Add #[loop_match] for improved DFA codegen by folkertdev · Pull Request #138780 · rust-lang/rust (original) (raw)

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

#[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