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The UnDelete visit arm never wrote p.delete_target, so every
is_delete_target check derived from it (in e_identifier, e_dot,
e_index, e_import_meta, handle_identifier, and
maybe_rewrite_property_access) was permanently false. The port carried
the field and all the guards over from esbuild but dropped the one
assignment that makes them meaningful.
Observable on main with minify_syntax (default for `bun run`):
delete [obj.p][0] -> folded to `delete obj.p` (runtime wrong)
delete {f: x}.f -> folded to `delete x`
delete E.A (TS enum) -> folded to `delete 1`
delete ns.prop (bundle) -> no "Cannot assign to import" error
delete exports.foo -> CJS->ESM unwrap not deoptimized
delete DEFINED (--define) -> substituted to `delete <value>`
The bun-specific "foo"[n] and [x][n] folds in e_index also ran with no
delete/assign-target guard at all, so the first case above stayed
broken even with delete_target wired. Those now bail when the index
expression is a delete target or an assignment target (`[y][0] = 5`
previously folded to `y = 5`).
RuntimeTranspilerCache version bumped since the runtime transpiler
enables minify_syntax.
[o.m][0]() has this = the temporary array; folding to o.m() makes this = o. Add !is_call_target to the same guard (delete/assign/call are the three positions where a property reference behaves differently from its value) and cover it in the existing tests.
The EImportMeta arm in maybe_rewrite_property_access produced values (undefined, require.main == module, string literals) where a property reference is required. `delete import.meta.hot` became `delete undefined` (strict-mode SyntaxError) and `delete import.meta.main` under --format=cjs became `(delete require.main) == module`. Gate the inline branches on !is_delete_target && assign_target == None and fall through to the plain E::Dot return, matching the neighbouring EObject/enum arms. Also add the parsedMin assertion for `delete E["A"]` suggested in review.
`delete "abc".length` folded to `delete 3` (true) instead of `false`. The EString arm in maybe_rewrite_property_access now matches its neighbouring EObject arm's guard. Also lock in the existing printer-side `(0, x)` wrap for `delete (true ? a.b : 0)` with a test; bun handles that case via WAS_ORIGINALLY_DELETE_OF_IDENTIFIER_OR_PROPERTY_ACCESS at print time rather than a visit-time before/after check.
…n printer wrap The HotEnabled/HotDisabled arm in maybe_rewrite_property_access returned EUndefined for `import.meta.hot.accept` etc. with HMR off, producing `delete undefined` / `undefined = fn`. Bail to the outer E::Dot so the output stays a property reference (`undefined.accept`, which throws at runtime exactly as the unfolded input would). Also add lock-in transpiler tests for `delete (0, a.b)` / `?? || && ?:`. bun handles those at print time via WAS_ORIGINALLY_DELETE_OF_IDENTIFIER_OR_PROPERTY_ACCESS (set in parse_prefix, checked in the printer) rather than esbuild's visit-time canBeDeleted before/after wrap; the output and runtime behaviour match esbuild.
…abled Under HMR-enabled (Bake dev server), every rewrite in this arm produces a valid hmr.<name> property reference; bailing left `ESpecial::HotEnabled` in the tree, which prints as `hmr.indirectHot` (a throwing Proxy), so `import.meta.hot.data = obj` regressed. The delete-undefined/assign-undefined hazard only exists on the !enabled branches.
`--drop=Bun` on `delete Bun.inspect()` replaces the call with EUndefined; without minify that prints as `delete undefined`, which parses back as delete-of-a-bare-identifier (strict-mode SyntaxError). The test passed on main only because the runtime transpiler then substituted `undefined` to `void 0` while is_delete_target was permanently false. is_identifier_or_numeric_constant_or_property_access already covers ENumber(inf/nan) for the same reason (they print as `Infinity`/`NaN`); add EUndefined so the bundled output is `delete (0, undefined)` and stays valid under strict mode. Fixes bundler_drop.test.ts drop/DeleteAssignTarget.
…er-delete-target # Conflicts: # src/js_parser/visit/visit_expr.rs # src/js_printer/lib.rs # src/jsc/RuntimeTranspilerCache.rs # test/bundler/bundler_cjs2esm.test.ts # test/bundler/transpiler/transpiler.test.js
…tant value `X = 5` with `--define X='"abc"'` printed `"abc" = 5`. After deciding not to substitute a non-assignable define value into an assignment target, e_identifier still handed the define's value text to handle_identifier as a symbol name, so find_symbol allocated an unbound symbol literally named `"abc"` and rebound the identifier to it. Keep the original identifier instead, as esbuild does; reads are still replaced.
`bun test` keeps the runtime transpiler cache enabled when user defines are set, so entries transpiled before this fix can hold the invalid assignment output. Bumping the version makes them miss.
The EString arm of maybe_rewrite_property_access folded "foo".length to a number with only a minify_syntax check, so `"foo".length = 4` printed `3 = 4`, which is an early SyntaxError. The runtime transpiler runs with this fold on, so `bun file.js` failed to load a valid program. Skip the fold when the access is an assignment target, as esbuild does.
…sign-target' into farm/a1513e28/assign-target-substitution # Conflicts: # src/jsc/RuntimeTranspilerCache.rs
WalkthroughChangesThe parser now preserves delete and assignment targets during define substitution, property inlining, and syntax minification. Module metadata, Assignment Target Preservation
Suggested reviewers: Merge Risk: 🟡 Moderate · up to This change preserves assignment and delete targets, but the current implementation may incorrectly affect later identifier reads after a delete expression, producing wrong transformed output. Merge should wait for the target-state restoration and regression test. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Full details: Linked Issues checkExplanation The changes satisfy issue Full details: Out of Scope Changes checkExplanation The additional delete-target, property-folding, import.meta, module metadata, CommonJS, and cache-version changes are directly related to preserving reference semantics and are covered by the stated PR objectives and tests. Comment |
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Status: fix complete, ready for review. CI on 55fbf03 (build 108097): all lanes green except Reproduced with the released binary (1.4.1): This PR unifies #36734 (delete targets, |
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Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/js_parser/fold.rs— pre-existing: siblingEImportMetaarm of the same match still foldsimport.meta.main/.hot/.dir/.file/.path/.urlto a constant with noidentifier_opts.assign_target()guard — the exact class this PR adds for theEStringarm at line 448, left unfixed in the parallel switch arm REVIEW.md names as same-class scopeExtended reasoning...
With
bun build entry.js(ParseTask.rs:2598 setsimport_meta_main_value = Some(true)for the entry), sourceimport.meta.main = x;reachesmaybe_rewrite_property_accessvia the EDot visitor (visit_expr.rs:1426 forwardsin_.assign_target = Replace); theEImportMetaarm at fold.rs:499 returnsEBoolean(true)unconditionally, so the printer emitstrue = x;— an earlySyntaxError: Left side of assignment is not a reference, whereas the input is valid JS that throws a run-time TypeError. Same forimport.meta.dir = x/.file/.path/.urlunder--format=cjsor Bake (lines 521-540 returnEStringconstants), andimport.meta.hot = x(line 504 returnsESpecial). Base branch behaves identically; the fix is to gate the wholeEImportMetaarm onidentifier_opts.assign_target() == AssignTarget::None && !identifier_opts.is_delete_target(), mirroring theEIdentifier/EObject/EDot/EIndexarms and the guard this PR adds forEString.Verification: pre-existing — The
EImportMetaarm at src/js_parser/fold.rs:498-554 is a parallel switch arm in the samematchthat this PR patches (EString at :445-456), and it has noidentifier_opts.assign_target()guard. At :499-501name == b"main"returnsp.value_for_import_meta_main(...), which per src/js_parser/p.rs:5229-5235 returnsExprData::EBoolean(known)whenever… | pre-existing — the…
…arget' into farm/a1513e28/assign-target-substitution # Conflicts: # src/js_parser/fold.rs # src/jsc/RuntimeTranspilerCache.rs
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robobun
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On the two findings about
The comment-cop findings are addressed in 0ed4ad4: the four multi-line comments are down to one line each or removed. |
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Updated 7:40 PM PT - Aug 28th, 2026
❌ @robobun, your commit 55fbf03 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 40804That installs a local version of the PR into your bun-40804 --bun |
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Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/js_parser/visit/visit_expr.rs— pre-existing: dot-define substitution ine_dotstill fires when the dot is adeleteoperand — the sibling of the identifier-define path this PR just guardedExtended reasoning...
Bun auto-injects
process.env.NODE_ENV(and users pass--define a.b=<constant>) as dot defines. With this PRp.delete_targetis now set (line 1222) sois_delete_targetat line 1338 is finally correct, and the identifier-define path at line 255 now skips substitution underdelete. But the dot-define path here still gates only onin_.assign_target == js_ast::AssignTarget::Noneand ignoresis_delete_target, sodelete process.env.NODE_ENV(or--define window.DEBUG=0withdelete window.DEBUG) is folded todelete "development"/delete 0. That evaluates totruewith no side effect, whereas the original deletes the property (and can returnfalse/ throw for a non-configurable one). Output is unchanged from the base branch — the guard was unreachable before becauseis_delete_targetwas always false — but the PR fixes this exact class for identifier defines and property-access folds while leaving the dot-define arm inconsistent. Fix: add&& !is_delete_targetto the condition at line 1348 so a constant dot-define value is not substituted into adeleteoperand.Verification: pre-existing — the code observation is accurate but merging changes nothing on this path, and the output is valid JS (unlike the SyntaxError cases this PR fixes). At
src/js_parser/visit/visit_expr.rs:1348, the dot-define substitution gates only onif in_.assign_target == js_ast::AssignTarget::Noneand does not consultis_delete_target(computed at line 1338).value_for_define…
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On the dot-define finding ( |
`X ??= 1` with --define X=0 printed `0 ??= 1` and `"foo".length ??= 1` printed `3 ??= 1` under --minify-syntax. Both go through the same assign_target guard as `X += 1`. Add `??=`, `||=` and `&&=` rows so the matrix covers them on every entry point.
The three arms in maybe_rewrite_property_access checked assign_target only, so `delete module.id` bundled to `delete "lib.cjs"`. Add the is_delete_target guard that the neighbouring module.exports arm uses. Test the import.meta writes on ESM output from a non-entry module, where `import.meta.main` reads inline to `false`, instead of on --format=cjs output, where import.meta itself is not valid.
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Actionable comments posted: 2
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Inline comments:
In `@src/js_parser/visit/visit_expr.rs`:
- Line 1220: In the delete-expression handling around p.delete_target, save the
existing target before visiting the operand, restore it immediately afterward,
and preserve the operand’s visit behavior so later identifier resolution is
unaffected. Add a regression test covering delete X followed by console.log(X)
with --define X=0.
In `@test/bundler/transpiler/assign-target-substitution.test.ts`:
- Line 97: Replace the parameterized test.each() and test.concurrent.each()
calls in the affected test matrices with describe.each() blocks, including the
cases around stringLengthTargets and the other referenced matrices. Keep each
individual test inside the generated describe block, retaining concurrency only
for nested tests where it is currently required.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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📒 Files selected for processing (8)
src/js_parser/fold.rssrc/js_parser/visit/visit_expr.rssrc/jsc/RuntimeTranspilerCache.rstest/bundler/bundler_cjs2esm.test.tstest/bundler/esbuild/default.test.tstest/bundler/transpiler/assign-target-substitution.test.tstest/bundler/transpiler/assign-to-import.test.tstest/bundler/transpiler/transpiler.test.js
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On the coverage note for the |
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Closing: this change is now a commit of #30936 ( |
Problem
"foo".length = 4prints3 = 4under--minify-syntax(always on in the runtime transpiler).X = 1prints0 = 1under--define X=0.bun file.jsthen fails withSyntaxError: Left side of assignment is not a reference.for a valid program.delete "foo".lengthfolds todelete 3anddelete module.idbundles todelete "lib.cjs".maybe_rewrite_property_access(src/js_parser/fold.rs) folds"str".length,import.meta.*andmodule.idwith no target check, andOp::UnDeletenever setp.delete_target, so everyis_delete_targetguard was dead.e_identifier(src/js_parser/visit/visit_expr.rs) refuses to substitute a constant into a target, but then passes the define's value text (0) tohandle_identifierasoriginal_name, which renames the identifier to0.Fix
"str".length,import.meta.*,module.id/filename/pathand[x][n]folds for an assignment or delete target, and setp.delete_targetbefore visiting adeleteoperand. Reads still fold.--define X=FOO) is still substituted there, as in esbuild'sdefineValueCanBeUsedInAssignTarget.p.delete_targetchanges three more behaviors, each as esbuild does it:delete ns.xon an import namespace is a build error (assign-to-import.test.ts),delete exports.adeoptimizes the CommonJS to ESM unwrap (bundler_cjs2esm.test.ts), anddelete E.Aordelete FOOno longer inlines the enum member or define (transpiler.test.js). The runtime transpiler cache moves to version 28 so old entries miss.test/bundler/transpiler/assign-target-substitution.test.ts, 62 cases over every entry point, 47 fail on the released binary. The other suites run are listed in Notes.Background
AssignTargetmarks an expression that is written to.p.delete_targetis the operand of thedeletebeing visited. Both reachmaybe_rewrite_property_accessthroughIdentifierOpts.--define K=V) replaces each read of the unbound identifierKwith the parsed valueV.DefineData.original_namekeeps the raw text ofV.handle_identifieris the final step for every identifier reference. Itsoriginal_nameargument re-resolves the identifier under that name, for synthesized identifiers only.assign-to-definewarning here only at debug level. bun suppresses that class of warning for every file, so no message is added.Notes
Suites run with the fix:
transpiler.test.js,assign-to-import.test.ts,bundler_cjs2esm,bundler_cjs,bundler_edgecase,esbuild/default,esbuild/dce. Entry points in the matrix:Bun.Transpiler,bun buildwith and without--no-bundle, bundledimport.metaandmodule.id,bun file.js,bun --define.This PR unifies two open PRs and adds the test matrix:
p.delete_target, the"str".length,import.meta.*,import.meta.hot.*and[x][n]guards) and js_parser: keep the identifier when assigning to a define with a constant value #38563 (the define half, fixesbun.build--defineshould not substitution a variable that is assigned to with a substitution that is not assignable #21210). Both branches are merged in as they are, so their commits and tests are included unchanged. Both conflicted with main on the cache version (26 vs 27), resolved here as 28. Both are closed in favor of this PR.test/bundler/transpiler/assign-target-substitution.test.ts, the matrix over every assignment-target kind for both rewrites on every entry point, and themodule.id/filename/pathdelete guard.Related open PRs that touch the same code: #40857 carries the same
e_identifierchange (itsdefine_value_can_be_assign_targetalso admits member-chain values) and listsFixes #21210. Its hunk rebases cleanly on this PR. #38200 lowersimport.metain cjs and iife output, which is why theimport.metatest here uses esm output. #40829 touches the same fold sites for call targets.Not changed, on purpose: a dot define under
deletestill substitutes.--define window.DEBUG=0withdelete window.DEBUGprintsdelete 0, which is what esbuild prints too (its dot-define path checksassignTargetonly). One residual of that:--define window.FOO=undefinedwithdelete window.FOOprintsdelete undefined, an early error in strict mode, where esbuild printsdelete void 0. Gating the dot-define path onis_delete_targetwould makedelete process.env.NODE_ENVthrow at run time in a browser bundle, so the fix for that residual belongs in the printer or in #40857's define work, not here.The matrix also covers
??=,||=and&&=on every entry point. Without the fix,X ??= 1printed0 ??= 1and"foo".length ??= 1printed3 ??= 1.Repro before the fix:
After the fix both print the source unchanged except
console.log(0), which is what esbuild 0.18 and 0.25 print.bun sl.jsnow runs and throws the run-timeTypeError: Attempted to assign to readonly property.on the first line, as node does in strict mode.The define guard itself was never the problem:
NaN = 2andundefined = 1go through the same path and were always left alone, because the builtin defines have an emptyoriginal_name.The dot-define path (
--define window.DEBUG=0withwindow.DEBUG = 1) already checksassign_targetbefore substituting and is unchanged. esbuild additionally substitutes an identifier-valued dot define in target position; bun does not, and this PR does not change that.esbuild folds
delete "foo".lengthtodelete 3. This PR keeps the reference instead, sincedelete 3istruewhere the original isfalse(sloppy) or a TypeError (strict).Not in this PR: the printer emits a leading space when the output starts with a prefix
++or--(++x;). The new test trims both ends because of it. It is tracked separately.The two
bytecode:tests inbun-build-api.test.tstime out at 5 s under the local debug build (128 bytecode builds per variant). They do not touch this code.[review] gate passed · iteration 0 · 8 files touched
fails on main (without fix)
passes on PR (with fix)
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gate history · 3 passed · 0 rejected · iteration 0
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