Promote references to constants instead of statics

r? @oli-obk

Rollup of 8 pull requests

Successful merges:

 - #67756 (Collector tweaks)
 - #67889 (Compile some CGUs in parallel at the start of codegen)
 - #67930 (Rename Result::as_deref_ok to as_deref)
 - #68018 (feature_gate: Remove `GateStrength`)
 - #68070 (clean up E0185 explanation)
 - #68072 (Fix ICE #68058)
 - #68114 (Don't require `allow_internal_unstable` unless `staged_api` is enabled.)
 - #68120 (Ban `...X` pats, harden tests, and improve diagnostics)

Failed merges:

r? @ghost

Ban `...X` pats, harden tests, and improve diagnostics

Follow up to https://github.com/rust-lang/rust/pull/67258#issuecomment-565656155 and https://github.com/rust-lang/rust/pull/67258#discussion_r357879932.

r? @cramertj @oli-obk

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of #63770)
cc @Centril

Fix ICE #68058

Fixes #68058

r? @petrochenkov

clean up E0185 explanation

r? @Dylan-DPC

feature_gate: Remove `GateStrength`

The "soft feature gating" from `feature_gate/check.rs` is unused, and even if it were used, hardcoded warning is not a good solution and [deny-by-default lint](https://github.com/rust-lang/rust/issues/64266) would be a better way to do this.

cc https://github.com/rust-lang/rust/pull/67806#discussion_r363091701
r? @Centril

Rename Result::as_deref_ok to as_deref

Addresses https://github.com/rust-lang/rust/issues/50264#issuecomment-533952247
r? @dtolnay

Compile some CGUs in parallel at the start of codegen

This brings the compilation time for `syntex_syntax` from 11.542s to 10.453s with 6 threads in non-incremental debug mode. Just compiling `n` CGUs in parallel at the beginning of codegen seems sufficient to get rid of the staircase effect, at least for `syntex_syntax`.

Based on https://github.com/rust-lang/rust/pull/67777.

r? @michaelwoerister
cc @alexcrichton @Mark-Simulacrum

Collector tweaks

r? @Mark-Simulacrum

Also fix a bug with the span passed in `mk_range`.

Point at the span for the definition of crate foreign ADTs

Follow up to #65421. Partially addresses #65386. Blocked on #53081.

Co-Authored-By: NMazdak Farrokhzad <twingoow@gmail.com>

Rollup of 8 pull requests

Successful merges:

 - #67666 (make use of pointer::is_null)
 - #67806 (Extract `rustc_ast_passes`, move gating, & refactor linting)
 - #68043 (Add some missing timers)
 - #68074 (Add `llvm-skip-rebuild` flag to `x.py`)
 - #68079 (Clarify suggestion for E0013)
 - #68084 (Do not ICE on unicode next point)
 - #68102 (Inline some conversion methods around OsStr)
 - #68106 (Fix issue with using `self` module via indirection)

Failed merges:

r? @ghost

Fix issue with using `self` module via indirection

Fixes https://github.com/rust-lang/rust/issues/68103.

Inline some conversion methods around OsStr

Diff on the assembly of this snippet before and after this PR: https://www.diffchecker.com/NeGMjaJ2
```rust
use std::env;
use std::io;
use std::path::{Path, PathBuf};

pub fn cargo_home_with_cwd(cwd: &Path) -> io::Result<PathBuf> {
    match env::var_os("CARGO_HOME").filter(|h| !h.is_empty()) {
        Some(home) => {
            let home = PathBuf::from(home);
            if home.is_absolute() {
                Ok(home)
            } else {
                Ok(cwd.join(&home))
            }
        }
        _ => env::home_dir()
            .map(|p| p.join(".cargo"))
            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "could not find cargo home dir")),
    }
}
```

Do not ICE on unicode next point

Use `shrink_to_hi` instead of `next_point` and fix `next_point`.

Fix #68000, fix #68091, fix #68092.

Clarify suggestion for E0013

Fixes https://github.com/rust-lang/rust/issues/68038.

Add `llvm-skip-rebuild` flag to `x.py`

This PR follows on from #67437 to complete the feature request from #65612.

Specifically it adds a new command-line flag, `--llvm-skip-rebuild`, which overrides both any value set in `config.toml` and the default value (`false`).

I'm not 100% confident that I've implemented the override in the "best" way, but I've checked it locally and it seems to work at least.

This option isn't currently mentioned in the Guide to Rustc Development. I'd be happy to write something on it if folk think that's worthwhile.

Add some missing timers

Based on https://github.com/rust-lang/rust/pull/67988

r? @wesleywiser

Extract `rustc_ast_passes`, move gating, & refactor linting

Based on https://github.com/rust-lang/rust/pull/67770.

This PR extracts a crate `rustc_ast_passes`:

- `ast_validation.rs`, which is contributed by `rustc_passes` (now only has HIR based passes) -- the goal here is to improve recompilation of the parser,
- `feature_gate.rs`, which is contributed by `syntax` and performs post-expansion-gating & final erroring for pre-expansion gating,
- `show_span`, which is contributed by `syntax`.

To facilitate this, we first have to also:

- Move `{leveled_}feature_err{_err}` from `syntax::feature_gate::check` into `rustc_session::parse`.
- Move `get_features` into `rustc_parse::config`, the only place it is used.
- Move some some lint datatypes and traits, e.g. `LintBuffer`, `BufferedEarlyLint`, `BuiltinLintDiagnostics`, `LintPass`, and `LintArray` into `rustc_session::lint`.
- Move all the hard-wired lint `static`s into `rustc_session::lint::builtin`.

make use of pointer::is_null

r? @Mark-Simulacrum

This flag opts out of the min-const-fn checks entirely, which is usually
not what we want. The few cases where the flag is still necessary have
been annotated.