Continuation of rust-lang#94376.

cc rust-lang#53667

* Recover from invalid `'label: ` before block.
* Make suggestion to enclose statements in a block multipart.
* Point at `match`, `while`, `loop` and `unsafe` keywords when failing
  to parse their expression.
* Do not suggest `{ ; }`.
* Do not suggest `|` when very unlikely to be what was wanted (in `let`
  statements).

fix typo in btree/vec doc: Self -> self

this pr fixes rust-lang#92345
the documentation refers to the object the method is called for, not the type, so it should be using the lower case self.

Tweak diagnostics

* Recover from invalid `'label: ` before block.
* Make suggestion to enclose statements in a block multipart.
* Point at `match`, `while`, `loop` and `unsafe` keywords when failing
  to parse their expression. (Fix rust-lang#92705.)
* Do not suggest `{ ; }`.
* Do not suggest `|` when very unlikely to be what was wanted (in `let`
  statements).

…delay, r=petrochenkov

Fix ICE when passing block to while-loop condition

We were incorrectly delaying a bug when we passed _any_ block (that evaluated to `()`) to a while loop. This PR makes the check a bit more sophisticated.

We should only suppress the error here in cases that are equivalent to those we find in rust-lang#93574 (i.e. only while loop conditions that have destructuring assignment expressions in them).

Fixes rust-lang#93997
cc `@estebank` who added this code

I would not be opposed to removing the delay-bug altogether, and just emitting this error always. I much prefer duplicate errors over no errors.

Fix ICE when using Box<T, A> with large A

A sequel to rust-lang#94043 that fixes rust-lang#81270 and rust-lang#92054 (duplicate).

4 - Make more use of `let_chains`

Continuation of rust-lang#94376.

cc rust-lang#53667

…=Urgau

Add long explanation for E0726

This is the cleaned up version of rust-lang#87655 with the missing fixes.

Part of rust-lang#61137.

r? `@Urgau`