Remove vestigial diverging !-coercion

Such conversions are no longer permitted. Fixes #50350.

r? @nikomatsakis

Update stdsimd module

to include https://github.com/rust-lang-nursery/stdsimd/commit/a19ca1cd91cf97777af8268a6136bd2e4648e189

Rustdoc improvements

Fixes #50658. (last commit)

A lot of small improvements.

r? @QuietMisdreavus

Ensure derive(PartialOrd) is no longer accidentally exponential

Previously, two comparison operations would be generated for each field, each of which could delegate to another derived PartialOrd. Now we use ordering and optional chaining to ensure each pair of fields is only compared once, addressing https://github.com/rust-lang/rust/issues/49650#issuecomment-379467572.

Closes #49505.

r? @Manishearth (sorry for changing it again so soon!)

Close #50755

Update clippy

r? @Manishearth

@bors p=1

typeck: Save the index of private fields

Save the index of all fields regardless of their visibility. Problems
could occur later when attempting to index fields in error recovery if
they are not inserted.

Fixes: #50493

rustc: don't trip an assertion for enums with present but uninhabited variants.

Fixes #50731.

r? @nikomatsakis

Update stdsimd module to include https://github.com/rust-lang-nursery/stdsimd/commit/a19ca1cd91cf97777af8268a6136bd2e4648e189

stabilize macro_lifetime_matcher

This stabilizes `:lifetime` which has completed FCP in #34303.

Fix volatile_store and nontemporal_store

Fixes #50371.

update libcompiler_builtins

let's OpenBSD to use libcompiler_rt.a from system library. it unbreaks
the build from source on OpenBSD.

see https://github.com/rust-lang-nursery/compiler-builtins/pull/241 for related PR

note this PR brings some other changes (on `floatdisf`/`floatundisf`) with compiler-builtins update.

r? @alexcrichton

Fix self referential impl Trait substitutions

A high impact bug because a lot of common traits use a `Self` substitution by default. Should be backported to beta.

There was a check for this which wasn't catching all cases, it was made more robust.

Fixes #49376
Fixes #50626

r? @petrochenkov

Reduce span highlighted code in unused_variables lint

Fixes #50472
- [X] reduce var span
- [ ] mark applicable

Before:
```
mut unused_mut_var
^^^^^^^^^^^^^^^^^^
```
After:
```
mut unused_mut_var
    ^^^^^^^^^^^^^^
```

Save the index of all fields regardless of their visibility. Problems
could occur later when attempting to index fields in error recovery if
they are not inserted.

Macros: Add a 'literal' fragment specifier

See: https://github.com/rust-lang/rust/issues/35625

```rust

macro_rules! test_literal {
    ($l:literal) => {
        println!("literal: {}", $l);
    };
    ($e:expr) => {
        println!("expr: {}", $e);
    };
}

fn main() {
    let a = 1;
    test_literal!(a);
    test_literal!(2);
    test_literal!(-3);
}
```

Output:

```
expr: 1
literal: 2
literal: -3
```

ToDo:

* [x] Feature gate
* [x] Basic tests
* [x] Tests for range patterns
* [x] Tests for attributes
* [x] Documentation
* [x] Fix for `true`/`false`
* [x] Fix for negative number literals

Implements RFC 1576.

See: https://github.com/rust-lang/rfcs/blob/master/text/1576-macros-literal-matcher.md

Changes are mostly in libsyntax, docs, and tests. Feature gate is
enabled for 1.27.0.

Many thanks to Vadim Petrochenkov for following through code reviews
and suggestions.

Example:

````rust

macro_rules! test_literal {
    ($l:literal) => {
        println!("literal: {}", $l);
    };
    ($e:expr) => {
        println!("expr: {}", $e);
    };
}

fn main() {
    let a = 1;
    test_literal!(a);
    test_literal!(2);
    test_literal!(-3);
}
```

Output:

```
expr: 1
literal: 2
literal: -3
```

This reverts commit a8809711.

Rollup of 8 pull requests

Successful merges:

 - #50624 (fs::write: Add example writing a &str)
 - #50634 (Do not silently truncate offsets for `read_at`/`write_at` on emscripten)
 - #50644 (AppVeyor: Read back trace from crash dump on failure.)
 - #50661 (Ignore non .rs files for tidy libcoretest)
 - #50663 (rustc: Allow an edition's feature on that edition)
 - #50667 (rustc: Only suggest deleting `extern crate` if it works)
 - #50670 (rustc: Include semicolon when removing `extern crate`)
 - #50678 (Update openbsd targets)

Failed merges:

Update openbsd targets

- add a new target `aarch64-unknown-openbsd`
- update `i686-unknown-openbsd` to use lld with clang, in order to correctly link binaries with `i128`

rustc: Include semicolon when removing `extern crate`

Currently the lint for removing `extern crate` suggests removing `extern crate`
most of the time, but the rest of the time it suggest replacing it with `use
crate_name`. Unfortunately though when spliced into the original code you're
replacing

    extern crate foo;

with

    use foo

which is syntactically invalid! This commit ensure that the trailing semicolon
is included in rustc's suggestion to ensure that the code continues to compile
afterwards.

rustc: Only suggest deleting `extern crate` if it works

This commit updates one of the edition lints to only suggest deleting `extern
crate` if it actually works. Otherwise this can yield some confusing behavior
with rustfix specifically where if you accidentally deny the `rust_2018_idioms`
lint in the 2015 edition it's suggesting features that don't work!

rustc: Allow an edition's feature on that edition

This commit fixes a hard error where the `#![feature(rust_2018_preview)]`
feature was forbidden to be mentioned when the `--edition 2018` flag was passed.
This instead silently accepts that feature gate despite it not being necessary.
It's intended that this will help ease the transition into the 2018 edition as
users will, for the time being, start off with the `rust_2018_preview` feature
and no longer immediately need to remove it.

Closes #50662

Ignore non .rs files for tidy libcoretest

Previously, any file would be read, which is both unnecessary, and causes issues if irrelevant non-Unicode files were read (e.g. `.DS_STORE`).