perf(parser): peek instead of checkpoint/rewind for export default modifier#23124
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…modifier (#23124) ## What `parse_export_default_declaration_kind` took a parser-level `checkpoint()` and consumed the contextual keyword (`abstract`/`async`/`interface`) with `bump_any()` *before* knowing whether it begins a declaration, then `rewind`-ed to un-consume it when the keyword turned out to be an identifier expression (`export default async;`, `export default abstract + 1`). ## Why it's removable - The disambiguation is single-token lookahead, so `self.lexer.peek_token()` answers it without consuming the modifier. - No diagnostics were ever discarded by the rewind: the only `self.error(...)` calls on the speculative span live in the `async function` and `interface` branches, both of which `return` (commit) and never reach the rewind. `parse_ts_interface_declaration` always returns `Declaration::TSInterfaceDeclaration`, so its `if let` never fails. The rewind only ever un-consumed a single token. ## Change Peek the following token, consume the modifier only on the commit branches, and fall through to the expression parse otherwise. This drops the parser-level `checkpoint()`/`rewind()` — including the fragile `fatal_error.take()` that was dropped on the commit paths — and only pays a lexer-level peek when the token actually is a contextual keyword. Behavior-preserving: - `cargo coverage -- parser` — test262 100%, snapshots unchanged. - `just allocs` — unchanged. 🤖 Generated with [Claude Code](https://claude.com/claude-code)
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### 💥 BREAKING CHANGES - 7a24911 codegen: [**BREAKING**] Borrow sourcemaps from codegen (#23422) (Boshen) - bb0ed44 transformer: [**BREAKING**] Disable styled-components transpileTemplateLiterals by default (#23171) (Boshen) ### 🚀 Features - 1490a0a linter/react: Implement react-compiler rule (#23202) (Boshen) - 6c0bdf0 transformer/react-refresh: Support `module.property.useHook()` (#23190) (Dunqing) - 47991bd semantic: Report TS1228 for invalid type predicates (#23174) (camc314) - 1d3af58 parser: Add TS2398 parameter property diagnostic (#23216) (camc314) - 44313da semantic: Add `scope_is_descendant_of` api (#22313) (camc314) - e5050c0 parser: Improve diagnostic for rest initializer (#23205) (camc314) - ec266bb transformer: Run React Compiler as a feature-gated transform pass (#23201) (Boshen) - e7374fe parser: Report error for `const` modifier on interface type parameter (#23173) (camc314) - a7c1c9b parser: Report ambient definite variable assertions (#23165) (camc314) - d169fcd parser: Report invalid class definite assertions (#23164) (camc314) - 00244d8 parser: Report definite property initializer errors (#23160) (camc314) ### 🐛 Bug Fixes - 52d0c31 transformer: Replace ambient dot defines (#23231) (camc314) - 2c28748 transformer/class: Parent generated constructors to class scope (#23222) (camc314) - 8edd234 parser: Report accessor definite assertion on token (#23203) (camc314) - de38a3f react_compiler: Keep imports referenced only by a local re-export (#23176) (Boshen) - f5721c2 codegen: Preserve parentheses around `intrinsic` type reference (#23156) (Boshen) - e89f81d parser: Don't emit TS1477 for parenthesized instantiation expression (#23147) (Boshen) - 8a04149 parser: Reject module-referencing imports/exports in a namespace body (#22829) (Boshen) ### ⚡ Performance - 2783295 parser: Table-driven operator precedence lookup (#23346) (Boshen) - 231d5de parser: Single-match member expression dispatch (#23347) (Boshen) - e89729b codegen: Accept one-shot wrap closures (#23265) (camc314) - a6c11fa parser: Force-inline read_non_decimal to fold per-digit number dispatch (#23157) (Boshen) - d74964c parser: Store class definite assertion offset (#23170) (camc314) - f0fda4d parser: Shrink-wrap cold diagnostic tails out of hot frames (#23159) (Boshen) - a082180 parser: Store definite assertion offset (#23167) (camc314) - 534f9c6 oxc: Conditionally rebuild semantic in compiler pipeline (#23153) (Boshen) - b435c6a parser: Skip checkpoint for `infer T extends U` constraint in disallow context (#23128) (Boshen) - 7464dce parser: Peek instead of checkpoint/rewind for `export default` modifier (#23124) (Boshen) - 80a9a32 parser: Fast-path single-keyword TS declarations (#23083) (Boshen) - da1a6c6 diagnostics: Migrate to allocation-optimized oxc-miette (#23094) (Boshen) - b7b08ce parser: Peek once for the static modifier disambiguation (#23079) (Boshen) - e7e07a3 parser: Fold unary dispatch into a single match (#23076) (Boshen) ### 📚 Documentation - d241add semantic: Add `AGENTS.md` test guidance for agents (#23441) (camc314) - 026f1ae parser: Add `AGENTS.md` test guidance for agents (#23440) (camc314) - 09755ac transformer: Add `AGENTS.md` test guidance for agents (#23439) (camc314) - e6bdfd4 lexer: Correct reference link for `byte_handlers!` (#23313) (Dunqing) - 65b6d7a allocator: Fix memory leaks in `Arena` examples (#23257) (overlookmotel) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>
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What
parse_export_default_declaration_kindtook a parser-levelcheckpoint()and consumed the contextual keyword (abstract/async/interface) withbump_any()before knowing whether it begins a declaration, thenrewind-ed to un-consume it when the keyword turned out to be an identifier expression (export default async;,export default abstract + 1).Why it's removable
self.lexer.peek_token()answers it without consuming the modifier.self.error(...)calls on the speculative span live in theasync functionandinterfacebranches, both of whichreturn(commit) and never reach the rewind.parse_ts_interface_declarationalways returnsDeclaration::TSInterfaceDeclaration, so itsif letnever fails. The rewind only ever un-consumed a single token.Change
Peek the following token, consume the modifier only on the commit branches, and fall through to the expression parse otherwise. This drops the parser-level
checkpoint()/rewind()— including the fragilefatal_error.take()that was dropped on the commit paths — and only pays a lexer-level peek when the token actually is a contextual keyword.Behavior-preserving:
cargo coverage -- parser— test262 100%, snapshots unchanged.just allocs— unchanged.🤖 Generated with Claude Code