`file_metadata_raw` interns the strings `"<unknown>"` and `""` very
frequently. This commit avoids that, which reduces the number of symbols
interned significantly and reduces instruction counts by up to 0.5% on
some workloads.

`find_attr_val(&line, "since")` returns `Some(", issue = ")` when
`line` is set to the following line:

```
[unstable(feature = "checked_duration_since", issue = "58402")]
```

Make `find_attr_val` use regex that is a little bit more
precise (requires `=` after key name).

It still does not handle all cases (e.g., extra leading chars in key
name, or escaped quotes in value), but is good enough for now.

Make deprecation lint `ambiguous_associated_items` deny-by-default

As requested by r? @Centril

cc rust-lang#57644

…rrors, r=QuietMisdreavus

report fatal errors during doctest parsing

Fixes rust-lang#59557.

… r=Centril

Tidy: ensure lang features are sorted by since

This is the tidy side of rust-lang#60361.

What is left is actually splitting features into groups and sorting by since.

This PR also likely to produce a small (a couple of lines) merge conflict with rust-lang#60362.

r? @Centril

…ikomatsakis

Disallow non-explicit elided lifetimes in async fn

Fix rust-lang#60203

r? @nikomatsakis

 Do not suggest incorrect syntax on pattern type error due to borrow

Fix rust-lang#55174.

Rename `RUST_LOG` to `RUSTC_LOG`

cc: rust-lang#57985

I think we should also change these submodules:

- rustc-guide
- Cargo (rename to `CARGO_LOG`, cc: rust-lang/cargo#6605, rust-lang/cargo#6189)
- miri
- rls
- rustfmt

r? @davidtwco

…e, r=oli-obk

Require a trait in the bounds of existential types

Fixes rust-lang#53090

r? @oli-obk

…vidtwco

Resolve match arm ty when arms diverge

Fix rust-lang#58695.

… r=oli-obk

Const prop refactoring

This is rebased on top of rust-lang#60428 so only the top commit is new.

This is the refactoring to remove the `mir` field from `ConstPropagator` which is necessary before we can begin to actually propagate constants.

r? @oli-obk

…r=davidtwco

Avoid repeated interning of static strings.

`file_metadata_raw` interns the strings `"<unknown>"` and `""` very
frequently. This commit avoids that, which reduces the number of symbols
interned significantly and reduces instruction counts by up to 0.5% on
some workloads.

minor compiler doc tweaks

…ts, r=cramertj

Propagate mutability from arguments to local bindings in async fn

Fixes rust-lang#60498

cc @nikomatsakis
r? @davidtwco