Add edition 2021.

🎆 Happy new ~~year~~ Rust. 🍾

This adds --edition=2021, and updates suggestions about 2018 to say "2018 *or later*".

Related Cargo PR: https://github.com/rust-lang/cargo/pull/8922

---

Edit: This adds the new edition as *unstable*. Without `-Z unstable-options`, `--edition=2021` results in:
```
$ rustc --edition=2021
error: edition 2021 is unstable and only available with -Z unstable-options.
```

Rollup of 8 pull requests

Successful merges:

 - #80323 (Update and improve `rustc_codegen_{llvm,ssa}` docs)
 - #80368 (rustdoc: Render visibilities succinctly)
 - #80514 (Fix broken ./x.py install)
 - #80519 (Take type defaults into account in suggestions to reorder generic parameters)
 - #80526 (Update LLVM)
 - #80532 (remove unnecessary trailing semicolon from bootstrap)
 - #80548 (FIx ICE on wf check for foreign fns)
 - #80551 (support pattern as const parents in type_of)

Failed merges:

 - #80547 (In which we start to parse const generics defaults)

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

support pattern as const parents in type_of

nice to know that there's still stuff about rust i didn't know about 😆

fixes #80531

r? `@varkor`

FIx ICE on wf check for foreign fns

Fixes #80468

r? `@davidtwco` `@lcnr`

remove unnecessary trailing semicolon from bootstrap

Update LLVM

- [GlobalISel][IRTranslator] Fix a crash when the use of an extractvalue is a non-dominated metadata use.
- [asan] Use dynamic shadow memory position on Apple Silicon macOS

r? ``@cuviper``

Take type defaults into account in suggestions to reorder generic parameters

Fixes #80512

Fix broken ./x.py install

During my tarball refactorings in https://github.com/rust-lang/rust/pull/79788 I changed the directory layout used by the tarball generation code, and that broke the other parts of rustbuild which hardcoded the paths of those directories. Namely, `./x.py install` relied on the uncompressed copy of the tarball left behind by `fabricate`/`rust-installer`, causing https://github.com/rust-lang/rust/issues/80494.

While the easy fix for https://github.com/rust-lang/rust/issues/80494 would've been to just update the hardcoded paths to match the new structure, that fix would leave us in the same situation if we were to change the directory layout again in the future. Instead I refactored the code to return a `GeneratedTarball` struct as the output of all the dist steps, and I put all the paths the rest of rustbuild needs to care about in its fields. That way, future changes to `src/bootstrap/tarball.rs` will not break other stuff.

This PR is best reviewed commit-by-commit.
r? `@Mark-Simulacrum`
`@rustbot` modify labels: beta-nominated beta-accepted T-release

rustdoc: Render visibilities succinctly

Fixes #79139.

r? `@jyn514`

Update and improve `rustc_codegen_{llvm,ssa}` docs

Fixes #75342.

These docs were very out of date and misleading. They even said that
they codegen'd the *AST*!

For some reason, the `rustc_codegen_ssa::base` docs were exactly
identical to the `rustc_codegen_llvm::base` docs. They didn't really
make sense, because they had LLVM-specific information even though
`rustc_codegen_ssa` is supposed to be somewhat generic. So I removed
them as they were misleading.

r? ``@pnkfelix`` maybe?

Implement edition-based macro :pat feature

This PR does two things:
1. Fixes the perf regression from https://github.com/rust-lang/rust/pull/80100#issuecomment-750893149
2. Implements `:pat2018` and `:pat2021` matchers, as described by `@joshtriplett`  in https://github.com/rust-lang/rust/issues/54883#issuecomment-745509090 behind the feature gate `edition_macro_pat`.

r? `@petrochenkov`

cc `@Mark-Simulacrum`

The return of the GroupBy and GroupByMut iterators on slice

According to https://github.com/rust-lang/rfcs/pull/2477#issuecomment-742034372, I am opening this PR again, this time I implemented it in safe Rust only, it is therefore much easier to read and is completely safe.

This PR proposes to add two new methods to the slice, the `group_by` and `group_by_mut`. These two methods provide a way to iterate over non-overlapping sub-slices of a base slice that are separated by the predicate given by the user (e.g. `Partial::eq`, `|a, b| a.abs() < b.abs()`).

```rust
let slice = &[1, 1, 1, 3, 3, 2, 2, 2];

let mut iter = slice.group_by(|a, b| a == b);
assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
assert_eq!(iter.next(), Some(&[3, 3][..]));
assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
assert_eq!(iter.next(), None);
```

[An RFC](https://github.com/rust-lang/rfcs/pull/2477) was open 2 years ago but wasn't necessary.

The struct will allow to store more context on the generated tarballs.

Files in the temporary directory are used by ./x.py install.

Rustdoc render public underscore_imports as Re-exports

Fixes #61592

Fixes #61592

Remove all doc_comment!{} hacks by using #[doc = expr] where needed.

This replaces about 200 cases of

`````rust
        doc_comment! {
            concat!("The smallest value that can be represented by this integer type.

# Examples

Basic usage:

```
", $Feature, "assert_eq!(", stringify!($SelfT), "::MIN, ", stringify!($Min), ");",
$EndFeature, "
```"),
            #[stable(feature = "assoc_int_consts", since = "1.43.0")]
            pub const MIN: Self = !0 ^ ((!0 as $UnsignedT) >> 1) as Self;
        }
`````
by
```rust
        /// The smallest value that can be represented by this integer type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN, ", stringify!($Min), ");")]
        /// ```
        #[stable(feature = "assoc_int_consts", since = "1.43.0")]
        pub const MIN: Self = !0 ^ ((!0 as $UnsignedT) >> 1) as Self;
```

---

**Note:** For a usable diff, make sure to enable 'ignore whitspace': https://github.com/rust-lang/rust/pull/79150/files?diff=unified&w=1

Add `#[track_caller]` to `bug!` and `register_renamed`

Before:

```
thread 'rustc' panicked at 'compiler/rustc_lint/src/context.rs:267:18: invalid lint renaming of broken_intra_doc_links to rustdoc::broken_intra_doc_links', compiler/rustc_middle/src/util/bug.rs:34:26
```

After:

```
thread 'rustc' panicked at 'src/librustdoc/core.rs:455:24: invalid lint renaming of broken_intra_doc_links to rustdoc::broken_intra_doc_links', compiler/rustc_middle/src/util/bug.rs:35:26
```

The reason I added it to `register_renamed` too is that any panic in
that function will be the caller's fault.

Co-authored-by: NJoshua Nelson <jyn514@gmail.com>

Only produce .xz tarballs on CI

This PR adds a `./configure` option to choose which tarball compression formats to produce, and changes our CI configuration to only produce `.xz` tarballs. The release process will then recompress everything into `.gz` when producing a release.

This will drastically reduce our storage costs for CI artifacts, as we'd stop storing the same data twice. **Stable, beta and nightly releases will not be affected by this at all.**

Before landing this we'll need to increase the VM size of our release process, to recompress everything in a reasonable amount of time.
r? `@Mark-Simulacrum`

Rollup of 9 pull requests

Successful merges:

 - #78934 (refactor: removing library/alloc/src/vec/mod.rs ignore-tidy-filelength)
 - #79479 (Add `Iterator::intersperse`)
 - #80128 (Edit rustc_ast::ast::FieldPat docs)
 - #80424 (Don't give an error when creating a file for the first time)
 - #80458 (Some Promotion Refactoring)
 - #80488 (Do not create dangling &T in Weak<T>::drop)
 - #80491 (Miri: make size/align_of_val work for dangling raw ptrs)
 - #80495 (Rename kw::Invalid -> kw::Empty)
 - #80513 (Add regression test for #80062)

Failed merges:

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

Add regression test for #80062

Closes #80062.
r? `@lcnr`