lexer: only treat __PURE__ as an annotation when it starts a // comment by robobun · Pull Request #41284 · oven-sh/bun

robobun · 2026-09-03T11:58:30Z

Problem

A // comment that mentions /*#__PURE__*/ as text marks the next call as pure. Bun removes an unused pure call at runtime and in bun build, without minification. So // `/*#__PURE__*/` followed by console.log(...) deletes the console.log. In @angular/build this deleted source.appendRight(...) and corrupted the Angular build output (advancedOptimizations (oxc-transform) corrupts vendor bundle output — related to #33699, but fix in #33700 doesn't fully resolve it, and worker pool concurrency setting also implicated angular/angular-cli#33973).
The cause is scan_single_line_comment in src/js_parser/lexer.rs:2087. On any # or @ in a // comment it calls scan_pragma, which sets has_pure_comment_before when __PURE__ follows. The position in the comment does not matter.

Fixes #41275. Fixes #19355.

Fix

In a // comment, @__PURE__ or #__PURE__ is an annotation only when it is the first word: only whitespace may sit between // and the marker. Text after the marker is still allowed. Block comments are unchanged.
This deviates from esbuild, terser and rollup, which match the marker anywhere in a comment. Those tools only act on it when minifying or tree shaking. Bun acts on it always, so a false match deletes user code. Line directives such as // @ts-ignore, //# sourceMappingURL= and // eslint-disable-next-line use the same first-word rule. Block comments keep the esbuild rule because a /* */ comment cannot quote a /*#__PURE__*/, and /** @__PURE__ */ must keep working.
Four existing tests in test/bundler/bundler_comments.test.ts put text before the marker and asserted removal. They came in with the SIMD lexer change (Use Highway SIMD #19134) to exercise the scanner, and now assert that the call is kept. test/bundler/transpiler/transpiler.test.js:1589 was a victim of this bug: the @__PURE__ text in the commented-out lines above it deleted the first err(...) call, which asserted a TSX-only error for the ts loader. It now asserts the ts behavior, as in esbuild's ts_parser_test.go.
Verified: test/bundler/bundler_comments.test.ts (4 new cases) and test/cli/install/bun-run.test.ts (1 new runtime case). Both fail on the release binary. Also ran test/bundler/esbuild/dce.test.ts, default.test.ts, tsconfig.test.ts, bundler_minify.test.ts, bundler_jsx.test.ts, transpiler/transpiler.test.js, transpiler/react-compiler.test.ts.

Background

A pure annotation (/* @__PURE__ */ f()) tells a bundler that the call has no side effects. If the result is unused, the call can be removed.
Bun's dead_code_elimination parser feature is on by default, also for bun run. The SExpr visitor (src/js_parser/visit/visit_stmt.rs:1430) calls SideEffects::simplify_unused_expr with no minify gate, and scan_side_effects.rs drops a call marked IfUnused whose arguments have no side effects. The docs describe pure annotation support as "Always active".
has_pure_comment_before is reset at the start of Lexer::next() and read in parse_expr_common (src/js_parser/parse/mod.rs:75), which marks the next ECall or ENew as CanBeUnwrapped::IfUnused.

Notes

Repro under bun 1.4.1:

$ bun -e 'function repro() {
    // `/*#__PURE__*/`
    console.log("This should not be deleted");
}
console.log(repro.toString());'
function repro() {}

esbuild 0.21.5 attaches the annotation to the same input but keeps the call unless --minify-syntax is passed.

Alternative considered: gate the removal of a pure-marked expression statement on minify_syntax, as esbuild does. That changes documented runtime behavior and unminified bun build output for every correctly annotated call, so this PR narrows what counts as an annotation instead.

A /* see @__PURE__ for details */ block comment before a call still marks it pure. That is the esbuild rule and is not changed here. A quoted /*#__PURE__*/ cannot occur inside a block comment, so the real-world reports all come from // comments.

Self-reviewed: the review found the dead transpiler.test.js assertion and asked for the esbuild deviation to be stated. Both are addressed above. It also suggested gating bun test on ignore_dce_annotations. That is a separate change and is not in this PR.

Unrelated failures seen locally with the debug build: two bun-build-api.test.ts bytecode tests that time out at 5s under the debug build.

no test proof · iteration 5 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/install/bun-run.test.ts

A `#__PURE__` or `@__PURE__` marker anywhere in a `//` comment marked the next call as pure. Bun removes an unused pure call without minification, so a comment that quoted `/*#__PURE__*/` deleted the statement after it. The marker now has to be the first word of the comment. Block comments are unchanged. In transpiler.test.js the commented-out TSX lines above the `const` type parameter assertions contain `@__PURE__`, so the first assertion after them never ran. It asserted a TSX-only error for the ts loader. Replace it with the ts expectations.

coderabbitai · 2026-09-03T12:03:04Z

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Walkthrough

Changes

The lexer now recognizes __PURE__ in block comments and only at the first marker position in line comments. Regression tests cover prose, Unicode, and CLI execution. Transpiler tests accept async arrow functions with const type parameters.

Pure annotation parsing

Layer / File(s)	Summary
Lexer pragma recognition `src/js_parser/lexer.rs`	Pragma scanning uses comment and pure-annotation policies. Line comments enable `__PURE__` recognition only at the annotation position.
Pure annotation regression coverage `test/bundler/bundler_comments.test.ts`, `test/cli/install/bun-run.test.ts`	Tests verify that non-leading marker text preserves code and valid leading annotations remove unused calls.

TypeScript const parameters

Layer / File(s)	Summary
Async const parameter transpilation `test/bundler/transpiler/transpiler.test.js`	Tests expect async arrow functions with single and duplicate `const` type parameters to transpile successfully.

Suggested reviewers: jarred-sumner

Merge Risk: ⚪ Minimal · up to fb3ca

This change narrows line-comment pure-annotation recognition so prose or quoted markers no longer remove following side-effecting calls. No current merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address both linked issues [`#41275`] and [`#19355`] by restricting line-comment PURE recognition and adding regression coverage for preserved side-effecting code and dead-code elimination…
Out of Scope Changes check	✅ Passed	The lexer changes and test updates are related to the linked objectives. The transpiler test correction is in scope because it removes an assertion affected by the same incorrect annotation parsing.
Title check	✅ Passed	The title clearly and concisely describes the main lexer change: recognizing PURE only when it starts a // comment.
Description check	✅ Passed	The description explains the problem, fix, behavior for line and block comments, affected tests, verification steps, and known unrelated failures. It does not use the exact template headings, but it p…

Full details: Linked Issues check

Explanation

The changes address both linked issues [#41275] and [#19355] by restricting line-comment PURE recognition and adding regression coverage for preserved side-effecting code and dead-code elimination.

Full details: Description check

Explanation

The description explains the problem, fix, behavior for line and block comments, affected tests, verification steps, and known unrelated failures. It does not use the exact template headings, but it provides the required information and is mostly complete.

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In `@test/cli/install/bun-run.test.ts`:
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robobun · 2026-09-03T12:04:21Z

^{Updated 3:30 PM PT - Sep 3rd, 2026}

❌ @robobun, your commit fb3ca35 has 2 failures in Build #109825 (All Failures):

test/js/bun/test/parallel/test-integration-rspack.ts - code 1 on 🪟 2019 x64
test/cli/install/bunx.test.ts - code 1 on 🪟 11 aarch64

🧪 To try this PR locally:

bunx bun-pr 41284

That installs a local version of the PR into your bun-41284 executable, so you can run:

bun-41284 --bun

Only the first # or @ in a // comment can have an all-whitespace prefix. Checking the prefix on every marker was quadratic for a comment with many markers.

claude

The quadratic re-scan I flagged earlier is fixed — first_marker now gates is_all_whitespace so the prefix is checked at most once per // comment, and the new 4096-space/4096-@ bundler test covers it. No further bugs found.

Deferring to a human because this intentionally diverges from esbuild's __PURE__ matching (the ported-code reference per REVIEW.md), flips four existing bundler_comments.test.ts assertions to the opposite outcome, and rewrites an unrelated transpiler.test.js assertion — all well-argued in the description, but each is a judgment call a maintainer should sign off on.

Extended reasoning...

Overview

The PR narrows #__PURE__ / @ __PURE__ recognition in single-line comments so the marker is only honored when it is the first non-whitespace token after //. The lexer change is ~10 lines: scan_pragma gains an allow_pure parameter, scan_single_line_comment computes it from a first_marker flag plus a one-shot strings::is_all_whitespace check on the bytes between // and the marker, and scan_multiline_comment_body passes true unconditionally. Tests: four new itBundled cases and four flipped assertions in bundler_comments.test.ts, one runtime bun -e repro in bun-run.test.ts, and a rewrite of one transpiler.test.js assertion whose original form was itself a victim of this bug.

Security risks

None. This is a lexer heuristic for a tree-shaking annotation; no untrusted-input parsing beyond what the lexer already does, no auth/crypto/permissions surface. The prior revision's O(N·M) prefix re-scan (a mild parse-time DoS vector) was the only resource concern, and it is now bounded to a single linear scan per comment via the first_marker short-circuit.

Level of scrutiny

Medium-high. The code change is small and mechanically correct — self.start + 2 is always the byte after //, pragma_trigger_pos is the #/@ position from self.end, and && short-circuits so is_all_whitespace runs at most once. But the semantic change is a deliberate deviation from esbuild, terser, and rollup (which match the marker anywhere in a comment), and REVIEW.md states that for ported code the reference implementation is the spec. The PR description justifies the deviation convincingly (Bun applies DCE at runtime without a minify gate, so a false match silently deletes user code), but choosing to narrow the annotation rule rather than gate removal on minify_syntax is a design trade-off a maintainer should ratify.

Other factors

Four pre-existing tests had their expected output inverted. REVIEW.md flags "never mutate an existing test's input to a new case", and while here the inputs are unchanged and only the assertions are corrected to match the new intended behavior, a human should confirm those tests were exercising the SIMD scanner (as the description claims) rather than intentionally locking in match-anywhere semantics. The transpiler.test.js edit replaces an err() with two exp() cases for async <const T>() => {} under the ts loader — a distinct correctness fix riding along, plausible and consistent with esbuild's ts_parser_test.go, but separate from the headline change. No CODEOWNERS cover these paths. My earlier inline finding was addressed by commit 2fcbe29; nothing new blocks.

Jarred-Sumner

fix mordant. probably use an enum?

robobun · 2026-09-03T22:07:40Z

Done in fb3ca35. scan_pragma now takes CommentKind and PureAnnotation enums instead of the two bools.

claude

Code review found no issues

No high-confidence issues detected in this change.

github-actions Bot added the claude label Sep 3, 2026

robobun mentioned this pull request Sep 3, 2026

Transpiler treats /*#__PURE__*/ inside single-line // comment as pure annotation, causing incorrect dead code elimination #41275

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coderabbitai Bot reviewed Sep 3, 2026

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Comment thread test/cli/install/bun-run.test.ts Outdated

test: run the bun-run pure comment case with -e

f914725

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