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.

⬆ rust-analyzer

Implement `AsFd` etc. for `UnixListener`.

Implement `AsFd`, `From<OwnedFd>`, and `Into<OwnedFd>` for
`UnixListener`. This is a follow-up to #87329.

r? `@joshtriplett`

Add doctests for HashMap's into_values and into_keys methods

Fixes #87591

Test TAITs different lifetimes in defining uses fail

r? `@oli-obk`

Related to #86727

Stop emitting the `dso_local` LLVM attribute for external symbols under the static relocation model on macOS.

This matches Clang's behavior:

https://github.com/llvm/llvm-project/blob/973cb2c326be9f256da0897c4d2ef117dc22761d/clang/lib/CodeGen/CodeGenModule.cpp#L1038-L1040

Even if `dso_local` were properly supported in this way on macOS, it seems
incorrect to add this annotation as liberally as we did. The `dso_local`
annotation is for symbols that ultimately end up in the same linkage unit, but
we were adding this annotation even for `static` values inside `extern` blocks
marked with `#[link(type="framework")]`, which should be considered dynamically
linked.  Note that Clang likewise avoids emitting `dso_local` for `dllimport`
symbols:

https://github.com/llvm/llvm-project/blob/973cb2c326be9f256da0897c4d2ef117dc22761d/clang/lib/CodeGen/CodeGenModule.cpp#L1005-L1007

This issue caused breakage in the `ring` crate, which links to a symbol defined
in `Security.framework` that ultimately resolves to address `0x0`:

https://github.com/briansmith/ring/blob/b94d61e044b42827fefd71d5f61e8c58a7659870/src/rand.rs#L390

For this symbol, the use of `dso_local` causes LLVM to emit a relocation of
type `X86_64_RELOC_SIGNED`, which is a 32-bit signed PC-relative offset. If the
binary is large enough, `0x0` might be out of range, and the link will fail.
Avoiding `dso_local` causes LLVM to use the GOT instead, emitting a relocation
of type `X86_64_RELOC_GOT_LOAD`, which will properly handle the large offset
and cause the link to succeed.

As a side note, the static relocation model is effectively deprecated for
security reasons on macOS, as it prohibits PIE. It's also completely
unsupported on Apple Silicon, so I don't think it's worth going to the effort
of properly supporting this model on that platform.

Rollup of 7 pull requests

Successful merges:

 - #86747 (Improve wording of the `drop_bounds` lint)
 - #87166 (Show discriminant before overflow in diagnostic for duplicate values.)
 - #88077 (Generate an iOS LLVM target with a specific version)
 - #88164 (PassWrapper: adapt for LLVM 14 changes)
 - #88211 (cleanup: `Span::new` -> `Span::with_lo`)
 - #88229 (Suggest importing the right kind of macro.)
 - #88238 (Stop tracking namespace in used_imports.)

Failed merges:

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

Stop tracking namespace in used_imports.

This changes `used_imports` from a `FxHashSet<(NodeId, Namespace)>` to a `FxHashSet<NodeId>`, as the Namespace information isn't used.

The only point that uses it did three lookups, `|=`'ing them together.

r? `@estebank`

Suggest importing the right kind of macro.

Fixes #88228.

r? `@estebank`

cleanup: `Span::new` -> `Span::with_lo`

Extracted from https://github.com/rust-lang/rust/pull/84373 as suggested in https://github.com/rust-lang/rust/pull/84373#issuecomment-857773867.
It turned out less useful then I expected, but anyway.
r? `@cjgillot`
`@bors` rollup

PassWrapper: adapt for LLVM 14 changes

These API changes appear to have all taken place in
https://reviews.llvm.org/D105007, which moved HWAddressSanitizerPass and
AddressSanitizerPass to only accept their options type as a ctor
argument instead of the sequence of bools etc. This required a couple of
parameter additions, which I made match the default prior to the
mentioned upstream LLVM change.

This patch restores rustc to building (though not quite passing all
tests, I've mailed other patches for those issues) against LLVM HEAD.