…attr, r=oli-obk

Add `#[loop_match]` for improved DFA codegen

tracking issue: rust-lang#132306
project goal: rust-lang/rust-project-goals#258

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

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`

…attr, r=oli-obk,traviscross

Add `#[loop_match]` for improved DFA codegen

tracking issue: rust-lang#132306
project goal: rust-lang/rust-project-goals#258

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

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`

Rollup of 15 pull requests

Successful merges:

 - #135731 (Implement parsing of pinned borrows)
 - #138780 (Add `#[loop_match]` for improved DFA codegen)
 - #142453 (Windows: make `read_dir` stop iterating after the first error is encountered)
 - #142633 (Error on invalid signatures for interrupt ABIs)
 - #142768 (Avoid a bitcast FFI call in transmuting)
 - #142825 (Port `#[track_caller]` to the new attribute system)
 - #142844 (Enable short-ice for Windows)
 - #142934 (Tweak `-Zmacro-stats` measurement.)
 - #142955 (Couple of test suite fixes for cg_clif)
 - #142977 (rustdoc: Don't mark `#[target_feature]` functions as ⚠)
 - #142980 (Reduce mismatched-lifetime-syntaxes suggestions to MaybeIncorrect)
 - #142982 (Corrected spelling mistake in c_str.rs)
 - #142983 (Taint body on invalid call ABI)
 - #142988 (Update wasm-component-ld to 0.5.14)
 - #142993 (Update cargo)

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

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

Add `#[loop_match]` for improved DFA codegen

tracking issue: #132306
project goal: rust-lang/rust-project-goals#258

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

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``