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Updated 12:30 PM PT - Sep 16th, 2026
✅ @robobun, your commit 38534e045606af18b050b8f3aaed6087de46d60b passed in 🧪 To try this PR locally: bunx bun-pr 42935That installs a local version of the PR into your bun-42935 --bun |
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Status How I reproduced it, on main ( mkdir -p node_modules/react && echo '{"name":"react","main":"index.js"}' > node_modules/react/package.json && : > node_modules/react/index.js
echo 'export const jsxDEV = (type, props, key) => ({ type, props, key });' > node_modules/react/jsx-dev-runtime.js
echo 'export const c = n => new Array(n).fill(Symbol.for("react.memo_cache_sentinel"));' > node_modules/react/compiler-runtime.js
cat > case.jsx <<'JSX'
function K(p) { const r = { n: p.n }; const el = <a key={`k${r.n}`} title={(() => { r.n = 10; return r.n; })()} />; return <div>{JSON.stringify([el.key, el.props])}</div>; }
console.log(K({ n: 2 }).props.children);
JSX
bun build case.jsx --target=browser --outfile=plain.js && bun plain.js
bun build case.jsx --target=browser --react-compiler --outfile=rc.js && bun rc.jsThe build without the compiler prints The new tests |
WalkthroughChangesJSX key ordering
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to JSX keys now evaluate after props and children while retaining the required code generation behavior. No merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@test/bundler/transpiler/react-compiler.test.ts`:
- Line 2970: Replace the target loop around the shared itBundled test with a
describe.each() parameterized suite for the "bun" and "browser" targets,
preserving the existing test body and target-specific behavior.
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📒 Files selected for processing (3)
src/react_compiler/lowering/build_hir/jsx.rstest/bundler/transpiler/react-compiler-fixtures.test.tstest/bundler/transpiler/react-compiler.test.ts
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The automatic runtime call is `jsx(type, props, key)`: the key runs after
the props and the children. Lowering put the key first, so the compiler
took it for the first thing the element evaluates. A key that stays inline
still printed in place and ran last. A key with a name (a dependency of a
memo block, or an operand that PromoteInterposedTemporaries names ahead of
a statement) ran first, ahead of the attributes and children it follows in
the output without the compiler.
The key is now lowered after the children. It stays the first attribute of
the HIR element, ahead of any spread, which is how codegen tells this call
shape from `createElement(type, {...spread, key})`.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline nit, I also checked the props.insert(0, …) ordering concern: the key Place is a temporary defined before the JsxExpression instruction like every other attribute, so its list position does not affect definedness, and codegen's is_key_after_spread check in src/react_compiler/codegen.rs:2578 still routes the key-first list to the jsx(type, props, key) shape while the createElement branch is untouched by this diff.
Extended reasoning...
The diff moves the lowering of args[2] (key) in lower_jsx_call from before the props loop to after it, and inserts the resulting attribute at index 0. All attribute places are temporaries lowered before the JSX instruction that consumes them, so attribute list order is a codegen-shape concern, not an SSA-definition concern. Codegen selects the createElement shape only when a key attribute appears after a spread; inserting at index 0 preserves the jsx() shape for this call form, and the createElement(tag, props, ...children) branch of lower_jsx_call is not modified. The remaining question of whether the new _c(36) divergence entry was captured from correct output was not independently rebuilt here, which is why a human look is still worthwhile alongside the inline test-vacuity nit.
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I re-reviewed after the latest commit and found no bugs; the compiled-precondition check now covers all five forms. Because this changes HIR lowering order on purpose (a deliberate deviation from upstream's build_hir with a recorded fixture divergence), a compiler maintainer's look is still worthwhile.
What was reviewed:
props.insert(0, key)vs codegen:codegen.rsonly routes tocreateElementwhenkeyfollows a spread, so a front-inserted key always prints asjsx(type, props, key)— consistent with the now-last lowering position.outline_jsx.rshandleskeyby name, not position, so the reordering does not affect it; the classic/createElementbranch is untouched.- Fixture divergence entry format matches how
SLOT_COUNT_DIVERGENCEis consumed (number[], asserted exactly).
Extended reasoning...
Overview
The PR moves the lowering of the hoisted JSX key argument (args[2]) in lower_jsx_call (/home/claude/bun/src/react_compiler/lowering/build_hir/jsx.rs) from before the props/children loop to after it, so HIR instruction order matches the runtime evaluation order of jsx(type, props, key). The resulting JsxAttribute is inserted at index 0 of the props list so it stays ahead of any spread. Tests add an itBundled case for bun and browser targets with five component shapes, and the fixtures test records one accepted slot-count divergence (method-call-scope-merge-mutable-range-sync, 36 vs 37).
Security risks
None. This is compiler transform code operating on the user's own source; there is no untrusted input parsing, no I/O, and no allocation or bounds arithmetic on external data beyond a Vec::insert(0, ..).
Level of scrutiny
Moderate. The diff is small, but it changes HIR instruction ordering, which feeds reactive scope inference and codegen inlining decisions, and it deliberately departs from the upstream reference implementation. I confirmed the two consumers that care about the key attribute (codegen.rs shape selection around line 2578 and outline_jsx.rs around lines 537/591) are keyed on the attribute name or on spread-relative position, not on lowering order, so the front insertion preserves the jsx() call shape. The PR's own differential testing claims are author claims I could not reproduce here (no debug build present), so a maintainer familiar with the React Compiler port should confirm the fixture divergence is acceptable.
Other factors
My earlier nit (the runtime assertion alone could pass with uncompiled functions) was addressed in commit 5c863be by an onAfterBundle check that each of the five functions starts with a memo cache on the browser target; the bun/ssr target legitimately has no memo blocks, so the early return there is correct. No third-party objections are outstanding (the coderabbit thread was resolved by a non-author). The change is well-scoped and tested, but it is a semantic change to a ported compiler pass, so I am deferring rather than approving.
…e the props object
The parser inlines `<a {...{ key: k, title: t }} />` into the props object,
where that `key` stays a property and runs in order. Lowering turned it
into a `key` attribute, and codegen moves every `key` attribute to args[2]
and keeps only the last one. The compiled call ran the key after the other
props, and with a second `key` attribute it dropped one of the two
expressions. Such a function now stays uncompiled, as one with an accessor
in an inlined spread already does.
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Problem
bun build --react-compiler, a JSXkeycan run ahead of the attributes and children that it follows without the compiler.<a key={r.n} title={(() => { r.n = 10; return r.n; })()} />gives key2. The build without the compiler gives10.jsx(type, props, key), so the key runs last.lower_jsx_call(src/react_compiler/lowering/build_hir/jsx.rs:252) lowered it first. A key that stays inline prints in place. A key with a name runs where it was lowered: first.PromoteInterposedTemporaries. react-compiler: keep operands in source order around a statement inside their expression #42859 widened the second case: in ssr mode the key`k${r.n}`was right before it and is wrong after it.Fix
createElement(type, {...spread, key}), where the key runs in attribute order.keythat the parser inlines from<a {...{ key: k }} />stays inside the props object. Codegen moved it to the third argument too. Such a function now stays uncompiled (Notes)._c(37)to_c(36)(Notes). The fixtures test records it.test/bundler/transpiler/react-compiler.test.ts(2 new tests, they fail without the fix),react-compiler-fixtures.test.ts,bundler_jsx.test.ts. Self-reviewed: 1 concern raised, 1 addressed (the inlinedkey).Background
if ($[0] !== dep) {...} else {...}. Only client mode has them.Notes
Forms (
p = { n: 2 },r = { n: p.n }, the stubjsxDEVreturns{ type, props, key }). "before" is main with #42859.<a key={`k${r.n}`} title={(() => { r.n = 10; return r.n; })()} />"k10""k2""k2"(right before #42859)<a title={(() => { r.n = 10; return r.n; })()} key={r.n} />1022(also before #42859)<a key={`k${r.n}`}>{(() => { r.n = 10; return r.n; })()}{r.n}</a>"k10""k2""k2"<a key={bump()} title={r.n} id={[r.n]} />withbump = () => ++r.n23<a key={++r.n} title={[r.n, p.n]} id={r.n} />[2,2][3,2]All of them match the build without the compiler on this branch, in both modes.
<a {...rest} key={k} />is acreateElementcall with the key in the props. It was right before and is unchanged.A
keyinside the props object. The parser inlines a lone object literal spread, so<a {...{ key: bump(), title: r.n }} />isjsxDEV("a", { key: bump(), title: r.n }): thekeyis a property and runs in order. Lowering made it akeyattribute like the hoisted one, and codegen moves everykeyattribute to the third argument and keeps the last one. The compiled call wasjsxDEV("a", { title: r.n }, bump()), and<a key={bump()} {...{ key: bump(), title: r.n }} />lost one of its twobump()calls. This was the same before this PR. The HIR attribute cannot say which kind ofkeyit is, so lowering records a Todo and the function stays uncompiled, as it already does for an accessor in such a spread (record_accessor_prop). The test has both forms, and the browser variant checks that they are the only forms without a memo cache.Why the old order was there. The comment said the key is lowered ahead of the children to match upstream, which lowers the attributes of the opening element and then the children. Upstream prints JSX syntax, and a later JSX transform decides where the key runs. Bun's compiler runs after that transform, so the call shape is known here.
The fixture.
method-call-scope-merge-mutable-range-synchas<div key={store.id}>around many children. The load ofstore.idwas lowered ahead of the children, inside the memo block that computes them, which cached it in a slot. It is now lowered next to the element, and the memo block of thedivdepends onstore.iditself. The element is rebuilt in the same cases. I built every fixture that builds (1,585 in client mode, 1,622 in ssr mode, and client mode withminify.syntax) before and after. This is the only file that differs.Differential runs. The generator from #42859, with a
keyon the element, first or last among the attributes. 2,000 components, two targets, four renders with a fresh memo cache and four with a kept one. The counts are results that differ from the build without the compiler.The 2 are a
letthat is reassigned inside a memo block and is stale on a cache hit (#42482 is open for it). Two generated components that compiled before now stay uncompiled with "This value cannot be modified": the key stores to an object that an earlier attribute already passed to JSX. In the order the call runs, that is a mutation of a frozen value.The other report from the review of #42859. It named a second form, a reassigned parameter ahead of a memo block. I tried ten shapes of it on main before #42859, on main and on this branch. All of them match the build without the compiler everywhere.
[human-review] gate passed · iteration 1 · 3 files touched
fails on main (without fix)
passes on PR (with fix)
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