Rollup of 4 pull requests

Successful merges:

 - #50177 (mark std::str::replace(,n) as #[must_use])
 - #50207 (Hash EntryKind::AssociatedConst const data)
 - #50214 (Js improvements)
 - #50219 (Added missing `.` in docs.)

Failed merges:

 - #50229 (Add setting to go to item if there is only one result)

Allow variant discriminant initializers to refer to other initializer…

…s of the same enum

r? @EddyB

fixes the 2.4 failure of https://github.com/rust-lang/rust/issues/49765

cc @durka @retep998

Added missing `.` in docs.

Js improvements

r? @QuietMisdreavus

Hash EntryKind::AssociatedConst const data

Related to #49991
r? @michaelwoerister
cc @EddyB

mark std::str::replace(,n) as #[must_use]

let x = "a b c c";
x.replacen("c", "d", 2");
might not do what people might think it does.

Make incremental compilation thread-safe

r? @michaelwoerister

Update rustfmt and rls

r? @alexcrichton

Allow crate:: in local paths

Currently if you want to use `crate` locally you have to do `::crate::`. This shouldn't be necessary

(will fix up tests later)

r? @petrochenkov

Make Binder's field private and clean up its usage

AKA "tour de rustc"

Closes #49814.

Introduce compile-pass

r? @alexcrichton

The plan is to move things that cannot fail (no assert, unwrap, etc) out so we don't have to run them, and in the long term we can also stop running LLVM for them.

Out of 3215 tests...

```
Language            Files        Lines         Code     Comments       Blanks
Rust                 3215       119254        64688        35135        19431
```

16% of them has an empty main (which is already moved in this PR).

```
grep -rnPzl 'fn main\(\)\s*{\s*}' | xargs rg --files-without-match cfg | wc -l
547
```

And only 50% of the tests contains assertions:

```
rg -e assert -e unwrap -e expect -e panic -l | wc -l
1600
```

The remainder is likely able to get moved, but they need check by a human so I didn't touch them in PR.

cc @rust-lang/compiler

* [ ] Update documentation

make rustdoc test follow the jobserver limit of threads

fix that to many threads is executing at the same time
when rustdoc test is executed.

Warn on all erroneous constants

fixes #49791
fixes #47054

@Zoxc this PR triggers the nondeterministic errors of https://github.com/rust-lang/rust/pull/49950#issuecomment-383074959 really often (at least on stage1).

Speed up `nearest_common_ancestor`.

`nearest_common_ancestor` can be made faster.

Here are all the benchmarks where one of the measurements improved by at least 1%.
```
clap-rs-check
	avg: -4.5%	min: -8.8%	max: -0.3%
clap-rs
	avg: -2.6%	min: -4.5%	max: 0.5%
script-servo
	avg: -1.7%	min: -3.6%	max: 0.0%
regression-31157
	avg: -1.5%	min: -2.6%	max: -0.4%
hyper
	avg: -1.2%	min: -2.5%	max: -0.0%
piston-image
	avg: -1.6%	min: -2.5%	max: 0.1%
regex
	avg: -1.2%	min: -2.2%	max: 0.0%
issue-46449
	avg: -1.8%	min: -2.1%	max: -0.7%
crates.io
	avg: -1.2%	min: -2.1%	max: 0.0%
hyper-check
	avg: -1.0%	min: -2.1%	max: -0.1%
clap-rs-opt
	avg: -1.4%	min: -2.0%	max: -0.3%
piston-image-check
	avg: -1.2%	min: -1.9%	max: -0.1%
regex-check
	avg: -0.5%	min: -1.8%	max: -0.1%
syn
	avg: -1.1%	min: -1.7%	max: -0.1%
tokio-webpush-simple-check
	avg: -1.1%	min: -1.6%	max: -0.3%
tokio-webpush-simple
	avg: -1.2%	min: -1.6%	max: -0.0%
helloworld-check
	avg: -1.4%	min: -1.6%	max: -1.2%
deeply-nested
	avg: -1.2%	min: -1.4%	max: -0.8%
encoding-check
	avg: -0.8%	min: -1.3%	max: -0.3%
unify-linearly-check
	avg: -1.0%	min: -1.3%	max: -0.8%
script-servo-check
	avg: -0.6%	min: -1.3%	max: 0.0%
regression-31157-check
	avg: -0.9%	min: -1.2%	max: -0.7%
script-servo-opt
	avg: -0.5%	min: -1.2%	max: 0.1%
deeply-nested-check
	avg: -0.8%	min: -1.2%	max: -0.7%
encoding
	avg: -0.7%	min: -1.1%	max: -0.3%
issue-46449-check
	avg: -0.9%	min: -1.1%	max: -0.6%
parser-check
	avg: -0.9%	min: -1.1%	max: -0.8%
html5ever
	avg: -0.5%	min: -1.0%	max: -0.0%
```

Edition breakage lint for absolute paths starting with modules

We plan to enable `extern_absolute_paths` in the 2018 edition. To allow for that, folks must transition their paths in a previous edition to the new one. This makes paths which import module contents via `use module::` or `::module::` obsolete, and we must edition-lint these.

https://internals.rust-lang.org/t/the-great-module-adventure-continues/6678/205?u=manishearth is the current plan for paths.

r? @nikomatsakis

Fixes #48722

Related to #49991

Fixes #20664.

Provide better names for builtin deriving-generated attributes

First attempt at fixing #49967

Not in love with any choices here, don't be shy if you aren't happy with anything :)

I've tested that this produces nicer names in documentation, and that it no longer has issues conflicting with constants with the same name. (I guess we _could_ make a test for that... unsure if that would be valuable)

In all cases I took the names from the methods as declared in the relevant trait.

In some cases I had to prepend the names with _ otherwise there were errors about un-used variables. I'm uneasy with the inconsistency... do they all need to be like that? Is there a way to generate an alternate impl or use a different name (`_`?) in the cases where the arguments are not used?

Lastly the gensym addition to Ident I implemented largely as suggested, but I want to point out it's a little circuitous (at least, as far as I understand it). `cx.ident_of(name)` is just `Ident::from_str`, so we create an Ident then another Ident from it. `Ident::with_empty_ctxt(Symbol::gensym(string))` may or may not be equivalent, I don't know if it's important to intern it _then_ gensym it. It seems like either we could use that, or if we do want a new method to make this convenient, it could be on Ident instead (`from_str_gensymed`?)