This shows up to 5% less instruction counts on multiple benchmarks, and up to
19% wins on the -j1 wall times for rustc self-compilation.

We can afford to spend the extra cycles building LLVM essentially once more for
the x86_64-unknown-linux-gnu CI build today. The builder finishes in around 50
minutes on average, and this adds just 10 more minutes. Given the sizeable
improvements in compiler performance, this is definitely worth it.

Stabilize and document `--force-warn`

This PR will stabilize and document the `--force-warn` command line option. It is currently a draft, pending an FCP.

I've taken the liberty of tidying up the lint level command line options a bit as part of this. The changes are quite minor and should only affect rustc's help output. I'm making them here because they're trivial and, in one case, necessary to unify the way `--force-warn` with the way the other options are displayed.

I also want to mention that `@rylev` has done a ton of work on moving this along and deserves most of the credit. I'm just the one who landed up writing this particular PR.

Resolves #86516.

This updates tests to reflect that `force-warn` is now stable.

Co-authored-by: NMark Rousskov <mark.simulacrum@gmail.com>

Use bound vars for GAT params in param_env in check_type_bounds

Fixes #87429

Allow specifying an deployment target version for all iOS llvm targets

Closes: https://github.com/rust-lang/rust/issues/79408

This pull requests adds the same procedure to define the iOS-version for the LLVM-target as was used for the simulator target and the desktop target.

This then closes the original problem mentioned in the above issue. The problem with incompatible bitcode remains, but is probably not easy fixable.

I realised that something is still not right. Try to fix that.

r? `@petrochenkov`

Remove `Session.used_attrs` and move logic to `CheckAttrVisitor`

Instead of updating global state to mark attributes as used,
we now explicitly emit a warning when an attribute is used in
an unsupported position. As a side effect, we are to emit more
detailed warning messages (instead of just a generic "unused" message).

`Session.check_name` is removed, since its only purpose was to mark
the attribute as used. All of the callers are modified to use
`Attribute.has_name`

Additionally, `AttributeType::AssumedUsed` is removed - an 'assumed
used' attribute is implemented by simply not performing any checks
in `CheckAttrVisitor` for a particular attribute.

We no longer emit unused attribute warnings for the `#[rustc_dummy]`
attribute - it's an internal attribute used for tests, so it doesn't
mark sense to treat it as 'unused'.

With this commit, a large source of global untracked state is removed.

Warn about unreachable code following an expression with an uninhabited type

This pull request fixes #85071. The issue is that liveness analysis currently is "smarter" than reachability analysis when it comes to detecting uninhabited types: Unreachable code is detected during type checking, where full type information is not yet available. Therefore, the check for type inhabitedness is quite crude:
https://github.com/rust-lang/rust/blob/fc81ad22c453776de16acf9938976930cf8c9401/compiler/rustc_typeck/src/check/expr.rs#L202-L205

i.e. it only checks for `!`, but not other, non-trivially uninhabited types, such as empty enums, structs containing an uninhabited type, etc. By contrast, liveness analysis, which runs after type checking, can benefit from the more sophisticated `tcx.is_ty_uninhabited_from()`:
https://github.com/rust-lang/rust/blob/fc81ad22c453776de16acf9938976930cf8c9401/compiler/rustc_passes/src/liveness.rs#L981
https://github.com/rust-lang/rust/blob/fc81ad22c453776de16acf9938976930cf8c9401/compiler/rustc_passes/src/liveness.rs#L996

This can lead to confusing warnings when a variable is reported as unused, but the use of the variable is not reported as unreachable. For instance:
```rust
enum Foo {}
fn f() -> Foo {todo!()}

fn main() {
    let x = f();
    let _ = x;
}
```
currently leads to
```
warning: unused variable: `x`
 --> t1.rs:5:9
  |
5 |     let x = f();
  |         ^ help: if this is intentional, prefix it with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: 1 warning emitted
```
which is confusing, because `x` _appears_ to be used in line 6. With my changes, I get:
```
warning: unreachable expression
 --> t1.rs:6:13
  |
5 |     let x = f();
  |             --- any code following this expression is unreachable
6 |     let _ = x;
  |             ^ unreachable expression
  |
  = note: `#[warn(unreachable_code)]` on by default
note: this expression has type `Foo`, which is uninhabited
 --> t1.rs:5:13
  |
5 |     let x = f();
  |             ^^^

warning: unused variable: `x`
 --> t1.rs:5:9
  |
5 |     let x = f();
  |         ^ help: if this is intentional, prefix it with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: 2 warnings emitted
```
My implementation is slightly inelegant because unreachable code warnings can now be issued in two different places (during type checking and during liveness analysis), but I think it is the solution with the least amount of unnecessary code duplication, given that the new warning integrates nicely with liveness analysis, where unreachable code is already implicitly detected for the purpose of finding unused variables.

Get piece unchecked in `write`

We already use specialized `zip`, but it seems like we can do a little better by not checking `pieces` length at all.

`Arguments` constructors are now unsafe. So the `format_args!` expansion now includes an `unsafe` block.

<details>
<summary>Local Bench Diff</summary>

```text
 name                        before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 fmt::write_str_macro1       22,967          19,718               -3,249  -14.15%   x 1.16
 fmt::write_str_macro2       35,527          32,654               -2,873   -8.09%   x 1.09
 fmt::write_str_macro_debug  571,953         575,973               4,020    0.70%   x 0.99
 fmt::write_str_ref          9,579           9,459                  -120   -1.25%   x 1.01
 fmt::write_str_value        9,573           9,572                    -1   -0.01%   x 1.00
 fmt::write_u128_max         176             173                      -3   -1.70%   x 1.02
 fmt::write_u128_min         138             134                      -4   -2.90%   x 1.03
 fmt::write_u64_max          139             136                      -3   -2.16%   x 1.02
 fmt::write_u64_min          129             135                       6    4.65%   x 0.96
 fmt::write_vec_macro1       24,401          22,273               -2,128   -8.72%   x 1.10
 fmt::write_vec_macro2       37,096          35,602               -1,494   -4.03%   x 1.04
 fmt::write_vec_macro_debug  588,291         589,575               1,284    0.22%   x 1.00
 fmt::write_vec_ref          9,568           9,732                   164    1.71%   x 0.98
 fmt::write_vec_value        9,516           9,625                   109    1.15%   x 0.99
```
</details>

Rollup of 6 pull requests

Successful merges:

 - #87976 (Account for tabs when highlighting multiline code suggestions)
 - #88174 (Clarify some wording in Rust 2021 lint docs)
 - #88188 (Greatly improve limitation handling on parallel rustdoc GUI test run)
 - #88230 (Fix typos “a”→“an”)
 - #88232 (Add notes to macro-not-found diagnostics to point out how things with the same name were not a match.)
 - #88259 (Do not mark `-Z thir-unsafeck` as unsound anymore)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Do not mark `-Z thir-unsafeck` as unsound anymore

The initial implementation of the THIR unsafety checker is now complete (rust-lang/project-thir-unsafeck#7).

r? `@oli-obk`

Add notes to macro-not-found diagnostics to point out how things with the same name were not a match.

This adds notes like:
```
error: cannot find derive macro `Serialize` in this scope
  --> $DIR/issue-88206.rs:22:10
   |
LL | #[derive(Serialize)]
   |          ^^^^^^^^^
   |
note: `Serialize` is imported here, but it is not a derive macro
  --> $DIR/issue-88206.rs:17:11
   |
LL | use hey::{Serialize, Deserialize};
   |           ^^^^^^^^^
```

Fixes https://github.com/rust-lang/rust/issues/88206

Includes https://github.com/rust-lang/rust/pull/88229

r? `@estebank`

Fix typos “a”→“an”

Fix typos in comments; found using a regex to find some easy instance of incorrect usage of a vs. an.

While automation was used to find these, every change was checked manually.

Changes in submodules get separate PRs:
* https://github.com/rust-lang/stdarch/pull/1201
* https://github.com/rust-lang/cargo/pull/9821
* https://github.com/rust-lang/miri/pull/1874
* https://github.com/rust-lang/rls/pull/1746
* https://github.com/rust-analyzer/rust-analyzer/pull/9984
  _folks @ rust-analyzer are fast at merging…_
  * https://github.com/rust-analyzer/rust-analyzer/pull/9985
  * https://github.com/rust-analyzer/rust-analyzer/pull/9987
  * https://github.com/rust-analyzer/rust-analyzer/pull/9989

_For `clippy`, I don’t know if the changes should better better be moved to a PR to the original repo._

<hr>

This has some overlap with #88226, but neither is a strict superset of the other.

If you want multiple commits, I can split it up; in that case, make sure to suggest a criterion for splitting.

Greatly improve limitation handling on parallel rustdoc GUI test run

Follow-up of https://github.com/rust-lang/rust/pull/88082.

r? `@dns2utf8`

Clarify some wording in Rust 2021 lint docs

Also added some inline code styling.

Account for tabs when highlighting multiline code suggestions

Address `'\t'` case in #87972.

Before:

![Screen Shot 2021-08-12 at 8 52 27 AM](https://user-images.githubusercontent.com/1606434/129228214-e5cfd203-9aa8-41c7-acd9-ce255ef8a21e.png)

After:

![Screen Shot 2021-08-12 at 8 52 15 AM](https://user-images.githubusercontent.com/1606434/129228236-57c951fc-c8cf-4901-989f-b9b5aa5eebca.png)

2229: Handle MutBorrow/UniqueImmBorrow better

We only want to use UniqueImmBorrow when the capture place is truncated and we
drop Deref of a MutRef.

r? `@nikomatsakis`

Fixes: https://github.com/rust-lang/project-rfc-2229/issues/56

Improve error reporting for closure return type mismatches

Fixes #87461.