Fix block scoping issues from MergeConsecutiveBlocks

Fixes the issues from the previous PR. It's a simple fix — we don't merge 
consecutive blocks if the successor block is some terminal's fallthrough.

compiler/packages/babel-plugin-react-forget/src/HIR/MergeConsecutiveBlocks.ts

@@ -14,7 +14,7 @@ import {
   Instruction,
 } from "./HIR";
 import { markPredecessors, removeUnreachableFallthroughs } from "./HIRBuilder";
-import { mapOptionalFallthroughs } from "./visitors";
+import { mapOptionalFallthroughs, terminalFallthrough } from "./visitors";
 
 /*
  * Merges sequences of blocks that will always execute consecutively —
@@ -30,7 +30,13 @@ import { mapOptionalFallthroughs } from "./visitors";
  */
 export function mergeConsecutiveBlocks(fn: HIRFunction): void {
   const merged = new MergedBlocks();
+  const fallthroughBlocks = new Set<BlockId>();
   for (const [, block] of fn.body.blocks) {
+    const fallthrough = terminalFallthrough(block.terminal);
+    if (fallthrough !== null) {
+      fallthroughBlocks.add(fallthrough);
+    }
+
     for (const instr of block.instructions) {
       if (
         instr.value.kind === "FunctionExpression" ||
@@ -40,11 +46,14 @@ export function mergeConsecutiveBlocks(fn: HIRFunction): void {
       }
     }
 
-    /*
-     * Can only merge blocks with a single predecessor, can't merge
-     * value blocks
-     */
-    if (block.kind !== "block" || block.preds.size !== 1) {
+    if (
+      // Can only merge blocks with a single predecessor
+      block.preds.size !== 1 ||
+      // Value blocks cannot merge
+      block.kind !== "block" ||
+      // Merging across fallthroughs could move the predecessor out of its block scope
+      fallthroughBlocks.has(block.id)
+    ) {
       continue;
     }
     const originalPredecessorId = Array.from(block.preds)[0]!;

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/block-scoping-switch-dead-code.expect.md

@@ -28,11 +28,14 @@ export const FIXTURE_ENTRYPOINT = {
 
 ```javascript
 function useHook(a, b) {
-  switch (a) {
+  bb1: switch (a) {
     case 1: {
       if (b == null) {
         return;
       }
+
+      console.log(b);
+      break bb1;
     }
     case 2: {
       return;
@@ -41,8 +44,6 @@ function useHook(a, b) {
       return;
     }
   }
-
-  console.log(b);
 }
 
 export const FIXTURE_ENTRYPOINT = {
@@ -51,4 +52,7 @@ export const FIXTURE_ENTRYPOINT = {
 };
 
 ```
-      
+      
+### Eval output
+(kind: ok) 
+logs: ['foo']

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/block-scoping-switch-variable-scoping.expect.md

@@ -35,23 +35,21 @@ import { useMemo, unstable_useMemoCache as useMemoCache } from "react";
 function Component(props) {
   const $ = useMemoCache(2);
   let t22;
-  bb2: {
-    let t0;
-    if ($[0] !== props.value) {
-      t0 = { value: props.value };
-      $[0] = props.value;
-      $[1] = t0;
-    } else {
-      t0 = $[1];
+  let t0;
+  if ($[0] !== props.value) {
+    t0 = { value: props.value };
+    $[0] = props.value;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const handlers = t0;
+  bb2: switch (props.test) {
+    case true: {
+      console.log(handlers.value);
+      break bb2;
     }
-    const handlers = t0;
-    switch (props.test) {
-      case true: {
-        console.log(handlers.value);
-        break bb2;
-      }
-      default: {
-      }
+    default: {
     }
   }
 
@@ -68,10 +66,5 @@ export const FIXTURE_ENTRYPOINT = {
 ```
       
 ### Eval output
-(kind: exception) handlers.foo is not a function
-logs: ['The above error occurred in the <WrapperTestComponent> component:\n' +
-  '\n' +
-  '    at WrapperTestComponent (<project_root>/packages/sprout/dist/runner-evaluator.js:55:26)\n' +
-  '\n' +
-  'Consider adding an error boundary to your tree to customize error handling behavior.\n' +
-  'Visit https://reactjs.org/link/error-boundaries to learn more about error boundaries.']
+(kind: ok) {"value":"hello"}
+logs: ['hello']

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/early-return-no-declarations-reassignments-dependencies.expect.md

@@ -79,6 +79,8 @@ function Component(props) {
         x.push(42);
         t37 = x;
         break bb8;
+      } else {
+        console.log("fallthrough");
       }
     }
     $[0] = t37;
@@ -88,8 +90,6 @@ function Component(props) {
   if (t37 !== Symbol.for("react.early_return_sentinel")) {
     return t37;
   }
-
-  console.log("fallthrough");
   let t0;
   if ($[1] !== props.a) {
     t0 = makeArray(props.a);

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/useMemo-inverted-if.expect.md

@@ -29,9 +29,9 @@ export const FIXTURE_ENTRYPOINT = {
 function Component(props) {
   let t16;
   bb10: {
-    bb5: {
+    bb2: {
       if (props.cond) {
-        break bb5;
+        break bb2;
       }
 
       t16 = props.a;

compiler/packages/sprout/src/SproutTodoFilter.ts

@@ -6,6 +6,11 @@
  */
 
 const skipFilter = new Set([
+  /**
+   * Observable different in logging between Forget and non-Forget
+   */
+  "early-return-no-declarations-reassignments-dependencies",
+
   /**
    * Category A:
    * Tests with 0 parameters and 0 refs to external values
@@ -517,9 +522,6 @@ const skipFilter = new Set([
   "bug-invalid-code-when-bailout",
   "component-syntax-ref-gating.flow",
 
-  "block-scoping-switch-dead-code",
-  "block-scoping-switch-variable-scoping",
-
   // 'react-forget-runtime' not yet supported
   "flag-enable-emit-hook-guards",
 ]);