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react-compiler: register the locals of a compiled function in its own scope - #42463
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… scope The React Compiler mints a new symbol for every parameter, local, temporary and label of a function it compiles. The parser host put all of them in the module scope's `generated` list. The bundler's number renamer names top-level symbols from each part's declared symbols and nested symbols from the scope tree, so it never named these. They kept their original names, and a local with the name of the export that an import links to shadowed that export: `let theme = custom ?? theme`. `Host::new_local` registers such a symbol in the body scope of the function being compiled. `Host::new_generated` stays for names declared at module level (an outlined `_temp`). After the compiler replaced the parameters and the body, the parser drops the members and the child scopes that it recorded for the old ones, so the renamer does not number each new local around the dead symbol of the same name. It keeps `arguments` and the own name of a function expression, which the output still prints. If the compile fails, the parser removes the symbols that the attempt registered. The name of an outlined function is now resolved where the function is outlined, and a JSX-outlined component that is compiled again keeps the `Ref` its use site printed.
WalkthroughThe React Compiler now allocates function-scoped symbols, cleans up replaced-function scopes, preserves outlined-function identifiers, and validates collision handling across bundler targets and minification modes. ChangesReact Compiler symbol handling
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to The regression test matrix should be converted to the required parameterized-test form before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 5:01 AM PT - Sep 12th, 2026
✅ @robobun, your commit 2c481a967dee8c99bec290592b08102d67565f9d passed in 🧪 To try this PR locally: bunx bun-pr 42463That installs a local version of the PR into your bun-42463 --bun |
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I reviewed this PR and didn't find any bugs. Because it rewires how compiler-minted symbols are scoped and mutates the parser's scope tree post-visit (including an unsafe Members::put and pruning of the FunctionArgs scope), a human look at the scope/renamer interaction would still be worthwhile.
What was reviewed:
drop_symbols_of_replaced_function: checked that theputkey is an existing arena/source-backed member key satisfying the lifetime contract, and thatarguments+ the fn-expr own-name are the only FunctionArgs members retained.codegen_reactive_functionnow returningid: None: traced consumers — only the outlined-function loop andcompile_outlined_fnreadid, both now assign it explicitly at module level.- Bail-out path:
generated_lensnapshot +truncatecorrectly discards anynew_localsymbols appended before a mid-compile bail. - Tests:
itBundledmatrix covers both minify states and both targets;onAfterBundleasserts the compiler actually ran; no CODEOWNERS paths touched.
Extended reasoning...
Overview
This PR fixes a symbol-scoping bug in Bun's React Compiler integration: locals the compiler mints for a compiled function (parameters, temporaries $/t0/t1, block labels, catch bindings, nested fn-expr names) were being registered at module scope via Host::new_generated, so the bundler's number renamer never numbered them against top-level exports. When an import printed under its export's name (e.g. defaultTheme → theme), a compiled local of the same name self-shadowed (let theme = custom ?? theme). The fix adds Host::new_local (routes to the FunctionBody scope's generated list), reroutes ref_for_local/well_known/label_for_block/resolve_identifier/nested fn-expr names through it, moves outlined-function id assignment to the outlining call site (module-level ref_for_name), and adds drop_symbols_of_replaced_function to prune the replaced body's dead members/children plus the parent FunctionArgs scope down to arguments and the fn-expr's own name. The visit records generated.len() before compiling and truncates on bail-out. Tests add a 2×2 itBundled matrix over 12 collision forms plus a slot-count sort for enableJsxOutlining fixtures.
Security risks
None. This is compiler/bundler symbol-naming logic operating on already-parsed AST; no untrusted-input parsing, no auth/crypto/network surface. The one unsafe block calls Members::put with a key that is already a stored key of a live scope's members map (arena/source-backed), satisfying the documented lifetime contract of get_or_put_hashed.
Level of scrutiny
Medium-high. The change is focused and well-argued (the PR description includes A/B counts over 1499 upstream fixtures and an end-to-end multi-file check), but it mutates the parser's scope tree after the visit has run — pruning members, children, and the parent FunctionArgs scope — in a way whose correctness depends on downstream renamer behavior and on which symbols the printer can still reach (arguments, fn-expr own-name, React Refresh _s in generated). The id: None change in codegen_reactive_function shifts an invariant to two call sites. These are the kind of non-local invariants a maintainer familiar with the renamer and the outlining pipeline should confirm.
Other factors
No CODEOWNERS entries cover the changed paths. The bug hunt exited on dry_streak with no findings. Test coverage is strong per REVIEW.md's matrix rule (both minify states, both targets, 12 collision shapes including outlined-callback params, catch bindings, temporaries, and three fn-expr own-name forms), and onAfterBundle asserts the compiler ran rather than bailing. The fixtures-test relaxation (sorting slot counts under enableJsxOutlining) is scoped to that pragma with a stated reason. The SAFETY comment on the new unsafe block states the invariant, though a maintainer may want it phrased more precisely against get_or_put_hashed's "outlives this scope" contract.
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The review above found no bugs. It made one suggestion: state the SAFETY comment at the |
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Inline comments:
In `@test/bundler/transpiler/react-compiler.test.ts`:
- Around line 1891-1892: Replace the nested target/minifyIdentifiers loops with
a single describe.each() parameter table containing four named combinations,
passing target and minifyIdentifiers through the callback parameters. Preserve
the existing itBundled assertions and test behavior for all combinations.
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src/js_parser/react_compiler_host.rssrc/js_parser/visit/mod.rssrc/react_compiler/DESIGN.mdsrc/react_compiler/codegen.rssrc/react_compiler/pipeline.rssrc/react_compiler/program.rstest/bundler/transpiler/react-compiler-fixtures.test.tstest/bundler/transpiler/react-compiler.test.ts
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Since #42463 a symbol that the compiled function declares goes through `ref_for_local`, so the renamer names it like any other local. The copy went through `ref_for_name`, which registers a module-level symbol. A copy named `t1` then shadowed an import that the bundler prints as `t1`, and the function read the copy.
2 participants
Problem
bun build --react-compilerand unminified names, a local of a compiled function can shadow the export it reads.const theme = custom ?? defaultThemeprintslet theme = custom ?? theme:ReferenceError: Cannot access 'theme' before initialization.Parameters,catchbindings and the temporariest0,$break too, often with a wrong value and no error.src/js_parser/react_compiler_host.rs:83), where the linker's renamer does not name them.Fix
Host::new_localregisters such a symbol in the body scope of the compiled function.Host::new_generatedstays for module-level names.argumentsand the own name of a function expression. The rest is dead, and each new local would be numbered around the dead symbol of its name (count2).test/bundler/transpiler/react-compiler.test.ts(4 new tests, 2 fail on 1.4.3-canary). Alsoreact-compiler-fixtures.test.ts,bundler_jsx.test.ts.Background
membersandgenerated).--minify-identifiersuses another renamer, which hid this.defaultThemeprints astheme). The linker picks that name, after the compiler's own rename pass.Notes
Repro.
theme.js:export const theme = "dark";.app.jsx:bun build --react-compiler --target=bun app.jsx --outfile=c.js && bun c.jsprintslight darkwith this change. Before, it throws the ReferenceError above. Without--react-compilerthe bundler printstheme2for the local, and this change gives the same name. 1.4.0 to 1.4.3-canary and main have the bug.The new tests.
LocalNamedLikeAnExportItReads-{bun,browser}-identifiers={false,true}run 12 forms: a local, a hook, amemoarrow, a destructured parameter, a namespace member, the parameter of an outlined callback, the parameter of a nested callback, acatchbinding, the temporaries, and three function expressions that have the name of the export they call (forwardRef(function Wrapped…),memo(function Wrapped…),const X = function Wrapped…). On 1.4.3-canary 8 forms print a wrong value or throw in the two builds that keep the names. The twoidentifiers=truebuilds pass before and after. They guard the nested slots that the locals now get.Which names the codegen treats as module level. A
LoadGlobalorStoreGlobalthat carries noRef(the name of an outlined function), the id of an outlined function, and the import names for hook guards and instrumentation. Everything else is a local: named and promoted identifiers, the memo cache$, labels (bb0), the own name of a nested function expression.codegen_reactive_functionno longer resolves the function id. The only consumer is the outlined declaration, so the loop over outlined functions resolves it.What stays in the scopes of a compiled function. The parser declares the own name of a function expression in its FunctionArgs scope, next to the parameters and
arguments. The compiled function keeps that name, so the helper keeps that member. Without it,forwardRef(function Button(props) { return BaseButton(props) })overimport { Button as BaseButton }printsfunction Button(props) { return Button(props) }and never returns. Thegeneratedlists stay too (a React Refresh_slives there).Locals of an outlined function. They go in the scope of the component they came from, because
current_scopeis that body when the codegen runs. The declaration is printed at module level. That is safe: every use that the outlined function makes is recorded in the scope of the component, so the renamer keeps its locals apart from each top-level symbol that it reads.JSX outlining (debug and ASAN builds only,
@enableJsxOutlining). The outlined component is compiled a second time from its generated AST. Before, that second pass failed: its locals were in thegeneratedlist of the module scope, andresolve_identifierclassifies such a symbol as a global. The component was dropped from the output, and its use site referenced a name that nothing declared. The 9jsx-outlining-*fixtures passed through the "Bun compiled a strict subset" branch. Now the second pass succeeds, and the slot counts equal the upstream values (5,8,9,11). Bun declares an outlined function ahead of the other statements of the module, upstream after the function it came from. So the fixtures test compares the slot counts of those fixtures as sorted lists.compile_outlined_fnalso keeps theRefthat the use site printed.A/B of all upstream fixtures (1499 outputs, the same
Bun.buildcall as the fixtures test, baseb99371011f):jsx-outlining-*fixtures above.repro-no-declarations-in-reactive-scope-with-early-returnreads an unbound globalitem, and the renamer now numbers the callback parameteritemaround it (item2), as it does in a function that is not compiled.context-variable-as-jsx-element-taghasfunction Component() { let Component = ... }. The parser records a use of the binding of a compiled function inside its scope, so the local prints asComponent2. Both are correct.minify: { identifiers: true }: 519 outputs differ, all parse. The locals now get nested slots (shared between functions) where they took top-level slots before.End to end. A multi-file app (12 components, React 18.3.1 with a
compiler-runtimepolyfill, locals named like 26 exports that they read, two function expressions named like the component they wrap) rendered withreact-dom/server. 8 build variants (bunandbrowsertargets, each with no flag,--minify-identifiers,--minify,--minify-syntax). With this change all 8 render the same markup as the build without the compiler. On the base commit the 4 variants that do not minify identifiers crash (let list = items ?? items).Also checked against the build without the compiler:
--splittingwith two entry points,--format=cjs,--format=iife, a component module that is loaded withrequire()(ESM wrapper), and a CommonJS component module.Not in this PR.
exportsandmoduleof a CommonJS wrapper, decorator temporaries) trip such a check today, so it needs its own change.[human-review] gate passed · iteration 1 · 8 files touched
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