Fix rust-lang#82524.

…f the value is provided or not

See @JohnTitor's [comment](rust-lang#82720 (comment))

@rustbot label +C-cleanup

…ake transition over hir::ItemKind simpler

…, r=jyn514

Make ItemKind::ExternCrate looks like hir::ItemKind::ExternCrate to make transition over hir::ItemKind simpler

It was surprisingly difficult to make this change, mostly because of two issues:

* We now store the `ExternCrate` name in the parent struct (`clean::Item`), which forced me to modify the json conversion code a bit more than expected.
* The second problem was that, since we now have a `Some(name)`, it was trying to render it, ending up in a panic because we ended up in a `unreachable` statement. The solution was simply to add `!item.is_extern_crate()` in `formats::renderer` before calling `cx.item(item, &cache)?;`.

I'll continue to replace all the `clean::ItemKind` variants one by one until it looks exactly like `hir::ItemKind`. Then we'll simply discard the rustdoc type. Once this done, we'll be able to discard `clean::Item` too to use `hir::Item`.

r? `@jyn514`

… r=Manishearth

Warn on `#![doc(test(...))]` on items other than the crate root and use future incompatible lint

Part of rust-lang#82672.

This PR does multiple things:
 * Create a new `INVALID_DOC_ATTRIBUTE` lint which is also "future incompatible", allowing us to use it as a warning for the moment until it turns (eventually) into a hard error.
 * Use this link when `#![doc(test(...))]` isn't used at the crate level.
 * Make rust-lang#82702 use this new lint as well.

r? `@jyn514`

Detect match arm body without braces

Fix rust-lang#82524.

…es, r=oli-obk

Bump optimization from mir_opt_level 2 to 3 and 3 to 4 and make "release" be level 2 by default

r? `@oli-obk`

…anewok

Update tests names to start with `issue-`

See `@JohnTitor's` [comment](rust-lang#82720 (comment))

`@rustbot` label +C-cleanup