Instead, use raw pointers to accomplish internal mutability throughout.

And avoid duplicating logic for visiting `Item`s with different kinds (regular, associated, foreign).

Co-authored-by: joboet <[email protected]>

…=fee1-dead

Fix substitution parts having a shifted underline in some cases

If two suggestions parts are side by side, the underline's offset:
(WIP PR as an example, not yet pushed)
```
error: expected a pattern, found an expression
 --> ./main.rs:4:9
  |
4 |         1 + 2 => 3
  |         ^^^^^ arbitrary expressions are not allowed in patterns
  |
help: check the value in an arm guard
  |
4 |         n if n == 1 + 2 => 3
  |         ~     +++++++++++++
```
The emitter didn't take into account that the string had shrunk/grown if two substitution parts were side-by-side (surprisingly, there was only one case in the ui testsuite.)

```
help: check the value in an arm guard
  |
4 |         n if n == 1 + 2 => 3
  |         ~ +++++++++++++
```

`@rustbot` label +A-suggestion-diagnostics

ast: Generalize item kind visiting

And avoid duplicating logic for visiting `Item`s with different kinds (regular, associated, foreign).

The diff is better viewed with whitespace ignored.

…-thread-locals, r=joboet

thread_local: be excruciatingly explicit in dtor code

Use raw pointers to accomplish internal mutability, and clearly split references where applicable. This reduces the likelihood that any of these parts are misunderstood, either by humans or the compiler's optimizations.

Fixes rust-lang#124317

r? `@joboet`