This adds a new lint to `rustc` that is used in rustdoc when a code
block is empty or cannot be parsed as valid Rust code.

Previously this was unconditionally a warning. As such some
documentation comments were (unknowingly) abusing this to pass despite
the `-Dwarnings` used when compiling `rustc`, this should not be the
case anymore.

This also gives a better error message when a span is missing.

- Simplify boolean expression
- Give an example of invalid syntax
- Remove explanation of why code block is text

missing space in "rootseparator"

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

The coverage body_span doesn't always match the function body_span.

Some code cleanup extracted from future (but unfinished) commit to fix
coverage in attr macro functions.

I ran into an error trying to fix dead block coverage and realized the
`coverageinfo` query is getting a different MIR compared to the
codegenned MIR, which can sometimes be a problem during mapgen.

I changed that query to use the `InstandeDef` (which includes the
generic parameter substitutions, prosibly specific to const params)
instead of the `DefId` (without unknown/default const substitutions).

When having the order

```
foo.bar(); // we can now use this method since i32 implements the Foo trait

[...]

impl Foo for i32
```

the `// we can now use this method` comment is less clear to me.

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

…meGomez

rustdoc: Remove unnecessary `StripItem` wrapper

…pider

rustdoc: Make "rust code block is empty" and "could not parse code block" warnings a lint (`INVALID_RUST_CODEBLOCKS`)

Fixes rust-lang#79792. This already went through FCP in rust-lang#79816, so it only needs final review.

This is mostly a rebase of rust-lang#79816 - thank you ``@poliorcetics`` for doing most of the work!

…ennytm

Fix typo in comment

missing space in "rootseparator"

…ks, r=tmandry

coverage bug fixes and some refactoring

This replaces the relevant commits (2 and 3) from PR rust-lang#85082, and also corrects an error querying for coverageinfo.

1. `coverageinfo` query needs to use the same MIR as codegen

I ran into an error trying to fix dead block coverage and realized the
`coverageinfo` query is getting a different MIR compared to the
codegenned MIR, which can sometimes be a problem during mapgen.

I changed that query to use the `InstandeDef` (which includes the
generic parameter substitutions, prosibly specific to const params)
instead of the `DefId` (without unknown/default const substitutions).

2. Simplified body_span and filtered span code

  Some code cleanup extracted from future (but unfinished) commit to fix
  coverage in attr macro functions.

3. Spanview needs the relevant body_span used for coverage

The coverage body_span doesn't always match the function body_span.

r? ``@tmandry``

…iplett

dbg macro: Discuss use in tests, and slightly clarify

As discussed in a tangent in rust-lang#82778.

I chose to use [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/) in the source text but I don't mind reformatting it.

…=petrochenkov

Miner code formatting

swap function order for better read flow

I was reading this error message for the first time.

I was a little bit confused when reading that part:
```
foo.bar(); // we can now use this method since i32 implements the Foo trait
```

At the time I was reading `// we can now use this method` I wasn't sure why. It only made sense when reading on. So swapping these parts results in a better read flow.

…play, r=notriddle

Fix display for "implementors" section

Just saw this problem when going through docs:

![Screenshot from 2021-05-13 15-20-52](https://user-images.githubusercontent.com/3050060/118131978-766fc180-b3ff-11eb-86a8-7f6d22afa675.png)

This fix puts it back to normal:

![Screenshot from 2021-05-13 15-23-29](https://user-images.githubusercontent.com/3050060/118132006-7e2f6600-b3ff-11eb-9985-025a7b7c5216.png)

You can see it on the `TryFrom` page for example.

r? ``@Nemo157``

Use my real name

…=jackh726

Improve match statements

…mobile, r=jsha

Fix toggle position on mobile

Before:

![Screenshot from 2021-05-14 14-21-27](https://user-images.githubusercontent.com/3050060/118276475-fe210300-b4c7-11eb-94f8-4e2a4e10d91e.png)
![Screenshot from 2021-05-14 14-21-30](https://user-images.githubusercontent.com/3050060/118276479-feb99980-b4c7-11eb-85db-40e9df6e9abd.png)

After:

![Screenshot from 2021-05-14 15-16-54](https://user-images.githubusercontent.com/3050060/118276494-0416e400-b4c8-11eb-9479-d447928cfa62.png)
![Screenshot from 2021-05-14 15-16-59](https://user-images.githubusercontent.com/3050060/118276498-0416e400-b4c8-11eb-99f6-894276c62dfc.png)

r? ``@jsha``

…=jsha

Fix eslint errors

I cherry-picked the two non-CI commits from rust-lang#85285.

r? ``@jsha``