From b1006e12cd996debe341fa27db548d4c794bbdaa Mon Sep 17 00:00:00 2001
From: Jan Kassens <jkassens@meta.com>
Date: Thu, 13 Oct 2022 16:17:39 -0400
Subject: [PATCH] Invariant for use before definition of reactive values (#662)

---
 compiler/forget/src/BackEnd/SanityCheck.ts | 97 ++++++++++++++++++++++
 compiler/forget/src/BackEnd/index.ts       |  1 +
 compiler/forget/src/CompilerDriver.ts      |  1 +
 3 files changed, 99 insertions(+)
 create mode 100644 compiler/forget/src/BackEnd/SanityCheck.ts

diff --git a/compiler/forget/src/BackEnd/SanityCheck.ts b/compiler/forget/src/BackEnd/SanityCheck.ts
new file mode 100644
index 0000000000..c07ef52451
--- /dev/null
+++ b/compiler/forget/src/BackEnd/SanityCheck.ts
@@ -0,0 +1,97 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import type { NodePath } from "@babel/traverse";
+import * as t from "@babel/types";
+import type { CompilerContext } from "../CompilerContext";
+import { OutputKind } from "../CompilerOutputs";
+import * as LIR from "../LIR";
+import * as IR from "../IR";
+import { PassKind, PassName } from "../Pass";
+import { assertExhaustive } from "../Common/utils";
+import invariant from "invariant";
+import { isReactiveBlock, isRenderBlock } from "../LIR";
+
+/**
+ * An optional debug step when the output {@link OutputKind.IR} is enabled.
+ * Records the output IR in the compiler context.
+ */
+export default {
+  name: PassName.DumpIR,
+  kind: PassKind.LIRFunc as const,
+  run,
+};
+
+export function run(
+  lirFunc: LIR.Func,
+  _func: NodePath<t.Function>,
+  _context: CompilerContext
+) {
+  const irFunc = lirFunc.ir;
+
+  const reactiveValues = new Set<string>();
+  function defineReactiveVal(value: IR.ReactiveVal) {
+    const name = value.binding.identifier.name;
+    reactiveValues.add(name);
+  }
+
+  function useReactiveValues(values: Set<IR.ReactiveVal>) {
+    for (const reactiveValue of values) {
+      const name = reactiveValue.binding.identifier.name;
+      invariant(
+        reactiveValues.has(name),
+        `Reactive value "${name}" not yet defined.`
+      );
+    }
+  }
+
+  for (const param of irFunc.params) {
+    if (IR.isReactiveVal(param)) {
+      defineReactiveVal(param);
+    }
+  }
+
+  function visitJSX(expr: IR.ExprVal) {
+    if (IR.isJSXTagVal(expr)) {
+      expr.children.forEach((child) => visitJSX(child));
+    }
+    if (!expr.stable) {
+      const entry = lirFunc.memoCache.entries.get(expr);
+      if (entry) {
+        invariant(LIR.MemoCache.isExprEntry(entry), "");
+        const { inputs } = irFunc.depGraph.getOrCreateVertex(entry.value);
+        useReactiveValues(inputs);
+      }
+    }
+  }
+
+  for (const block of lirFunc.blocks) {
+    switch (block.kind) {
+      case LIR.BlockKind.Render:
+        invariant(isRenderBlock(block), "Expected render block");
+        for (const instr of block.body) {
+          for (const decl of instr.ir.decls) {
+            if (IR.isReactiveVal(decl)) {
+              defineReactiveVal(decl);
+            }
+          }
+        }
+        break;
+      case LIR.BlockKind.Reactive:
+        invariant(isReactiveBlock(block), "Expected reactive block");
+        useReactiveValues(block.inputs);
+        for (const instr of block.body) {
+          instr.ir.jsxTreeRoots.forEach((root) => {
+            visitJSX(root);
+          });
+        }
+        break;
+      default:
+        assertExhaustive(block.kind, `Unhandled block ${block}`);
+    }
+  }
+}
diff --git a/compiler/forget/src/BackEnd/index.ts b/compiler/forget/src/BackEnd/index.ts
index 8a4a5de40b..2c2e1c49f5 100644
--- a/compiler/forget/src/BackEnd/index.ts
+++ b/compiler/forget/src/BackEnd/index.ts
@@ -14,3 +14,4 @@ export { default as DumpLIR } from "./DumpLIR";
 export { default as JSGen } from "./JSGen";
 export { default as LIRGen } from "./LIRGen";
 export { default as MemoCacheAlloc } from "./MemoCacheAlloc";
+export { default as SanityCheck } from "./SanityCheck";
diff --git a/compiler/forget/src/CompilerDriver.ts b/compiler/forget/src/CompilerDriver.ts
index a7342242a7..8a883eb70b 100644
--- a/compiler/forget/src/CompilerDriver.ts
+++ b/compiler/forget/src/CompilerDriver.ts
@@ -53,6 +53,7 @@ export function createCompilerDriver(
       passManager.addPass(BE.LIRGen);
       passManager.addPass(BE.MemoCacheAlloc);
       passManager.addPass(BE.DumpLIR);
+      passManager.addPass(BE.SanityCheck);
 
       // JS Generation.
       passManager.addPass(BE.JSGen);