This patch makes it clear in std::process::Child::kill()'s API
documentation that an error is returned if the child process has
already cleanly exited.  This is implied by the example, but not
called out explicitly.

Add documentation links to the original type for various OS-specific
extension traits and normalize the language for introducing such traits.
Also, remove some outdated comments around the extension trait
definitions.

Per FCP:

* rust-lang#27802 (comment)
* rust-lang#33906 (comment)

The unstable-feature attribute requires an issue (neglecting it is
E0547), which gets used in the error messages. Unfortunately, there are
some cases where "0" is apparently used a placeholder where no issue
exists, directing the user to see the (nonexistent) issue #0. (It would
have been better to either let `issue` be optional—compare to how issue
is an `Option<u32>` in the feature-gate declarations in
libsyntax/feature-gate.rs—or actually require that an issue be created.)
Rather than endeavoring to change how `#[unstable]` works at this time
(given competing contributor and reviewer priorities), this simple patch
proposes the less-ambitious solution of just not adding the "(see
issue)" note when the number is zero.

Resolves rust-lang#49983.

Closes rust-lang#43751.

Fixes rust-lang#48878

Because it's faster than HashMap.

This change reduces the time taken for a few of the rustc-perf
benchmarks, mostly the small ones, by up to 5%.

…-Simulacrum

std: Child::kill() returns error if process has already exited

This patch makes it clear in std::process::Child::kill()'s API
documentation that an error is returned if the child process has
already cleanly exited.  This is implied by the example, but not
called out explicitly.

Add Cell::update

This commit adds a new method `Cell::update`, which applies a function to the value inside the cell.

Previously discussed in: rust-lang/rfcs#2171

### Motivation

Updating `Cell`s is currently a bit verbose. Here are several real examples (taken from rustc and crossbeam):

```rust
self.print_fuel.set(self.print_fuel.get() + 1);

self.diverges.set(self.diverges.get() | Diverges::Always);

let guard_count = self.guard_count.get();
self.guard_count.set(guard_count.checked_add(1).unwrap());
if guard_count == 0 {
    // ...
}
```

With the addition of the new method `Cell::update`, this code can be simplified to:

```rust
self.print_fuel.update(|x| x + 1);

self.diverges.update(|x| x | Diverges::Always);

if self.guard_count.update(|x| x.checked_add(1).unwrap()) == 1 {
    // ...
}
```

### Unresolved questions

1. Should we return the old value instead of the new value (like in `fetch_add` and `fetch_update`)?
2. Should the return type simply be `()`?
3. Naming: `update` vs `modify` vs `mutate` etc.

cc @SimonSapin

…akis

Fix revision support for UI tests.

Fixes rust-lang#48878

Add doc links to `std::os` extension traits

Addresses a small subset of rust-lang#29367.

This adds documentation links to the original type for various OS-specific extension traits, and uses a common sentence for introducing such traits (which now consistently ends in a period).

Stabilize `std::hint::unreachable_unchecked`.

Closes rust-lang#43751.

Deprecate Read::chars and char::decode_utf8

Per FCP:

* rust-lang#27802 (comment)
* rust-lang#33906 (comment)

don't see issue #0

The unstable-feature attribute requires an issue (neglecting it is
E0547), which gets used in the error messages. Unfortunately, there are
some cases where "0" is apparently used a placeholder where no issue
exists, directing the user to see the (nonexistent) issue #0. (It would
have been better to either let `issue` be optional—compare to how issue
is an `Option<u32>` in the feature-gate declarations in
libsyntax/feature-gate.rs—or actually require that an issue be created.)
Rather than endeavoring to change how `#[unstable]` works at this time
(given competing contributor and reviewer priorities), this simple patch
proposes the less-ambitious solution of just not adding the "(see
issue)" note when the number is zero.

Resolves rust-lang#49983.

…ietMisdreavus

fix search bar bug

Fixes rust-lang#50064.

r? @QuietMisdreavus

encourage descriptive issue titles

There are two sides to avoiding duplicate issues, searching for existing ones, and making sure existing ones can be searched for. This addresses the later.

r? @steveklabnik

…ichaelwoerister

Use FxHashMap in syntax_pos::symbol::Interner::intern.

Because it's faster than HashMap.

This change reduces the time taken for a few of the rustc-perf
benchmarks, mostly the small ones, by up to 5%.
```
coercions
        avg: -1.3%      min: -5.5%      max: -0.0%
helloworld-check
        avg: -2.3%      min: -3.5%      max: -1.8%
deeply-nested-check
        avg: -1.4%      min: -3.2%      max: -0.5%
tuple-stress-opt
        avg: -0.7%      min: -2.0%      max: -0.1%
unify-linearly-check
        avg: -1.2%      min: -1.9%      max: -0.6%
coercions-check
        avg: -0.8%      min: -1.3%      max: -0.4%
unused-warnings-check
        avg: -1.0%      min: -1.3%      max: -0.8%
deeply-nested-opt
        avg: -0.5%      min: -1.2%      max: -0.2%
deeply-nested
        avg: -0.7%      min: -1.2%      max: -0.4%
helloworld
        avg: -0.8%      min: -1.1%      max: -0.7%
tuple-stress-check
        avg: -0.5%      min: -1.0%      max: -0.1%
unused-warnings
        avg: -0.8%      min: -1.0%      max: -0.7%
unused-warnings-opt
        avg: -0.8%      min: -1.0%      max: -0.7%
coercions-opt
        avg: -0.5%      min: -1.0%      max: -0.1%
helloworld-opt
        avg: -0.7%      min: -1.0%      max: -0.6%
```

core: Fix overflow in `int::mod_euc` when `self < 0 && rhs == MIN`

This commit removes usage of `abs`, which overflows when `self == MIN`.