<[T]>::reverse: Fix pointer provenance rules

Should fix #78749

Stabilize clamp

Tracking issue: https://github.com/rust-lang/rust/issues/44095

Clamp has been merged and unstable for about a year and a half now. How do we feel about stabilizing this?

More consistently use spaces after commas in lists in docs

This PR changes instances of lists that didn't use spaces after commas, like `vec![1,2,3]`, to `vec![1, 2, 3]` to be more consistent with idiomatic Rust style (the way these were looks strange to me, especially because there are often lists that *do* use spaces after the commas later in the same code block 😬).

I noticed one of these in an example in the stdlib docs and went looking for more, but as far as I can see, I'm only changing those spots in user-facing documentation or rustc output, and the changes make no semantic difference.

Add "memset" as doc alias to slice::fill()

Similar to 53f969df and should make it easier for people coming from C to find this function.

Give a better error when rustdoc tests fail

- Run the default rustdoc against the current rustdoc
- Diff output recursively
- Colorize diff output

Closes https://github.com/rust-lang/rust/issues/78750.

## Resolved questions

- Should this be opt-in instead of on by default?
  + No
- Should this call through to `delta`? That's not a very common program to have installed, but I'm not sure how to do diffs after the fact. Maybe `compiletest` can take a `--syntax-highlighter` parameter or something?
  + I decided to use `delta` if available and `diff --color` otherwise. It prints a warning if delta isn't installed so you know you can get nicer diffs

## Open questions.

- What version of rustdoc would this compare against? Ideally it would compare against `$(git merge-base HEAD origin/master)` - maybe that's feasible if we install those artifacts from CI?
- Does it always make sense to compare the tests? Especially for new tests, I'm not sure how useful it would be ... but then again, one of the questions I want to know most as a reviewer is 'did it break before?'.

r? `@GuillaumeGomez`
cc `@Mark-Simulacrum`

Add support for custom allocators in `Vec`

This follows the [roadmap](https://github.com/rust-lang/wg-allocators/issues/7) of the allocator WG to add custom allocators to collections.

r? `@Amanieu`

This pull request requires a crater run.

### Prior work:
- #71873: Crater-test to solve rust-lang/wg-allocators#1
- [`alloc-wg`](https://github.com/TimDiekmann/alloc-wg)-crate

Rollup of 8 pull requests

Successful merges:

 - #77844 (clarify rules for ZST Boxes)
 - #79067 (Refactor the abi handling code a bit)
 - #79182 (Fix links to extern types in rustdoc (fixes #78777))
 - #79231 (Exhaustively match in variant count instrinsic)
 - #79238 (Direct RUSTC_LOG (tracing/log) output to stderr instead of stdout.)
 - #79256 (Fix typos)
 - #79264 (Get rid of some doctree items)
 - #79272 (Support building clone shims for arrays with generic size)

Failed merges:

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

Support building clone shims for arrays with generic size

Fixes #79269.

Get rid of some doctree items

They can be derived directly from the `hir::Item`, there's no special logic.

- TypeDef
- OpaqueTy
- Constant
- Static
- TraitAlias
- Enum
- Union
- Struct

Part of #78082 (the easiest part, I'm still debugging some other changes).
r? `@GuillaumeGomez`

Fix typos

Direct RUSTC_LOG (tracing/log) output to stderr instead of stdout.

Looks like this got missed in the initial implementation, AFAIK the old behavior was to output on stderr.
(Hit this while trying to debug `rustc` running inside a build script which was only letting stderr through)

r? ``@oli-obk`` cc ``@davidbarsky`` ``@hawkw``

Exhaustively match in variant count instrinsic

Fix #79137

Fix links to extern types in rustdoc (fixes #78777)

 r? `@jyn514`
 Fixes #78777.
The initial fix we tried was:
```diff
diff --git a/src/librustdoc/passes/collect_intra_doc_links.rs b/src/librustdoc/passes/collect_intra_doc_links.rs
index 8be9482acff..c4b7086fdb1 100644
--- a/src/librustdoc/passes/collect_intra_doc_links.rs
+++ b/src/librustdoc/passes/collect_intra_doc_links.rs
`@@` -433,8 +433,9 `@@` impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
             Res::PrimTy(prim) => Some(
                 self.resolve_primitive_associated_item(prim, ns, module_id, item_name, item_str),
             ),
-            Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum | DefKind::TyAlias, did) => {
+            Res::Def(kind, did) if kind.ns() == Some(Namespace::TypeNS) => {
                 debug!("looking for associated item named {} for item {:?}", item_name, did);
+
                 // Checks if item_name belongs to `impl SomeItem`
                 let assoc_item = cx
                     .tcx
```

However, this caused traits to be matched, resulting in a panic when `resolve_associated_trait_item` is called further down in this function.

This PR also adds an error message for that panic. Currently it will look something like:
```rust
thread 'rustc' panicked at 'Not a type: DefIndex(8624)', compiler/rustc_metadata/src/rmeta/decoder.rs:951:32
```
I wasn't sure how to get a better debug output than `DefIndex(...)`, and am open to suggestions.

Refactor the abi handling code a bit

I am not quite sure if all changes are improvements.

clarify rules for ZST Boxes

LLVM's rules around `getelementptr inbounds` with offset 0 are a bit annoying, and as a consequence we have no choice but say that a `Box<()>` pointing to previously allocated memory that has since been freed is UB. Clarify the docs to reflect this.

This is based on conversations on the LLVM mailing list.
* Here's my initial mail: https://lists.llvm.org/pipermail/llvm-dev/2019-February/130452.html
* The first email of the March part of that thread: https://lists.llvm.org/pipermail/llvm-dev/2019-March/130831.html
* First email of the April part: https://lists.llvm.org/pipermail/llvm-dev/2019-April/131693.html

The conclusion for me at least was that `getelementptr inbounds` with offset 0 is *not* the identity function, but can sometimes return `poison` even when the input is a regular pointer -- specifically, it returns `poison` when this pointer points into something that LLVM "knows has been deallocated", i.e., a former LLVM-managed allocation. It is however the identity function on pointers obtained by casting integers.

Note that there [are formal proposals](https://people.mpi-sws.org/~jung/twinsem/twinsem.pdf) for LLVM semantics where `getelementptr inbounds` with offset 0 isn't quite the identity function but never returns `poison` (it affects the provenance of the pointer but in a way that doesn't matter if this pointer is never used for memory accesses), and indeed this is likely necessary to consistently describe LLVM semantics. But with the informal LLVM LangRef that we have right now, and with LLVM devs insisting otherwise, it seems unwise to rely on this.

It is applied exactly when the return value has an indirect pass mode.
Except for InReg on x86 fastcall, arg attrs are now only used for
optimization purposes and thus are fine to ignore.

This makes it clearer that only PassMode::Indirect allows ByVal

Both flags are mutually exclusive

std: Update the bactrace crate submodule

This commit updates the `library/backtrace` submodule which primarily
pulls in support for split-debuginfo on macOS, avoiding the need for
`dsymutil` to get run to get line numbers and filenames in backtraces.

x.py: allow a custom string appended to the version

This adds `rust.description` to the config as a descriptive string to be
appended to `rustc --version` output, which is also used in places like
debuginfo `DW_AT_producer`. This may be useful for supplementary build
information, like distro-specific package versions.

For example, in Fedora 33, `gcc --version` outputs:

    gcc (GCC) 10.2.1 20201016 (Red Hat 10.2.1-6)

With this change, we can add similar vendor info to `rustc --version`.

They can be derived directly from the `hir::Item`, there's no special
logic.

- TypeDef
- OpaqueTy
- Constant
- Static
- TraitAlias
- Enum
- Union
- Struct

lint: Do not provide suggestions for non standard characters

Fixes #77273

Only provide suggestions if the case-fixed result is different than the original.

update miri

Fixes https://github.com/rust-lang/rust/issues/79226
Cc `@rust-lang/miri` r? `@ghost`

Add `core::slice::fill_with`

Tracking issue https://github.com/rust-lang/rust/issues/79221.

As suggested by `@m-ou-se` in https://github.com/rust-lang/rust/issues/70758#issuecomment-726838099 this implements `slice::fill_with` as a counterpart to `slice::fill`. This mirrors `Vec::resize` and `Vec::resize_with`. Thanks!

r? `@m-ou-se`

rustc_expand: Mark inner `#![test]` attributes as soft-unstable

Custom inner attributes are feature gated (https://github.com/rust-lang/rust/issues/54726) except for attributes having name `test` literally, which are not gated for historical reasons.

`#![test]` is an inner proc macro attribute, so it has all the issues described in https://github.com/rust-lang/rust/issues/54726 too.
This PR gates it with the `soft_unstable` lint.

Reworks Sccc computation to iteration instead of recursion

Linear graphs, producing as many scc's as nodes, would recurse once for every node when entered from the start of the list. This adds a test that exhausted the stack at least on my machine with error:

```
thread 'graph::scc::tests::test_deep_linear' has overflowed its stack
fatal runtime error: stack overflow
```

This may or may not be connected to #78567. I was only reminded that I started this rework some time ago. It might be plausible as borrow checking a long function with many borrow regions around each other—((((((…))))))— may produce the linear list setup to trigger this stack overflow ? I don't know enough about borrow check to say for sure.

This is best read in two separate commits. The first addresses only `find_state` internally. This is classical union phase from union-find. There's also a common solution of using the parent pointers in the (virtual) linked list to track the backreferences while traversing upwards and then following them backwards in a second path compression phase.

The second is more involved as it rewrites the mutually recursive `walk_node` and `walk_unvisited_node`. Firstly, the caller is required to handle the unvisited case of `walk_node` so a new `start_walk_from` method is added to handle that by walking the unvisited node if necessary. Then `walk_unvisited_node`, where we would previously recurse into in the missing case, is rewritten to construct a manual stack of its frames. The state fields consist of the previous stack slots.

Arena: use specialization to avoid copying data

In several cases, a `Vec` or `SmallVec` is passed to `Arena::alloc_from_iter` directly. This PR makes sure those cases don't copy their data unnecessarily, by specializing the `alloc_from_iter` implementation.