Prune scopes whose deps always invalidate

Implements the optimization described in the previous PR: if we know that a 
scope's dependency will _always_ invalidate (it is not memoized and it is 
guaranteed to be a new object if the instruction executes, such as an array or 
object literal), then we can prune that scope. The invalidation is transitive: 
we track always-invalidating types from within scopes, and if their scope gets 
invalidated we prune downstream scope that depend on them. 

## Test Plan 

Tested via #2639 - see https://fburl.com/everpaste/3e3hjpjs. 91 files change 
output due to reactive scopes which would always invalidate due to always 
invalidating dependencies.

compiler/packages/babel-plugin-react-forget/src/Entrypoint/Pipeline.ts

@@ -63,6 +63,7 @@ import {
   pruneUnusedScopes,
   renameVariables,
 } from "../ReactiveScopes";
+import { pruneAlwaysInvalidatingScopes } from "../ReactiveScopes/PruneAlwaysInvalidatingScopes";
 import { eliminateRedundantPhi, enterSSA, leaveSSA } from "../SSA";
 import { inferTypes } from "../TypeInference";
 import {
@@ -319,6 +320,13 @@ function* runWithEnvironment(
     value: reactiveFunction,
   });
 
+  pruneAlwaysInvalidatingScopes(reactiveFunction);
+  yield log({
+    kind: "reactive",
+    name: "PruneAlwaysInvalidatingScopes",
+    value: reactiveFunction,
+  });
+
   promoteUsedTemporaries(reactiveFunction);
   yield log({
     kind: "reactive",

compiler/packages/babel-plugin-react-forget/src/ReactiveScopes/PruneAlwaysInvalidatingScopes.ts

@@ -0,0 +1,115 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  ReactiveFunctionTransform,
+  Transformed,
+  visitReactiveFunction,
+} from ".";
+import {
+  IdentifierId,
+  ReactiveFunction,
+  ReactiveInstruction,
+  ReactiveScopeBlock,
+  ReactiveStatement,
+} from "../HIR";
+
+/**
+ * Some instructions will *always* produce a new value, and unless memoized will *always*
+ * invalidate downstream reactive scopes. This pass finds such values and prunes downstream
+ * memoization.
+ *
+ * NOTE: function calls are an edge-case: function calls *may* return primitives, so this
+ * pass optimistically assumes they do. Therefore, unmemoized function calls will *not*
+ * prune downstream memoization. Only guaranteed new allocations, such as object and array
+ * literals, will cause pruning.
+ */
+export function pruneAlwaysInvalidatingScopes(fn: ReactiveFunction): void {
+  visitReactiveFunction(fn, new Transform(), false);
+}
+
+class Transform extends ReactiveFunctionTransform<boolean> {
+  alwaysInvalidatingValues: Set<IdentifierId> = new Set();
+  unmemoizedValues: Set<IdentifierId> = new Set();
+
+  override transformInstruction(
+    instruction: ReactiveInstruction,
+    withinScope: boolean
+  ): Transformed<ReactiveStatement> {
+    this.visitInstruction(instruction, withinScope);
+
+    const { lvalue, value } = instruction;
+    switch (value.kind) {
+      case "ArrayExpression":
+      case "ObjectExpression":
+      case "JsxExpression":
+      case "JsxFragment":
+      case "NewExpression": {
+        if (lvalue !== null) {
+          this.alwaysInvalidatingValues.add(lvalue.identifier.id);
+          if (!withinScope) {
+            this.unmemoizedValues.add(lvalue.identifier.id);
+          }
+        }
+        break;
+      }
+      case "StoreLocal": {
+        if (this.alwaysInvalidatingValues.has(value.value.identifier.id)) {
+          this.alwaysInvalidatingValues.add(value.lvalue.place.identifier.id);
+        }
+        if (this.unmemoizedValues.has(value.value.identifier.id)) {
+          this.unmemoizedValues.add(value.lvalue.place.identifier.id);
+        }
+        break;
+      }
+      case "LoadLocal": {
+        if (
+          lvalue !== null &&
+          this.alwaysInvalidatingValues.has(value.place.identifier.id)
+        ) {
+          this.alwaysInvalidatingValues.add(lvalue.identifier.id);
+        }
+        if (
+          lvalue !== null &&
+          this.unmemoizedValues.has(value.place.identifier.id)
+        ) {
+          this.unmemoizedValues.add(lvalue.identifier.id);
+        }
+        break;
+      }
+    }
+    return { kind: "keep" };
+  }
+
+  override transformScope(
+    scopeBlock: ReactiveScopeBlock,
+    _withinScope: boolean
+  ): Transformed<ReactiveStatement> {
+    this.visitScope(scopeBlock, true);
+
+    for (const dep of scopeBlock.scope.dependencies) {
+      if (this.unmemoizedValues.has(dep.identifier.id)) {
+        /*
+         * This scope depends on an always-invalidating value so the scope will always invalidate:
+         * prune it to avoid wasted comparisons
+         */
+        for (const [id, _decl] of scopeBlock.scope.declarations) {
+          if (this.alwaysInvalidatingValues.has(id)) {
+            this.unmemoizedValues.add(id);
+          }
+        }
+        for (const identifier of scopeBlock.scope.reassignments) {
+          if (this.alwaysInvalidatingValues.has(identifier.id)) {
+            this.unmemoizedValues.add(identifier.id);
+          }
+        }
+        return { kind: "replace-many", value: scopeBlock.instructions };
+      }
+    }
+    return { kind: "keep" };
+  }
+}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/prune-scopes-whose-deps-invalidate-array.expect.md

@@ -24,32 +24,15 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { unstable_useMemoCache as useMemoCache } from "react";
 import { useHook } from "shared-runtime";
 
 function Component(props) {
-  const $ = useMemoCache(4);
   const x = [];
   useHook();
   x.push(props.value);
-  let t0;
-  if ($[0] !== x) {
-    t0 = [x];
-    $[0] = x;
-    $[1] = t0;
-  } else {
-    t0 = $[1];
-  }
-  const y = t0;
-  let t1;
-  if ($[2] !== y) {
-    t1 = [y];
-    $[2] = y;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  return t1;
+
+  const y = [x];
+  return [y];
 }
 
 export const FIXTURE_ENTRYPOINT = {

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/prune-scopes-whose-deps-invalidate-new.expect.md

@@ -26,32 +26,15 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { unstable_useMemoCache as useMemoCache } from "react";
 import { useHook } from "shared-runtime";
 
 function Component(props) {
-  const $ = useMemoCache(4);
   const x = new Foo();
   useHook();
   x.value = props.value;
-  let t0;
-  if ($[0] !== x) {
-    t0 = { x };
-    $[0] = x;
-    $[1] = t0;
-  } else {
-    t0 = $[1];
-  }
-  const y = t0;
-  let t1;
-  if ($[2] !== y) {
-    t1 = { y };
-    $[2] = y;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  return t1;
+
+  const y = { x };
+  return { y };
 }
 
 class Foo {}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/prune-scopes-whose-deps-invalidate-object.expect.md

@@ -24,32 +24,15 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { unstable_useMemoCache as useMemoCache } from "react";
 import { useHook } from "shared-runtime";
 
 function Component(props) {
-  const $ = useMemoCache(4);
   const x = {};
   useHook();
   x.value = props.value;
-  let t0;
-  if ($[0] !== x) {
-    t0 = { x };
-    $[0] = x;
-    $[1] = t0;
-  } else {
-    t0 = $[1];
-  }
-  const y = t0;
-  let t1;
-  if ($[2] !== y) {
-    t1 = { y };
-    $[2] = y;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  return t1;
+
+  const y = { x };
+  return { y };
 }
 
 export const FIXTURE_ENTRYPOINT = {