[robobun]

Problem

• bun build strips a license banner written as /* @license ... */, /** @license ... */, /* @preserve ... */, // @license ... or // @preserve .... The lexer's only legal-comment test is text[2] == b'!' (src/js_parser/lexer.rs:1895, has_legal_annotation). esbuild keeps a comment that contains the word @license or @preserve. There is no --legal-comments flag, so a bundle of a package that carries its license this way loses it in every mode (Bun.build preserve important comments #9795, bundler: implement --legal-comments to control how licenses in source code is bundled. #8727).
• import() drops a legal comment that is still waiting for the next statement. parse_import_expr (src/js_parser/parse/parse_import_export.rs:45) set preserve_all_comments_before, consumed (, then cleared the whole buffer. const load = /*! L */ () => import("./x") lost /*! L */. The interior comments it collected were never used: E::Import has no field for them and the printer passes &[].

Fix

• is_legal_comment (lexer.rs) keeps the ! rule and adds esbuild's rule: an @ followed by license or preserve at a word boundary, anywhere in the comment. @licensee, @license_x and @copyright stay non-legal, as in esbuild.
• parse_import_expr no longer touches the legal comment buffer. The preserve_all_comments_before flag had no other user, so it is removed from the lexer and its snapshot.
• Correct because a legal comment inside import(...) is now handled like every other expression-position legal comment in bun: the next statement boundary prints it. Nothing else read the interior comments.
• Verified: test/bundler/bundler_comments.test.ts (new legal comments block, 4 tests) and test/bundler/transpiler/transpiler.test.js (2 new tests, 1 updated expectation). All 6 fail on bun 1.4.0. Also ran bundler_edgecase, bundler_cjs2esm, esbuild/default, esbuild/dce, esbuild/ts, inspect-error, build-error, and the source lints. bundler_npm and bundler_bytecode_portable pin outputs of real libraries and are updated (see Notes).

Background

• A legal comment is a comment the bundler must keep because it carries a license or copyright notice. The lexer pushes each one into comments_to_preserve_before. parse_stmts_up_to (src/js_parser/parse/mod.rs:1469) drains that buffer into S::Comment statements before each statement. The printer prints an S::Comment verbatim.
• Unlike esbuild, bun does not reset that buffer on every token. A legal comment in expression position waits until the next statement boundary and is printed after the statement that contains it. parser: keep legal comments across the speculative arrow parse in a conditional #40985 relies on this when it restores the buffer after a speculative arrow parse.
• --minify in esbuild also keeps legal comments. The default --legal-comments is eof when bundling and inline otherwise. Bun has only the inline behavior.
• Self-reviewed: 8 concerns raised, 5 addressed (word boundary, Bun.build preserve important comments #9795, docs, the dead if block, the parser: preserve comments inside function parameter lists #36705 note). Rejected: changing the CSS rule, removing the printer's leading_interior_comments stubs, and adding --legal-comments modes. See Notes.

Notes

• Differential against esbuild 0.21.5 (--legal-comments=inline, un-minified): 7 comment forms x 90 positions x 7 bun modes (bun build default, --minify, --minify-whitespace, --format=cjs, --target=node, --no-bundle, Bun.Transpiler). After this change every form is kept exactly once at every top-level statement position in every mode, and stays ahead of the statement it annotates. Bun keeps more than esbuild in two places: a legal comment inside JSX {} children and one in a switch case body. esbuild drops both.
• Before this change the same matrix lost every @license / @preserve form everywhere, and lost /*! only when import( was parsed while the comment waited in the buffer.
• The word-boundary rule is strings::has_prefix_with_word_boundary, the helper scan_pragma already uses for @__PURE__ and @jsx. Checked against esbuild: @license., @license*/, foo@license, @@license and // @license at end of file are kept. @licensee, @preserved, @licenses, @license_x, @license9, @license$ and @licenseé are dropped.
• CSS is unchanged. src/css/css_parser.rs:2613 keeps only the leading /*! comments of a stylesheet, which is the lightningcss rule bun's CSS parser follows. esbuild also keeps @license / @preserve in CSS. That gap belongs with bundler: implement --legal-comments to control how licenses in source code is bundled. #8727.
• The printer still has a leading_interior_comments parameter that every caller passes as &[] (src/js_printer/lib.rs:2694) and E::Import has a note about webpack magic comments (src/ast/e.rs:2456). Those were dead before this change and are the place a future implementation would print interior comments. Left for a dead-code sweep.
• The speculative arrow parse in a conditional (x = a ? /*! L */ (b) : T => { ... } : c) was fixed by parser: keep legal comments across the speculative arrow parse in a conditional #40985 and is not part of this change. Its test in transpiler.test.js that documented the import() loss now expects the comment.
• RuntimeTranspilerCache::EXPECTED_VERSION is not bumped. A cache entry written by an older bun only lacks comments, which has no runtime effect.
• parser: preserve comments inside function parameter lists #36705 (open) sets preserve_all_comments_before in parse_fn for a different purpose. Whichever of the two lands second needs a rebase: that branch must add the flag back.
• Tree shaking keeps a legal comment whose neighbor statement is removed, same as esbuild: /* @license */ export const unused = 1; keeps the comment when only another export is used.
• docs/bundler/minifier.mdx now names all three legal forms in the "Comment removal" section.
• react, react-dom, lodash and immutable start with a @license comment, so the bundled output of npm/ReactSSR (test/bundler/bundler_npm.test.ts) and of the library builds in test/bundler/bundler_bytecode_portable.test.ts now carries it. The generated positions, the file size, and the bytecode snapshot are updated. The new snapshot hashes are identical on linux x64 and on the aarch64 CI lanes, so the bytecode encoder output is still platform independent: only its input changed.

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no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bundler/bundler_bytecode_portable.test.ts

[robobun]

Status: reproduced on bun 1.4.0 and the canary with a differential against esbuild 0.21.5. bun build a.js on a file whose header is /* @license MIT */ prints no comment, esbuild prints it. const load = /*! L */ () => import("./x") loses /*! L */ in every mode. Both fixed here.

Tests: test/bundler/bundler_comments.test.ts (legal comments block) and test/bundler/transpiler/transpiler.test.js. Run bun bd test test/bundler/bundler_comments.test.ts. bundler_npm and bundler_bytecode_portable pin outputs of real libraries whose @license headers are now kept, so their expectations are updated (the new bytecode hashes match on linux x64 and the aarch64 lanes).

CI: the diff is green on every lane. The one remaining red test in build 108764 is test/cli/run/require-cache.test.ts on debian 13 x64-asan, a pre-existing 30s timeout that also fails on main and is reported to triage. The other failures in this and the previous builds passed on retry (docker image metadata fetch, install tests, fetch-leak).

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Walkthrough

The lexer now preserves comments marked with /*!, //!, @license, or @preserve. Dynamic import parsing no longer manages temporary comment state. Bundler and transpiler tests cover preservation, filtering, ordering, minification, and updated snapshots.

Changes

Legal comment preservation

Layer / File(s)	Summary
Lexer legal-comment detection src/js_parser/lexer.rs	The lexer uses is_legal_comment to preserve legal comments and reject prefix matches such as @licensee. Temporary preservation state was removed.
Dynamic import comment handling src/js_parser/parse/parse_import_export.rs, src/js_parser/p.rs	Dynamic import parsing no longer changes lexer comment state. Parser documentation now describes block-body comment draining.
Bundler, transpiler, and documentation coverage test/bundler/bundler_comments.test.ts, test/bundler/transpiler/transpiler.test.js, docs/bundler/minifier.mdx	Tests cover legal comment forms, output modes, ordering, transpilation, and dynamic imports. Minifier documentation lists the preserved forms.
Generated output snapshots test/bundler/bundler_bytecode_portable.test.ts, test/bundler/bundler_npm.test.ts	Bytecode hashes, byte sizes, source-map mappings, and bundled output size expectations were updated.

Suggested reviewers: dylan-conway

Merge Risk: ⚪ Minimal · up to 4ec72

This change preserves license and preserve comments in JavaScript builds, improving legal-notice retention without supplied evidence of a correctness or production-safety issue; it is merge-ready after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes both primary changes: preserving @license and @preserve comments and retaining legal comments across import().
Description check	✅ Passed	The description provides detailed problem, fix, background, verification, test coverage, and notes. It does not use the template headings exactly, but it contains the required information and is subst…

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Explanation

The description provides detailed problem, fix, background, verification, test coverage, and notes. It does not use the template headings exactly, but it contains the required information and is substantially complete.

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Inline comments:
In `@docs/bundler/minifier.mdx`:
- Line 760: Update the legal-comment description near the minifier documentation
to state that `@license` and `@preserve` are preserved only when used as standalone
annotations with a word boundary, not as prefixes in names such as `@licensee`,
`@license_x`, or `@preserved`; retain the existing /*! and //! rules.

In `@test/bundler/bundler_comments.test.ts`:
- Around line 470-471: Replace the for-loop over minify values surrounding the
“@license and `@preserve` comments are kept” test with a describe.each([false,
true]) suite, registering the existing itBundled case inside the callback while
preserving its test name and behavior.

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[robobun]

^{Updated 2:30 AM PT - Aug 31st, 2026}

❌ @robobun, your commit fc29adc has 1 failures in Build #108764 (All Failures):

• test/cli/run/require-cache.test.ts - code 1 on 🐧 13 x64-asan

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🧪   To try this PR locally:

bunx bun-pr 41013

That installs a local version of the PR into your bun-41013 executable, so you can run:

bun-41013 --bun

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.