Stabilize std::path::Path::ancestors

Closes #48581

r? @BurntSushi

Improve memoization and refactor NLL type check

I have a big branch that is refactoring NLL type check with the goal of introducing canonicalization-based memoization for all of the operations it does. This PR contains an initial prefix of that branch which, I believe, stands alone. It does introduce a few smaller optimizations of its own:

- Skip operations that are trivially a no-op
- Cache the results of the dropck-outlives computations done by liveness
- Skip resetting unifications if nothing changed

r? @pnkfelix

Add existential type definitions

Note: this does not allow creating named existential types, it just desugars `impl Trait` to a less (but still very) hacky version of actual `existential type` items.

r? @nikomatsakis

Fixed minor error in Compiler Error Index, E0603.

impl Hash for !

This was missing in some generic code I was writing and I figured that it would be worthwhile to add this. Blanket-requiring these traits to allow usage of errors in `HashSet`s and `HashMap`s isn't too unreasonable of a use case, and a prerequisite for allowing `!` as an error in such situations is this impl.

libstd: add an RAII utility for sys_common::mutex::Mutex

It is indeed debatable whether or not we should introduce more sophistication like this to the lowest layer of a system library. In fact, `Drop::drop()` cannot be `unsafe` (IIRC there was a discussion on introducing an unsafe variant of `Drop` whose entire scope must be within `unsafe`)

Removed two unused variables in os.rs

Issue #51419 suggested removing two unused variables in `libstd/sys/redox/os.rs`. This PR implements that change.

It compiles for me locally, but I haven't run any other tests.

Minor language change to CONTRIBUTING.md

In the `#building` section, the doc read that a default configuration "shall use around 3.5 GB of disk space." Changed "shall use" to "requires," as I don't think it's meant to sound so prescriptive.

Follow up to #51508, make parse_block public instead parse_block_expr

This is an follow up to #51508

I mistakenly made parse_block_expr public instead of parse_block.
This fixes this.

test: Ignore some problematic tests on powerpc and powerpc64*

This updates the list of tests which can be safely ignored on powerpc and powerpc64*.

refactor: create multiple HIR items for imports

When lowering `use` statements into HIR, they get a `Def` of the thing they're pointing at. This is great for things that need to know what was just pulled into scope. However, this is a bit misleading, because a `use` statement can pull things from multiple namespaces if their names collide. This is a problem for rustdoc, because if there are a module and a function with the same name (for example) then it will only document the module import, because that's that the lowered `use` statement points to.

The current version of this PR does the following:

* Whenever the resolver comes across a `use` statement, it loads the definitions into a new `import_map` instead of the existing `def_map`. This keeps the resolutions per-namespace so that all the target definitions are available.
* When lowering `use` statements, it looks up the resolutions in the `import_map` and creates multiple `Item`s if there is more than one resolution.
* To ensure the `NodeId`s are properly tracked in the lowered module, they need to be created in the AST, and pulled out as needed if multiple resolutions are available.

Fixes https://github.com/rust-lang/rust/issues/34843

Add Ref/RefMut map_split method

As proposed [here](https://internals.rust-lang.org/t/make-refcell-support-slice-splitting/7707).

TLDR: Add a `map_split` method that allows multiple `RefMut`s to exist simultaneously so long as they refer to non-overlapping regions of the original `RefCell`. This is useful for things like the slice `split_at_mut` method.

Signed-off-by: NNODA, Kai <nodakai@gmail.com>

Add lint for intra link resolution failure

This PR is almost done, just remains this note:

```
note: requested on the command line with `-W intra-link-resolution-failure`
```

I have no idea why my lint is considered as being passed through command line and wasn't able to find where it was set. If anyone has an idea, it'd be very helpful!

cc @QuietMisdreavus

Prevent Windows filesystem races in bootstrap tests

Fixes #51595.

This also makes bootstrap tests run near last in `./x.py test` invocations
since they are unlikely to fail.

r? @petrochenkov

rustc_codegen_llvm: don't treat i1 as signed, even for #[repr(i8)] enums.

Fixes #51582. r? @nagisa cc @nox @oli-obk

Fix my comment on editions

me = idiot

r? @oli-obk

rustdoc: process cross-crate glob re-exports

Turns out, we were deliberately ignoring glob re-exports when they were occurring across crates, even though we were fully processing them for local re-exports. This will at least bring the two into parity.

Fixes https://github.com/rust-lang/rust/issues/51252

Since they are unlikely to fail and are almost never going to fail
except with bootstrap changes (which would be tested locally anyway) it
makes sense to run these tests close to last.

Remove `?` macro separator compatibility note from 1.27 release notes

The implementation has been reverted in https://github.com/rust-lang/rust/pull/51417, so this no longer applies to 1.27.0.

r? @Mark-Simulacrum

The implementation has been reverted.

Stabilize #[repr(transparent)]

Tracking issue FCP: https://github.com/rust-lang/rust/issues/43036#issuecomment-394094318
Reference PR: https://github.com/rust-lang-nursery/reference/pull/353

rustc: rename ty::maps to ty::query.

Should've never been `maps` but "query system/engine" didn't fully settle from the start.

r? @michaelwoerister or @nikomatsakis

Keyword doc

Part of #51451.

r? @QuietMisdreavus

Follow up on https://github.com/rust-lang/rust/pull/51508, make parse_block public instead parse_block_expr

This is an follow up to #51508

I mistakenly made parse_block_expr public instead of parse_block.
This fixes this.

Speed up obligation forest code

Here are the rustc-perf benchmarks that get at least a 1% speedup on one or more of their runs with these patches applied:
```
inflate-check
        avg: -8.7%      min: -12.1%     max: 0.0%
inflate
        avg: -5.9%      min: -8.6%      max: 1.1%
inflate-opt
        avg: -1.5%      min: -2.0%      max: -0.3%
clap-rs-check
        avg: -0.6%      min: -1.9%      max: 0.5%
coercions
        avg: -0.2%?     min: -1.3%?     max: 0.6%?
serde-opt
        avg: -0.6%      min: -1.0%      max: 0.1%
coercions-check
        avg: -0.4%?     min: -1.0%?     max: -0.0%?
```

Refactor: Rename ExistentialPredicate::cmp to ExistentialPredicate::stable_cmp

See https://github.com/rust-lang/rust/pull/51276#discussion_r193549404 for
rationale.

Because stable_cmp takes three arguments and Ord::cmp takes two, I am confident that there is no shadowing happening here.

r? @nikomatsakis

Add error message for using >= 65535 hashes for raw string literal escapes

Fixes #50111.

Fix the recent spurious breakage on AppVeyor.

Fixed the spurious error introduced by d2b5b760 due to a wrongly resolved relative path on AppVeyor.

This only starts to happen today because we just entered the last week of the 6-week cycle.