Rollup merge of #138780 - trifectatechfoundation:loop_match_attr, r=oli-obk,traviscross

Add `#[loop_match]` for improved DFA codegen

tracking issue: https://github.com/rust-lang/rust/issues/132306
project goal: https://github.com/rust-lang/rust-project-goals/issues/258

This PR adds the `#[loop_match]` attribute, which aims to improve code generation for state machines. For some (very exciting) benchmarks, see https://github.com/rust-lang/rust-project-goals/issues/258#issuecomment-2732965199

Currently, a very restricted syntax pattern is accepted. We'd like to get feedback and merge this now before we go too far in a direction that others have concerns with.

## current state

We accept code that looks like this

```rust
#[loop_match]
loop {
    state = 'blk: {
        match state {
            State::A => {
                #[const_continue]
                break 'blk State::B
            }
            State::B => { /* ... */ }
            /* ... */
        }
    }
}
```

- a loop should have the same semantics with and without `#[loop_match]`: normal `continue` and `break` continue to work
- `#[const_continue]` is only allowed in loops annotated with `#[loop_match]`
- the loop body needs to have this particular shape (a single assignment to the match scrutinee, with the body a labelled block containing just a match)

## future work

- perform const evaluation on the `break` value
- support more state/scrutinee types

## maybe future work

- allow `continue 'label value` syntax, which `#[const_continue]` could then use.
- allow the match to be on an arbitrary expression (e.g. `State::Initial`)
- attempt to also optimize `break`/`continue` expressions that are not marked with `#[const_continue]`

r? ``@traviscross``

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -32,6 +32,7 @@ pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod inline;
 pub(crate) mod lint_helpers;
+pub(crate) mod loop_match;
 pub(crate) mod must_use;
 pub(crate) mod repr;
 pub(crate) mod semantics;