[compiler] Fix assignment within for update expression

When converting value blocks from HIR to ReactiveFunction, we have to drop StoreLocal assignments that represent the assignment of the phi, since ReactiveFunction supports compound expressions. These StoreLocals are only present to represent the conditional assignment of the value itself - but it's also possible for the expression to have contained an assignment expression. Before, in trying to strip the first category of StoreLocal we also accidentally stripped the second category. Now we check that the assignment is for a temporary, and don't strip otherwise.

ghstack-source-id: cdb81880f29d5be3313086bd9e3fcfef9db0f85e
Pull Request resolved: #30067

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -921,14 +921,17 @@ class Driver {
         });
       } else if (defaultBlock.instructions.length === 1) {
         const instr = defaultBlock.instructions[0]!;
-        let place: Place = instr.lvalue!;
+        let place: Place = instr.lvalue;
         let value: ReactiveValue = instr.value;
-        if (instr.value.kind === "StoreLocal") {
-          place = instr.value.lvalue.place;
+        if (
+          value.kind === "StoreLocal" &&
+          value.lvalue.place.identifier.name === null
+        ) {
+          place = value.lvalue.place;
           value = {
             kind: "LoadLocal",
-            place: instr.value.value,
-            loc: instr.value.value.loc,
+            place: value.value,
+            loc: value.value.loc,
           };
         }
         return {
@@ -939,14 +942,17 @@ class Driver {
         };
       } else {
         const instr = defaultBlock.instructions.at(-1)!;
-        let place: Place = instr.lvalue!;
+        let place: Place = instr.lvalue;
         let value: ReactiveValue = instr.value;
-        if (instr.value.kind === "StoreLocal") {
-          place = instr.value.lvalue.place;
+        if (
+          value.kind === "StoreLocal" &&
+          value.lvalue.place.identifier.name === null
+        ) {
+          place = value.lvalue.place;
           value = {
             kind: "LoadLocal",
-            place: instr.value.value,
-            loc: instr.value.value.loc,
+            place: value.value,
+            loc: value.value.loc,
           };
         }
         const sequence: ReactiveSequenceValue = {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/for-with-assignment-as-update.expect.md

@@ -0,0 +1,49 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  let x = props.init;
+  for (let i = 0; i < 100; i = i + 1) {
+    x += i;
+  }
+  return [x];
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ init: 0 }],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+function Component(props) {
+  const $ = _c(2);
+  let x = props.init;
+  for (let i = 0; i < 100; i = i + 1) {
+    x = x + i;
+  }
+  let t0;
+  if ($[0] !== x) {
+    t0 = [x];
+    $[0] = x;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ init: 0 }],
+};
+
+```
+      
+### Eval output
+(kind: ok) [4950]

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/for-with-assignment-as-update.js

@@ -0,0 +1,12 @@
+function Component(props) {
+  let x = props.init;
+  for (let i = 0; i < 100; i = i + 1) {
+    x += i;
+  }
+  return [x];
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ init: 0 }],
+};