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)

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.

Generate an iOS LLVM target with a specific version

This commit adds the `LC_VERSION_MIN_IPHONEOS` load command to the Mach-O header generated for `aarch64-apple-ios` binaries. The operating system will look for this load command to determine the minimum supported operating system version and will not allow the binary to run if it's absent. This logic already exists for the simulator toolchain.

I've been using `otool` from a [cctools](https://github.com/tpoechtrager/cctools-port) toolchain to parse the header and validate that this change adds the required load command.

This change appears to be enough to build Rust binaries that can run on a jailbroken iPhone.

Show discriminant before overflow in diagnostic for duplicate values.

This PR adds the value before overflow for explicit discriminant values in the error for duplicate discriminant values.
I found it rather confusing to see only the overflowed value.

It only does this for literals, since overflows in const evaluated arithmetic are already a hard error.

This is my first PR to the compiler, so please let me know if the implementation can be improved :)

Before:
![image](https://user-images.githubusercontent.com/786213/125850097-bf5fb7e0-d800-4386-a738-c30f41822964.png)

After:
![image](https://user-images.githubusercontent.com/786213/125850120-e2bb765d-ad86-4888-a6cb-dec34fba3fea.png)

Improve wording of the `drop_bounds` lint

This PR addresses #86653. The issue is sort of a false positive of the `drop_bounds` lint, but I would argue that the best solution for #86653 is simply a rewording of the warning message and lint description, because even if the lint is _technically_ wrong, it still forces the programmer to think about what they are doing, and they can always use `#[allow(drop_bounds)]` if they think that they really need the `Drop` bound.

There are two issues with the current warning message and lint description:
- First, it says that `Drop` bounds are "useless", which is technically incorrect because they actually do have the effect of allowing you e.g. to call methods that also have a `Drop` bound on their generic arguments for some reason. I have changed the wording to emphasize not that the bound is "useless", but that it is most likely not what was intended.
- Second, it claims that `std::mem::needs_drop` detects whether a type has a destructor. But I think this is also technically wrong: The `Drop` bound says whether the type has a destructor or not, whereas `std::mem::needs_drop` also takes nested types with destructors into account, even if the top-level type does not itself have one (although I'm not 100% sure about the exact terminology here, i.e. whether the "drop glue" of the top-level type counts as a destructor or not).

cc `@jonhoo,` does this solve the issue for you?

r? `@GuillaumeGomez`

canonicalize consts before calling try_unify_abstract_consts query

Fixes #88022
Fixes #86953
Fixes #77708
Fixes #82034
Fixes #85031

these ICEs were all caused by calling the `try_unify_abstract_consts` query with inference vars in substs

r? `@lcnr`

The information was tracked, but unused.

Fix clippy::collapsible_match with let expressions

This fixes rust-lang/rust-clippy#7575 which is a regression from #80357. I am fixing the bug here instead of in the clippy repo (if that's okay) because a) the regression has not been synced yet and b) I would like to land the fix on nightly asap.

The fix is basically to re-generalize `match` and `if let` for the lint implementation (they were split because `if let` no longer desugars to `match` in the HIR).

Also fixes rust-lang/rust-clippy#7586 and fixes rust-lang/rust-clippy#7591
cc `@rust-lang/clippy`
`@xFrednet` do you want to review this?

marker_traits: require `EvaluatedToOk` during winnowing

closes #84955, while it doesn't really fix it in a way that makes me happy it should prevent the issue for now and this
test can't be reproduced anyways, so it doesn't make much sense to keep it open.

fixes #84917 as only one of the impls depends on regions, so we now drop the ambiguous one instead of the correct one.

cc https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/winnowing.20soundly/near/247899832

r? `@nikomatsakis`

Fix example in `Extend<(A, B)>` impl

After looking over the examples in my last PR (#85835) on doc.rust-lang.org/nightly I realized that the example didn't actually show what I wanted it to show 😅
So here's the better example

add file_prefix method to std::path

This is an initial implementation of `std::path::Path::file_prefix`. It is effectively a "left" variant of the existing [`file_stem`](https://doc.rust-lang.org/std/path/struct.Path.html#method.file_stem) method. An illustration of the difference is

```rust
use std::path::Path;

let path = Path::new("foo.tar.gz");
assert_eq!(path.file_stem(), Some("foo.tar"));
assert_eq!(path.file_prefix(), Some("foo"));
```

In my own development, I generally find I almost always want the prefix, rather than the stem, so I thought it might be best to suggest it's addition to libstd.

Of course, as this is my first contribution, I expect there is probably more work that needs to be done. Additionally, if the libstd team feel this isn't appropriate then so be it.

There has been some [discussion about this on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/file_lstem/near/238076313) and a user there suggested I open a PR to see whether someone in the libstd team thinks it is worth pursuing.

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

Rollup of 13 pull requests

Successful merges:

 - #87604 (CI: Verify commits in beta & stable are in upstream branches.)
 - #88057 (Update RELEASES to clarify attribute macro values.)
 - #88072 (Allow the iOS toolchain to be built on Linux)
 - #88170 (Update release note for 1.55.0.)
 - #88172 (Test that type alias impl trait happens in a submodule)
 - #88179 (Mailmap entry for myself)
 - #88182 (We meant to use a trait instead of lifetime here)
 - #88183 (test TAIT in different positions)
 - #88189 (Add TAIT struct test)
 - #88192 (Use of impl trait in an impl as the value for an associated type in a dyn)
 - #88194 (Test use of impl Trait in an impl as the value for an associated type in an impl trait)
 - #88197 (Test tait use in a fn type)
 - #88201 (Test that incomplete inference for TAITs fail)

Failed merges:

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

Test that incomplete inference for TAITs fail

r? `@oli-obk`

Related to #86727

Test tait use in a fn type

r? `@oli-obk`

I thought this was going to work but doesn't, quickly checked with Niko and he told me that we ruled this out for now. I'm not exactly sure why and how but here we have a test with a FIXME :)

Related to #86727

Test use of impl Trait in an impl as the value for an associated type in an impl trait

r? `@oli-obk`

Related to #86727

Use of impl trait in an impl as the value for an associated type in a dyn

r? `@oli-obk`

Related to #86727