nodes in statement position.

Extended #[cfg] folder to allow removal of statements, and
of expressions in optional positions like expression lists and trailing
block expressions.

Extended lint checker to recognize lint levels on expressions and
locals.

rather being stored inline. Refactor (and rename) the visitor so that
(by default) it only visits the interior content of an item not nested
items.

This is a [breaking-change] for anyone who uses the HIR visitor. Besides
changing `visit::` to `intravisit::`, you need to refactor your visitor
in one of two ways, depending on what it requires:

1. If you just want to visit all items (most common), you should call
   `krate.visit_all_items(&mut visitor)`.

2. If you need to visit nested items in the middle of the parent items,
   you should override `visit_nested_item` with something like:
   `self.visit_item(self.tcx.map.expect_item(item.id))`, presuming you
   have access to a tcx (or at least a HIR map).

[breaking change]

removing a bunch of asserts

And use VariantData instead of P<VariantData> in Item_ and Variant_

PathSegment, PatIdent, ExprWhile, ExprLoop, ExprBreak and ExprAgain - they need Idents for resolve

Followup on #28440

This could be a [breaking-change] if your lint or syntax extension (is that even possible?) uses HIR attributes or literals.

Reapplied the changes from https://github.com/freebroccolo/rust/commit/dc64b731d7f66c2b43d5e5e8c721be7bd3b59540
to a clean branch of master

`LocalSource` indicated wether a let binding originated from for-loop desugaring to enable specialized error messages, but for-loop expansion has changed and this is now achieved through `MatchSource::ForLoopDesugar`.

- add feature gate
- add basic tests
- adjust parser to eliminate conflict between `const fn` and associated
constants
- allow `const fn` in traits/trait-impls, but forbid later in type check
- correct some merge conflicts

This changes the `ToTokens` implementations for expressions, statements,
etc. with almost-trivial ones that produce `Interpolated(*Nt(...))`
pseudo-tokens. In this way, quasiquote now works the same way as macros
do: already-parsed AST fragments are used as-is, not reparsed.

The `ToSource` trait is removed. Quasiquote no longer involves
pretty-printing at all, which removes the need for the
`encode_with_hygiene` hack. All associated machinery is removed.

A new `Nonterminal` is added, NtArm, which the parser now interpolates.
This is just for quasiquote, not macros (although it could be in the
future).

`ToTokens` is no longer implemented for `Arg` (although this could be
added again) and `Generics` (which I don't think makes sense).

This breaks any compiler extensions that relied on the ability of
`ToTokens` to turn AST fragments back into inspectable token trees. For
this reason, this closes #16987.

As such, this is a [breaking-change].

Fixes #16472.
Fixes #15962.
Fixes #17397.
Fixes #16617.

Changes the style guidelines regarding unit tests to recommend using a
sub-module named "tests" instead of "test" for unit tests as "test"
might clash with imports of libtest.