Parse spawn, select, and emit

[StreamDemon]

Closes #93.

Summary

The three reserved keywords had no parse productions — every actor spawn (§8.2/§8.7), multiplexed receive (§8.6), async task (§8.9), and event emission (§11.1/§11.3) failed, including §4.6's own spawn move || { ... } example. This was milestone 1's exit criterion: #67's round-trip and #68's corpus cover the spec's §8 examples, and nearly all of them spawn actors.

• AST: Spawn(Box<Expr>), SpawnAsync { body: Block } (raw block like Loop), Select { arms: Vec<SelectArm> } with SelectArm { pattern, source, body } (unboxed; block bodies wrapped per MatchArm); Stmt::Emit { event: String, fields } reusing field-init shorthand, spanless per the Stmt convention; plus ExprKind::Tuple for unit/tuple expressions (see below).
• Parser: spawn_expr() (operand via plain expr(0) — not cond_expr(), spawn is not a block-head position, spawn Worker { count: 0 } parses, pinned); select_expr()/select_arm() modeled on the match pair (same arm loop, recovery, comma discipline; deferred pattern? like match_arm; the = delimiter means full or-patterns apply with no closure-param collision); emit as a block_inner() arm next to return — statement-only, no prefix() arm, on-chain-only restriction semantic. can_begin_expr gains Spawn/Select (send-head consequence pinned). Spans anchor at each construct's keyword token (Parse closures and the closure pipe stage #92 lesson), pinned by test.
• Bonus discovery: the corpus fixture tripped over Ok(()) — the parser had no unit or tuple expression, though the spec's own §11.1 example ends with one and the shape pervades §6/§8/§11. () and (a, b) now parse as ExprKind::Tuple with (a) still grouping; §16's expr list has no explicit alternative for these — recorded as Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89 item 5.
• Tests: 15 new (91 in the crate) — every node shape structurally; §8.6 select verbatim (return-arm adapted, Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89 item 1's third occurrence); or-pattern select arm; struct-literal pins (spawn operand, select source); greedy spawn (x |> f) pin (§16 grouping ambiguity, noted on Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89); select {…} |> f and pipe-in-arm-body; spawn async with .await tail; emit shorthand + statement-only pin; send-head consequence; span anchors; grammar violations; recovery count; non-tail ; behavior.
• Corpus: tests/corpus/spawn_select_emit.sp — §4.6 spawn-move (verbatim at last), §8.2 constructor spawns, §8.2a/§8.7 worker pools, §8.9 spawn async + JoinHandle, §8.6 select, §11.1/§11.3 emit inside an onchain mod (18 fixtures total).
• Docs: crates/AGENTS.md — the three keywords moved to Accepted with parse-only caveats, plus the four sentences the design review flagged as going stale (patterns-wired list gains select arms and notes select's return-arm rejection; send-head list gains spawn/select; the non-tail-; set gains select/spawn async with the Parser: block-like expressions as statements without trailing ; #62 reconciliation note).

Filed on #89 this round: item 5 (unit/tuple expressions), the third item-1 occurrence (§8.6 + §8.8, correcting #89's stale §8.10.1 citation), and the spawn x |> f grouping ambiguity.

Spec Sections Affected

None — implements existing §8/§11/§16; docs/reference/grammar.md remains byte-identical to §16.

Build Targets Tested

N/A — parser crate only, no codegen.

Test Plan

• cargo fmt --all -- --check — pass
• cargo clippy --workspace --all-targets -- -D warnings — pass
• cargo test --workspace — 103 tests pass (11 lexer, 91 parser incl. 15 new, 1 corpus over 18 fixtures)
• Corpus fixture covers §4.6/§8.2/§8.2a/§8.6/§8.7/§8.9/§11.1/§11.3 shapes
• CI green (rust.yml triggers: crates/** and tests/corpus/** changed)

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Summary by cubic

Parses spawn, select, and emit so actor spawns, multiplexed receive, async tasks, and on-chain event emission now succeed. Previously these keywords had no productions; now spawn/select begin expressions and emit is statement-only. Side effects: send before spawn/select opens a send-statement; non-tail select, spawn async, and brace-final spawn require a trailing ;; spans anchor at each keyword.

• AST (sploosh-ast): adds ExprKind::Spawn, ExprKind::SpawnAsync { body }, ExprKind::Select { arms: Vec<SelectArm> } with SelectArm { pattern, source, body }, Stmt::Emit { event, fields } (spanless), and ExprKind::Tuple for () and (a, b).
• Parser (sploosh-parser): spawn takes an unrestricted operand and binds trailing pipes greedily; spawn async takes a raw block. select parses match-style arms using pattern = expr => with full patterns; expression bodies require a trailing comma, block bodies do not. emit parses only as a statement and reuses field-init shorthand. can_begin_expr now includes spawn and select.
• Behavior to review: send followed by spawn or select opens a send-statement and their operands fail the method-call check. Non-tail select, spawn async, and brace-final spawn forms (spawn { ... }, spawn Type { ... }) need ;. Spans anchor at the construct keyword.
• Docs/tests: AGENTS.md reflects parse support and clarifies the semicolon rule for brace-final spawn. Adds a corpus covering §4.6/§8/§11 shapes and unit/tuple expression tests.

^{Written for commit 03058f7. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Implements existing spec grammar for spawn/select/emit with 15 new tests and corpus fixtures; change is confined to the parser/AST and introduces no operational or policy tradeoffs.

Re-trigger cubic</sub>