* Remove the `float: right` fallback from the main header, which hasn't
  been needed since IE11 support was dropped.

* Remove `in-band` from low-level headers, which hasn't been needed since
  `.rightside` switched to `float: right` in
  593d6d1

* Remove unreachable `.in-band > code, .in-band > .code-header` CSS, since
  the `in-band` class was attached to the `code-header` itself, not nested
  directly below it.

* Use `rem` instead of `em` for code header margins.

* This results in a slight change in spacing around impls and item-info,
  but since it makes it more consistent with the way methods are presented,
  it's probably fine.

…, r=davidtwco

Clean up (sub)diagnostic derives

The biggest chunk of this is unifying the parsing of subdiagnostic attributes (`#[error]`, `#[suggestion(...)]`, `#[label(...)]`, etc) between `Subdiagnostic` and `Diagnostic` type attributes as well as `Diagnostic` field attributes.

It also improves a number of proc macro diagnostics.

Waiting for rust-lang#101558.

…gillot

Only generate closure def id for async fns with body

Fixes rust-lang#102219

…example, r=thomcc

Clarify Iterator::rposition code example

Fixes rust-lang#101095.

r? `@thomcc`

…umeGomez

rustdoc: clean up `.out-of-band`/`.in-band` CSS

|        | method | impl
|--------|--------|------
| before | ![image](https://user-images.githubusercontent.com/1593513/192164603-dea9befb-0f5f-4bd5-a44d-1f8328e27955.png) | ![image](https://user-images.githubusercontent.com/1593513/192164642-06f958cb-8fd5-4b73-bdb7-d2778f358f80.png)
| after  | ![image](https://user-images.githubusercontent.com/1593513/192164612-f72ee8db-c87c-477c-99e8-283b1cf0c14b.png) | ![image](https://user-images.githubusercontent.com/1593513/192164653-8e628dc3-5bf3-4ce8-829b-06e27a90fe06.png)

* Remove the `float: right` fallback from the main header, which hasn't been needed since IE11 support was dropped.

* Remove `in-band` from low-level headers, which hasn't been needed since `.rightside` switched to `float: right` in rust-lang@593d6d1

* Remove unreachable `.in-band > code, .in-band > .code-header` CSS, since the `in-band` class was attached to the `code-header` itself, not nested directly below it.

* Use `rem` instead of `em` for code header margins.

* This results in a slight change in spacing around impls and item-info, but since it makes it more consistent with the way methods are presented, it's probably fine.

Preview: http://notriddle.com/notriddle-rustdoc-demos/band/std/fs/struct.File.html