[compiler][bugfix] Fix hoisting of let declarations (#32724)

(Found when compiling Meta React code)

Let variable declarations and reassignments are currently rewritten to
`StoreLocal <varName>` instructions, which each translates to a new
`const varName` declaration in codegen.

```js
// Example input
function useHook() {
  const getX = () => x;
  let x = CONSTANT1;
  if (cond) {
    x += CONSTANT2;
  }
  return <Stringify getX={getX} />
}

// Compiled output, prior to this PR
import { c as _c } from "react/compiler-runtime";
function useHook() {
  const $ = _c(1);
  let t0;
  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
    const getX = () => x;
    let x = CONSTANT1;
    if (cond) {
      let x = x + CONSTANT2;
      x;
    }

    t0 = <Stringify getX={getX} />;
    $[0] = t0;
  } else {
    t0 = $[0];
  }
  return t0;
}
```

This also manifests as a babel internal error when replacing the
original function declaration with the compiler output. The below
compilation output fails with `Duplicate declaration "x" (This is an
error on an internal node. Probably an internal error.)`.
```js
// example input
let x = CONSTANT1;
if (cond) {
  x += CONSTANT2;
  x = CONSTANT3;
}

// current output
let x = CONSTANT1;
if (playheadDragState) {
  let x = x + CONSTANT2
  x;
  let x = CONSTANT3;
}
```

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

@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import {CompilerError} from '..';
 import {
   DeclarationId,
   InstructionKind,
@@ -27,7 +28,17 @@ export function pruneHoistedContexts(fn: ReactiveFunction): void {
   visitReactiveFunction(fn, new Visitor(), hoistedIdentifiers);
 }
 
-type HoistedIdentifiers = Map<DeclarationId, InstructionKind>;
+const REWRITTEN_HOISTED_CONST: unique symbol = Symbol(
+  'REWRITTEN_HOISTED_CONST',
+);
+const REWRITTEN_HOISTED_LET: unique symbol = Symbol('REWRITTEN_HOISTED_LET');
+
+type HoistedIdentifiers = Map<
+  DeclarationId,
+  | InstructionKind
+  | typeof REWRITTEN_HOISTED_CONST
+  | typeof REWRITTEN_HOISTED_LET
+>;
 
 class Visitor extends ReactiveFunctionTransform<HoistedIdentifiers> {
   override transformInstruction(
@@ -35,6 +46,10 @@ class Visitor extends ReactiveFunctionTransform<HoistedIdentifiers> {
     state: HoistedIdentifiers,
   ): Transformed<ReactiveStatement> {
     this.visitInstruction(instruction, state);
+
+    /**
+     * Remove hoisted declarations to preserve TDZ
+     */
     if (
       instruction.value.kind === 'DeclareContext' &&
       instruction.value.lvalue.kind === 'HoistedConst'
@@ -68,31 +83,75 @@ class Visitor extends ReactiveFunctionTransform<HoistedIdentifiers> {
       return {kind: 'remove'};
     }
 
-    if (
-      instruction.value.kind === 'StoreContext' &&
-      state.has(instruction.value.lvalue.place.identifier.declarationId)
-    ) {
+    if (instruction.value.kind === 'StoreContext') {
       const kind = state.get(
         instruction.value.lvalue.place.identifier.declarationId,
-      )!;
-      return {
-        kind: 'replace',
-        value: {
-          kind: 'instruction',
-          instruction: {
-            ...instruction,
+      );
+      if (kind != null) {
+        CompilerError.invariant(kind !== REWRITTEN_HOISTED_CONST, {
+          reason: 'Expected exactly one store to a hoisted const variable',
+          loc: instruction.loc,
+        });
+        if (
+          kind === InstructionKind.Const ||
+          kind === InstructionKind.Function
+        ) {
+          state.set(
+            instruction.value.lvalue.place.identifier.declarationId,
+            REWRITTEN_HOISTED_CONST,
+          );
+          return {
+            kind: 'replace',
             value: {
-              ...instruction.value,
-              lvalue: {
-                ...instruction.value.lvalue,
-                kind,
+              kind: 'instruction',
+              instruction: {
+                ...instruction,
+                value: {
+                  ...instruction.value,
+                  lvalue: {
+                    ...instruction.value.lvalue,
+                    kind,
+                  },
+                  type: null,
+                  kind: 'StoreLocal',
+                },
               },
-              type: null,
-              kind: 'StoreLocal',
             },
-          },
-        },
-      };
+          };
+        } else if (kind !== REWRITTEN_HOISTED_LET) {
+          /**
+           * Context variables declared with let may have reassignments. Only
+           * insert a `DeclareContext` for the first encountered `StoreContext`
+           * instruction.
+           */
+          state.set(
+            instruction.value.lvalue.place.identifier.declarationId,
+            REWRITTEN_HOISTED_LET,
+          );
+          return {
+            kind: 'replace-many',
+            value: [
+              {
+                kind: 'instruction',
+                instruction: {
+                  id: instruction.id,
+                  lvalue: null,
+                  value: {
+                    kind: 'DeclareContext',
+                    lvalue: {
+                      kind: InstructionKind.Let,
+                      place: {...instruction.value.lvalue.place},
+                    },
+                    loc: instruction.value.loc,
+                  },
+                  loc: instruction.loc,
+                },
+              },
+              {kind: 'instruction', instruction},
+            ],
+          };
+        }
+      }
     }
 
     return {kind: 'keep'};