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react-compiler: emit createElement calls for the classic JSX runtime and key after a spread - #42389
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The compiler decodes the call the visit pass made of each JSX element and
builds a new one from HIR. The decoder guessed the runtime from the arity and
from args[1] being an object, so `createElement(type, {..}, child)` was read
as `jsx(type, {..}, key)`: the child became the key. Codegen then always
emitted `jsx`/`jsxDEV`, which the classic runtime never imports.
Lowering now asks the parser's jsx_imports table which callee it is. Codegen
emits `factory(type, props | null, ...children)` for the classic runtime and
for `key` after a spread in the automatic runtime, like the visit pass.
…is a local Codegen resolves options.jsx.factory again when it rebuilds the call, so the compiler never sees the factory as an operand. A factory that is a parameter or a local of the component looked unused and was dropped.
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Status Reproduced on 1.4.3-canary.1+6a92015fc (same on 1.4.2 and main 4b5862f): cat > a.jsx <<'JSX'
/** @jsxRuntime classic */
import React from "react";
export function App({ a }) {
const o = { a };
return <div title={o.a}>hi</div>;
}
JSX
bun build --react-compiler --target=browser --external react --external 'react/*' a.jsx
# t2 = jsxDEV("div", { title: o.a }, "hi", false, undefined, this) <- no import, "hi" in the key positionThe automatic runtime has the same decode bug: Fail-before and pass-after: PR: #42389 CI on 23230d4 (build 114473): 180 of 181 jobs pass, and the new tests pass on every lane. The one red job is |
WalkthroughChangesThe React compiler now identifies JSX runtime imports through parser metadata, resolves classic JSX factories, lowers classic and automatic call shapes, and generates JSX runtime support
Suggested reviewers: Priority: ⬇️ Low Merge Risk: 🔵 Low · up to The new classic-runtime test cases do not follow the repository's required parameterized-test structure. Align them with describe.each() before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@test/bundler/transpiler/react-compiler.test.ts`:
- Line 1526: Replace the for...of loop over classicRuntimes with a
describe.each() parameterized test suite, preserving the existing prelude,
tsconfig, jsx values and repeated test behavior for every runtime case.
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src/js_parser/p.rssrc/js_parser/parser.rssrc/js_parser/react_compiler_host.rssrc/js_parser/visit/visit_expr.rssrc/react_compiler/DESIGN.mdsrc/react_compiler/codegen.rssrc/react_compiler/lowering/build_hir/jsx.rssrc/react_compiler/program.rstest/bundler/transpiler/react-compiler.test.ts
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I reviewed this PR and didn't find any bugs. Because it changes JSX call-shape detection in the React Compiler's HIR lowering and adds a parallel codegen path that must stay in lockstep with visit_expr.rs::e_jsx_element, a human look would still be worthwhile.
What was reviewed:
codegen_create_element_callmirrors the classic/is_key_after_spreadbranch ofvisit_expr.rs(target selection,nullfor empty props,can_be_unwrapped_if_unused,was_jsx_element).is_key_after_spreadin codegen recomputes correctly: automatic-runtime lowering pushes the hoistedkeybefore any spread, and thecreateElementbranch preserves source order, so the predicate matchesparse_jsx.rs.- The callee-based
jsx_import_kindlookup replaces the arity heuristic and the name-basedFragmentcheck;tag_ofis a straightforward inverse ofget_with_tag. - Local-factory bailout records a
Todo(leaves the function uncompiled) rather than panicking;member_expression_rootrecursion depth is bounded by the configured factory path.
Extended reasoning...
Overview
This PR fixes Bun's built-in React Compiler to correctly emit createElement(type, props, ...children) calls for the classic JSX runtime and for the automatic-runtime "key-after-spread" fallback, where it previously always emitted the automatic jsx()/jsxDEV() shape (producing an unresolved jsxDEV reference and misplacing the first child into the key slot). It touches the parser (p.rs, parser.rs, visit_expr.rs) to extract a shared jsx_classic_member_expression helper and add a JSXImportSymbols::tag_of reverse lookup; extends the bun_react_compiler::Host trait with is_jsx_classic, jsx_classic_factory, and jsx_import_kind; rewrites HIR lowering in build_hir/jsx.rs to distinguish call shapes by the callee symbol instead of arity; and adds codegen_create_element_call in codegen.rs. Five new itBundled cases in react-compiler.test.ts cover the classic runtime via pragma/tsconfig/CLI flags, automatic-runtime key-after-spread, and the local-factory bailout.
Security risks
None. This is transpiler output-shape logic operating on already-parsed AST; no network, filesystem, credential, or untrusted-size handling is involved. The one new user-influenced input is the configured options.jsx.factory path, which the extracted helper handles identically to the pre-existing code (moved verbatim, including the .expect("unreachable")).
Level of scrutiny
Moderate-to-high. The change is not mechanical: it introduces a second codegen path that must produce AST byte-identical to what visit_expr.rs::e_jsx_element would have produced for the same source, and it replaces a heuristic (arity + args[1] shape + ref_name().starts_with(b"jsxs")) with a symbol-table lookup whose correctness depends on jsx_imports being populated for every callee the visit pass can emit. Getting this wrong silently miscompiles React components rather than failing loudly. Someone familiar with the visit-pass/React-Compiler handoff should confirm the shape-choice invariants — in particular that automatic-runtime lowering always front-loads the hoisted key so codegen's is_key_after_spread never fires spuriously, and that JsxFragment truly cannot reach codegen under the classic runtime (guarded by debug_assert!).
Other factors
Test coverage is strong and follows repo conventions: variant matrix over pragma/tsconfig/flags, both call shapes exercised in one component, expect(JSON.parse(stdout)).toEqual(...) on structured output, and an explicit assertion that the local-factory case stays uncompiled. The PR description states the new tests fail on the base and lists the sibling test files that were rerun. No CODEOWNERS entries match the changed paths, and there are no prior reviews or unresolved threads on the timeline. The bug-hunting run completed on a dry streak with no candidate findings.
…he JSX children (#42393) ### Problem - With `bun build --react-compiler`, `<div children="x">y</div>` renders `["x","y"]`. A plain build renders `"y"`. Two `children` attributes and `<div {...{ children: "sp" }}>real</div>` merge the same way, and `<div children="x" {...p} />` ignores `p.children`. - The parser lowers the element to `jsx("div", { children: "x", children: "y" })` before the compiler runs. `lower_jsx_call` (`src/react_compiler/lowering/build_hir/jsx.rs:296`) read every `children` key of that object as a JSX child. - `<i {...{ get g() { return p.a } }} />` passes the getter function as the prop `g`. The parser inlines the spread into the props object, and the lowering ignored `prop.kind`. ### Fix - Only the last property of the props object is read as the JSX children. The parser always appends them last. An earlier `children` key stays an attribute in its position, so the printed keys keep their source order. - A getter or setter in the props object records a `Todo` error, so the function is left uncompiled. `lower_object_method` does the same for object literals. - Verified: `react-compiler/ChildrenAttributeIsNotAJsxChild` in `test/bundler/transpiler/react-compiler.test.ts` (8 components, fails on 1.4.2). Also all of `react-compiler.test.ts` and `react-compiler-fixtures.test.ts`. ### Background - Bun's visit pass turns JSX into `jsx(tag, props, key)` calls. The compiler runs after it and decodes each call back into a `JsxExpression` instruction with `props` and `children`. - In an object literal the last duplicate key wins, and a spread after a key can replace it. - Codegen prints the attributes in order and then the children as a final `children` key. <details><summary>Notes</summary> Results of the test components, plain build and compiled build after this change (stub `jsx` runtime that returns `{t, p}`): | source | `props.children` | | --- | --- | | `<div children="x">y</div>` | `"y"` | | `<div children={p.a}>{p.b}</div>` with `{a: 1, b: "x"}` | `"x"` | | `<div children="a" children="b" />` | `"b"` | | `<div {...{ children: "sp" }}>real</div>` | `"real"` | | `<div children="x" {...p} />` with `{children: "q"}` and `{}` | `"q"`, `"x"` | | `<div children="x" {...p}>{p.a}{p.b}</div>` | `[1, 2]` | | `<div {...p} children="x" />` | `"x"` | | `<i {...{ get g() { return p.a } }} />` with `{a: 5}` | `props.g === 5` | Before: `["x","y"]`, `[1,"x"]`, `["a","b"]`, `["sp","real"]`, `"x"`/`"x"`, `["x",1,2]`, `"x"`, and `props.g` was a function. A `children` attribute that is the last property and not a JSX child (`<div {...p} children="x" />`) is still read as a child. The output is the same object, because codegen prints the children last. The upstream compiler does not have this problem: it sees the JSX element itself and prints it back, and Babel's JSX transform then builds the same duplicate-key object as a plain build. #42389 rewrites how `lower_jsx_call` tells the `jsx` call shape from the `createElement` one. It does not touch the two property loops this PR changes. </details>
2 participants
Problem
bun build --react-compileron a file with the classic JSX runtime (@jsxRuntime classic, tsconfig"jsx": "react",--jsx-runtime=classic) printsjsxDEV(...)calls that nothing imports. The first render throwsReferenceError: jsxDEV is not defined.lower_jsx_call(src/react_compiler/lowering/build_hir/jsx.rs:218) guessed the runtime from the arity and an objectargs[1].createElement("div", {title}, "hi")looks likejsx(tag, props, key), so"hi"became the key.codegen_jsx_call(src/react_compiler/codegen.rs:2597) always emitted the automatic shape.<div {...props} key={k}>hi</div>compiled tojsxDEV("div", {...props}, k, ...)with no child.Fix
jsx_importstable which symbol the callee is (Host::jsx_import_kind). Onlyjsx,jsxsandjsxDEVdecode as the automatic shape.factory(type, props | null, ...children), withkeyleft inprops, for the classic runtime and forkeyafter a spread in the automatic runtime.visit_expr.rs::e_jsx_elementmakes the same choice.P::jsx_classic_member_expressionreplaces two copies of the factory lookup invisit_expr.rs, so the compiler host shares it.test/bundler/transpiler/react-compiler.test.ts(5 new cases, all fail on 1.4.3-canary). Alsoreact-compiler-fixtures.test.ts,bundler_jsx.test.ts,transpiler.test.js.Background
jsx(type, {...props, children}, key)fromreact/jsx-runtime, orcreateElementfromreactwhenkeyfollows a spread.React.createElement(type, props, ...children). Nothing is imported andkeyis a prop.Hostis the compiler crate's view of parser state.Notes
Repro (1.4.2, 1.4.3-canary.1+6a92015fc, main 4b5862f):
No
react/jsx-dev-runtimeimport is printed, and"hi"sits in the key position. With this change the same line isReact.createElement("div", { title: o.a }, "hi").The automatic runtime case:
Before:
jsxDEV("div", { ...props }, k, false, undefined, this). After:createElement("div", { ...props, key: k }, "hi"), the same as the function the compiler does not touch.Why the arity guess cannot work: the visit pass emits
createElement(tag, propsOrNull, ...children)for the classic runtime and for the fallback. One child gives 3 arguments, four children give 6. Both collide withjsx(tag, props, key)andjsxDEV(tag, props, key, isStatic, source, self)when the element has at least one attribute.What lowering keeps: the automatic shape pushes the hoisted
keyfirst, so it never follows a spread. ThecreateElementshape keepskeywhere the source has it. Codegen recomputes "key after spread" from that order, the same testparse_jsx.rsuses (lastkeyindex greater than first spread index).When codegen picks the
createElementshape in the automatic runtime: only when akeyattribute follows a spread in the IR. That cannot happen for an element the visit pass hoisted akeyout of. Akeyleft inside the props after a spread means a laterkeyexists, and then the parser already chose the fallback. The one other source is the spread-collapse of the visit pass:<div {...{ ...x, key: k }}>becomesjsxDEV("div", { ...x, key: k }). The compiled output used to hoist thatkey(jsxDEV("div", { ...x }, k)), which letsx.keywin. It is nowcreateElement("div", { ...x, key: k }), wherekwins as in the source.Classic fragments:
<>...</>arrives as a call whose tag isReact.Fragment(fromoptions.jsx.fragment). It lowers as a plain member-expression tag, soJsxFragmentonly exists for the automatic runtime'sFragmentimport. Slot counts are the same as for the automatic runtime.A factory that is a local of the component (
function App({ h })with@jsx h) is not an operand of the IR instruction. Codegen resolvesoptions.jsx.factoryagain when it rebuilds the call, so the compiler saw the local as unused and dropped it. Lowering now records aTodofor that case and the component stays uncompiled (ClassicRuntimeLocalFactoryIsNotCompiled). Babel after the upstream compiler has the same blind spot, because the factory only appears when the JSX transform runs.Other checks with the debug build:
import * as React,--target=bun(ssr output mode),--minify,NODE_ENV=production(jsx(type, props, key)with 3 arguments still decodes as automatic),esbuild/default.test.ts -t JSX,esbuild/tsconfig.test.ts -t jsx,jsx-production.test.ts,jsx-tsconfig-react-jsx.test.ts.