Rollup of 7 pull requests

Successful merges:

 - rust-lang#132795 (Check `use<..>` in RPITIT for refinement)
 - rust-lang#132944 (add parentheses when unboxing suggestion needed)
 - rust-lang#132993 (Make rustc consider itself a stable compiler when `RUSTC_BOOTSTRAP=-1`)
 - rust-lang#133130 (`suggest_borrow_generic_arg`: instantiate clauses properly)
 - rust-lang#133133 (rustdoc-search: add standalone trailing `::` test)
 - rust-lang#133143 (Diagnostics for let mut in item context)
 - rust-lang#133147 (Fixup some test directives)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#133143 - kornelski:let-mut-global, r=compiler-errors

Diagnostics for let mut in item context

The diagnostics for `let` at the top level did not account for `let mut`, which [made the error unclear](https://users.rust-lang.org/t/create-a-vector-of-constants-outside-main/121251/1).

I've made the diagnostic always display a link to valid items. I've added dedicated help for `let mut` case that suggests using a `Mutex` (to steer novice users away from the `static mut` trap). Unfortunately, neither the Rust book, nor libstd docs have dedicated section listing all other types for interior-mutable `static`s.

Document alternatives to `static mut`

In rust-lang#133143 I've noticed alternatives to `static mut` aren't documented anywhere.

Document alternatives to `static mut`

In rust-lang#133143 I've noticed alternatives to `static mut` aren't documented anywhere.

Rollup merge of rust-lang#133145 - kornelski:static-mutex, r=traviscross

Document alternatives to `static mut`

In rust-lang#133143 I've noticed alternatives to `static mut` aren't documented anywhere.