revealed that this was insufficient (#50303), and
the fix was extended to references, slices, &c. (#50327) But even this
proved inadequate, as the erroneous suggestions were still being issued
for patterns in local (`let`) bindings (#50804). Here, we yank the
shorthand-detection and variable/node registration code into a new
common function that can be called while visiting both match arms and
`let` bindings.

Resolves #50804.

It's redundant as Result already implements Encodable
as well as Decodable.

When we want to implement label-break-value,
we can't really decide whether to emit ScopeTarget::Loop or
ScopeTarget::Block in the code that is supposed to create it.
So we get rid of it and reconstruct the information when
needed.

This reverts commit a8809711.

Previously, unused variables would get a note that the warning could be
silenced by prefixing the variable with an underscore, but that doesn't
work for field shorthand patterns, which the liveness analysis didn't
know about.

The "to avoid this warning" verbiage seemed unnecessary.

Resolves #47390.

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.

The field init shorthand syntax was stabilized in 1.17.0 (aebd94fd); we
are now free to use it in the compiler.

Make a `CodeExtent<'tcx>` be something allocated in an arena
instead of an index into the `RegionMaps`.

Instead, thread around `Option<CodeExtent>` where applicable.