Refactor passes and pass execution to be more parallel

For `syntex_syntax` (with 16 threads and 8 cores):
- Cuts `misc checking 1` from `0.096s` to `0.08325s`.
- Cuts `misc checking 2` from `0.3575s` to `0.2545s`.
- Cuts `misc checking 3` from `0.34625s` to `0.21375s`.
- Cuts `wf checking` from `0.3085s` to `0.05025s`.

Reduces overall execution time for `syntex_syntax` (with 8 threads and cores) from `4.92s` to `4.34s`.

Subsumes https://github.com/rust-lang/rust/pull/58494
Blocked on https://github.com/rust-lang/rust/pull/58250

r? @michaelwoerister

look for python2 symlinks before bootstrap python

Before this commit, if you're running x.py directly on a system where
`python` is symlinked to Python 3, then the `python` config option will
default to a Python 3 interpreter. This causes debuginfo tests to fail
with an opaque error message, since they have a hard requirement on
Python 2.

This commit modifies the Python probe behavior to look for python2.7 and
python2 *before* using the interpreter used to execute `x.py`.

fixes rust-lang#52482

MaybeUninit: add read_initialized, add examples

This adds a new `read_initialized` method, similar to suggestions by @Amanieu and @shepmaster. I also added examples to this and other methods.

rust-lldb: fix crash when printing empty string

Fixes #52185.

~Re-enables the pretty-std debuginfo test and tweaks the test as necessary to get it to pass again. This reveals that lldb's formatting of enums is broken (#58492). I also removed the emoji from the test because I couldn't get the docker image's gdb to print the emoji, just octal escapes (https://github.com/rust-lang/rust/pull/53154/files#r208263904).~

rustdoc: add option to calculate "documentation coverage"

This PR adds a new flag to rustdoc, `--show-coverage`. When passed, this flag will make rustdoc count the number of items in a crate with documentation instead of generating docs. This count will be output as a table of each file in the crate, like this (when run on my crate `egg-mode`):

```
+-------------------------------------+------------+------------+------------+
| File                                | Documented |      Total | Percentage |
+-------------------------------------+------------+------------+------------+
| src/auth.rs                         |         16 |         16 |     100.0% |
| src/common/mod.rs                   |          1 |          1 |     100.0% |
| src/common/response.rs              |          9 |          9 |     100.0% |
| src/cursor.rs                       |         24 |         24 |     100.0% |
| src/direct/fun.rs                   |          6 |          6 |     100.0% |
| src/direct/mod.rs                   |         41 |         41 |     100.0% |
| src/entities.rs                     |         50 |         50 |     100.0% |
| src/error.rs                        |         27 |         27 |     100.0% |
| src/lib.rs                          |          1 |          1 |     100.0% |
| src/list/fun.rs                     |         19 |         19 |     100.0% |
| src/list/mod.rs                     |         22 |         22 |     100.0% |
| src/media/mod.rs                    |         27 |         27 |     100.0% |
| src/place/fun.rs                    |          8 |          8 |     100.0% |
| src/place/mod.rs                    |         35 |         35 |     100.0% |
| src/search.rs                       |         26 |         26 |     100.0% |
| src/service.rs                      |         74 |         74 |     100.0% |
| src/stream/mod.rs                   |         49 |         49 |     100.0% |
| src/tweet/fun.rs                    |         15 |         15 |     100.0% |
| src/tweet/mod.rs                    |         73 |         73 |     100.0% |
| src/user/fun.rs                     |         24 |         24 |     100.0% |
| src/user/mod.rs                     |         87 |         87 |     100.0% |
+-------------------------------------+------------+------------+------------+
| Total                               |        634 |        634 |     100.0% |
+-------------------------------------+------------+------------+------------+
```

Trait implementations are not counted because by default they "inherit" the docs from the trait, even though an impl can override those docs. Similarly, inherent impl blocks are not counted at all, because for the majority of cases such docs are not useful. (The usual pattern for inherent impl blocks is to throw all the methods on a type into a single impl block. Any docs you would put on that block would be better served on the type itself.)

In addition, `--show-coverage` can be combined with `--document-private-items` to get the coverage counts for everything in the crate, not just public items.

The coverage calculation is implemented as a late pass and two new sets of passes which strip out most of the work that rustdoc otherwise does when generating docs. The is because after the new pass is executed, rustdoc immediately closes instead of going on to generate documentation.

Many examples of coverage calculations have been included as `rustdoc-ui` tests.

r? @rust-lang/rustdoc

Use early unwraps instead of bubbling up errors just to unwrap in the end

r? @RalfJung

overhaul unused doc comments lint

This PR contains a number of improvements to the `unused_doc_comments` lint.

- Extends the span to cover the entire comment when using sugared doc comments.
- Triggers the lint for all unused doc comments on a node, instead of just the first one.
- Triggers the lint on macro expansions, and provides a help note explaining that doc comments must be expanded by the macro.
- Adds a label pointing at the node that cannot be documented.

Furthermore, this PR fixes any instances in rustc where a macro expansion was erroneously documented.

Rollup of 24 pull requests

Successful merges:

 - #58080 (Add FreeBSD armv6 and armv7 targets)
 - #58204 (On return type `impl Trait` for block with no expr point at last semi)
 - #58269 (Add librustc and libsyntax to rust-src distribution.)
 - #58369 (Make the Entry API of HashMap<K, V> Sync and Send)
 - #58861 (Expand where negative supertrait specific error is shown)
 - #58877 (Suggest removal of `&` when borrowing macro and appropriate)
 - #58883 (Suggest appropriate code for unused field when destructuring pattern)
 - #58891 (Remove stray ` in the docs for the FromIterator implementation for Option)
 - #58893 (race condition in thread local storage example)
 - #58906 (Monomorphize generator field types for debuginfo)
 - #58911 (Regression test for #58435.)
 - #58912 (Regression test for #58813)
 - #58916 (Fix release note problems noticed after merging.)
 - #58918 (Regression test added for an async ICE.)
 - #58921 (Add an explicit test for issue #50582)
 - #58926 (Make the lifetime parameters of tcx consistent.)
 - #58931 (Elide invalid method receiver error when it contains TyErr)
 - #58940 (Remove JSBackend from config.toml)
 - #58950 (Add self to mailmap)
 - #58961 (On incorrect cfg literal/identifier, point at the right span)
 - #58963 (libstd: implement Error::source for io::Error)
 - #58970 (delay_span_bug in wfcheck's ty.lift_to_tcx unwrap)
 - #58984 (Teach `-Z treat-err-as-bug` to take a number of errors to emit)
 - #59007 (Add a test for invalid const arguments)

Failed merges:

 - #58959 (Add release notes for PR #56243)

r? @ghost

Update cargo, rls, books

## cargo

10 commits in 5c6aa46e6f28661270979696e7b4c2f0dff8628f..95b45eca19ac785263fed98ecefe540bb47337ac
2019-02-22 19:32:35 +0000 to 2019-03-06 19:24:30 +0000
- Relax some rustdoc tests. (rust-lang/cargo#6721)
- Include build script execution in the fingerprint. (rust-lang/cargo#6720)
- part of the infrastructure for public & private dependencies in the resolver (rust-lang/cargo#6653)
- Bump to 0.36.0 (rust-lang/cargo#6718)
- Some test/bench-related tweaks (rust-lang/cargo#6707)
- Fix links to the permanent home of the edition guide. (rust-lang/cargo#6703)
- HTTPS all the things (rust-lang/cargo#6614)
- Cargo test quicker by not building untested examples when filtered (rust-lang/cargo#6683)
- Link from ARCHITECTURE.md to docs.rs and github (rust-lang/cargo#6695)
- Update how to install rustfmt (rust-lang/cargo#6696)

## rls

9 commits in 0d6f53e1a4adbaf7d83cdc0cb54720203fcb522e..6a1b5a9cfda2ae19372e0613e76ebefba36edcf5
2019-02-14 07:52:15 +0000 to 2019-03-04 20:24:45 +0000
- Update cargo and clippy. (rust-lang-nursery/rls#1388)
- catch up rust-lang/rust PR#58321 (rust-lang-nursery/rls#1384)
- Apply Clippy fixes (rust-lang-nursery/rls#1327)
- Various cosmetic improvements (rust-lang-nursery/rls#1326)
- Make test `RlsHandle` transport-agnostic (rust-lang-nursery/rls#1317)
- Translate remaining tests (rust-lang-nursery/rls#1320)
- Remove unnecessary #![feature]s (rust-lang-nursery/rls#1323)
- Update Clippy (rust-lang-nursery/rls#1319)
- Update Clippy (rust-lang-nursery/rls#1315)

cc @xanewok

## Books
See #58936 for details.

Fix segfaults in release build C-variadic fns

`va_start` and `va_end` must be called to initialize/cleanup the
"spoofed" `VaList` in a Rust defined C-variadic function even  if
the `VaList` is not used.

r? @alexreg
Fixes: #58980

Report the diagnostic on macro expansions, and add a label indicating
why the comment is unused.

Report all unused attributes on a given doc comment instead of just the
first one, and extend the span of sugared doc comments to encompass the
whole comment.

HirIdification: almost there

The next iteration of HirIdification (#57578).

Replaces a bunch of `NodeId` method calls (mostly `as_local_node_id`) with `HirId` ones.

Removes `NodeId` from:
- [x] `PathSegment`
- [x] `PatKind`
- [x] `Destination` (replaces it with `HirId`)

In addition this PR also removes `Visitor::visit_def_mention`, which doesn't seem to be doing anything.

Add a test for invalid const arguments

Closes https://github.com/rust-lang/rust/issues/58811.

Teach `-Z treat-err-as-bug` to take a number of errors to emit

`-Z treat-err-as-bug` will cause `rustc` to panic after the first error is reported, like previously. `-Z treat-err-as-bug=2` will cause `rustc` to panic after 2 errors have been reported.

Fix #58983.

delay_span_bug in wfcheck's ty.lift_to_tcx unwrap

Fix #58158

libstd: implement Error::source for io::Error

On incorrect cfg literal/identifier, point at the right span

CC #58462

Add self to mailmap

Remove JSBackend from config.toml

JSBackend is implied when building the emscripten backend, and not available for the standard llvm backend.  This commit also puts the example config in sync with the defaults in src/bootstrap/native.rs

Elide invalid method receiver error when it contains TyErr

Fix #58712.

Make the lifetime parameters of tcx consistent.

I have implemented `codegen_allocator` for my backend, but I've had to make a small change to its signature in `ExtraBackendMethods`. I wonder if this change is justified, or if it is too specific to my use case to be useful to anyone else.

`write_metadata` and `codegen_allocator` are both called from `codegen_crate` (in `librustc_codegen_ssa/base.rs`), and they both receive the same `tcx` as an argument:

https://github.com/rust-lang/rust/blob/c196097e588b05e86b5ce6de992b2a6e6a7027bd/src/librustc_codegen_ssa/base.rs#L555-L557

and:

https://github.com/rust-lang/rust/blob/c196097e588b05e86b5ce6de992b2a6e6a7027bd/src/librustc_codegen_ssa/base.rs#L640-L642

However, `codegen_allocator` accepts a `TyCtxt` with any lifetime parameters (`tcx: TyCtxt<'_, '_, '_>`), while `write_metadata` requires that the `tcx` argument is of type `TyCtxt<'b, 'gcx, 'gcx>`. In my implementation, I've found that it's necessary for `tcx` in `codegen_allocator` to also have the `<'b, 'gcx, 'gcx>` lifetime parameters.

Have I misunderstood the purpose of the parameters of `TyCtxt`? I've read [here](https://rust-lang.github.io/rustc-guide/ty.html) that the last two parameters only need to be distinct if the function needs to be used during type inference, but I don't think that is the case here.

Add an explicit test for issue #50582

This code no longer ICEs, and @yodaldevoid found that it was fixed by
commit fe5710a2. While that added a similar test, we can explicitly test
this reproducer too.

Closes #50582.

Regression test added for an async ICE.

Regression test for #57084 (as suggested in issue).

Fix release note problems noticed after merging.

Cherry picked from stable branch.

Regression test for #58813

Fix #58813

Regression test for #58435.

Fix #58435

Monomorphize generator field types for debuginfo

Fixes #58888

r? @Zoxc

race condition in thread local storage example

The example had a potential race condition that would still pass the test.
If the thread which was supposed to modify it's own thread local was slower than the instruction to
modify in the main thread, then the test would pass even in case of a failure.
This is would be minor if the child thread was waited for since it check using an `assert_eq` for the
same thing, but vice versa.
However, if the `assert_eq` failed this would trigger a panic, which is not at all caught by the
example since the thread is not waited on.
Signed-off-by: Nbenaryorg <binary@benary.org>

Remove stray ` in the docs for the FromIterator implementation for Option

Suggest appropriate code for unused field when destructuring pattern

Fix #56472.

Suggest removal of `&` when borrowing macro and appropriate

Fix #58815.

Expand where negative supertrait specific error is shown

Fix #58857.

r? @petrochenkov

Make the Entry API of HashMap<K, V> Sync and Send

Fixes #45219

Add librustc and libsyntax to rust-src distribution.

Fixes #58268.

On return type `impl Trait` for block with no expr point at last semi

Partial solution, doesn't actually validate that the last statement in the function body can satisfy the trait bound, but it's a good incremental improvement over the status quo.

```
error[E0277]: the trait bound `(): Bar` is not satisfied
  --> $DIR/impl-trait-return-trailing-semicolon.rs:3:13
   |
LL | fn foo() -> impl Bar {
   |             ^^^^^^^^ the trait `Bar` is not implemented for `()`
LL |     5;
   |      - consider removing this semicolon
   |
   = note: the return type of a function must have a statically known size
```

Partially addresses #54771.