Address rust-lang#106096.

Instead of

```
LL |    fn oom(
   |  __^
   | | _|
   | ||
LL | || ) {
   | ||_-
LL | |  }
   | |__^
```

emit

```
LL | // fn oom(
LL | || ) {
   | ||_-
LL | |  }
   | |__^
   ```

Fix rust-lang#56036, fix rust-lang#76589.

…illaumeGomez

docs/test: add UI test and long-form error docs for `E0461`

Might take a couple of tries to pass CI. The UI test is x86-linux only; I'm not sure how to generalize it to other architectures.

r? `@GuillaumeGomez`

…piler-errors

Suggest `impl Iterator` when possible for `_` return type

Address rust-lang#106096.

…hods, r=jackh726

Deduplicate `op` methods

There are some operator-checking flavored methods in `FnCtxt` that can be deduplicated.

… r=estebank

Recover `fn` keyword as `Fn` trait in bounds

`impl fn()` -> `impl Fn()`

Fixes rust-lang#82515

…jackh726

Account for multiple multiline spans with empty padding

Instead of

```
LL |    fn oom(
   |  __^
   | | _|
   | ||
LL | || ) {
   | ||_-
LL | |  }
   | |__^
```

emit

```
LL | // fn oom(
LL | || ) {
   | ||_-
LL | |  }
   | |__^
   ```

…ound, r=GuillaumeGomez

rustdoc: combine common sidebar background color CSS rules

…, r=compiler-errors

Silence knock-down errors on `[type error]` bindings

Fix rust-lang#56036, fix rust-lang#76589.