From c87a1916f435f36efa8873ddfc8e13a25d91c118 Mon Sep 17 00:00:00 2001
From: Joe Savona <joesavona@fb.com>
Date: Wed, 24 May 2023 09:54:07 -0700
Subject: [PATCH] Infer property types for destructuring

---
 .../forget/src/TypeInference/InferTypes.ts    | 32 ++++++++++++-
 ...destructuring-property-inference.expect.md | 48 +++++++++++++++++++
 .../destructuring-property-inference.js       |  7 +++
 3 files changed, 86 insertions(+), 1 deletion(-)
 create mode 100644 compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.js

diff --git a/compiler/forget/src/TypeInference/InferTypes.ts b/compiler/forget/src/TypeInference/InferTypes.ts
index c804ec8735..a00cb5b5f6 100644
--- a/compiler/forget/src/TypeInference/InferTypes.ts
+++ b/compiler/forget/src/TypeInference/InferTypes.ts
@@ -201,9 +201,39 @@ function* generateInstructionTypes(
       break;
     }
 
+    case "Destructure": {
+      const pattern = value.lvalue.pattern;
+      if (pattern.kind === "ArrayPattern") {
+        for (let i = 0; i < pattern.items.length; i++) {
+          const item = pattern.items[i];
+          if (item.kind === "Identifier") {
+            // To simulate tuples we use properties with `String(<index>)`, eg "0".
+            const propertyName = String(i);
+            yield equation(item.identifier.type, {
+              kind: "Property",
+              object: value.value.identifier.type,
+              propertyName,
+            });
+          } else {
+            break;
+          }
+        }
+      } else {
+        for (const property of pattern.properties) {
+          if (property.kind === "ObjectProperty") {
+            yield equation(property.place.identifier.type, {
+              kind: "Property",
+              object: value.value.identifier.type,
+              propertyName: property.name,
+            });
+          }
+        }
+      }
+      break;
+    }
+
     case "DeclareLocal":
     case "DeclareContext":
-    case "Destructure":
     case "NewExpression":
     case "TypeCastExpression":
     case "JsxExpression":
diff --git a/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.expect.md b/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.expect.md
new file mode 100644
index 0000000000..57536d03a9
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.expect.md
@@ -0,0 +1,48 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  const x = [];
+  x.push(props.value);
+  const { length: y } = x;
+  foo(y);
+  return [x, y];
+}
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react";
+function Component(props) {
+  const $ = useMemoCache(5);
+  const c_0 = $[0] !== props.value;
+  let x;
+  if (c_0) {
+    x = [];
+    x.push(props.value);
+    $[0] = props.value;
+    $[1] = x;
+  } else {
+    x = $[1];
+  }
+  const { length: y } = x;
+  foo(y);
+  const c_2 = $[2] !== x;
+  const c_3 = $[3] !== y;
+  let t0;
+  if (c_2 || c_3) {
+    t0 = [x, y];
+    $[2] = x;
+    $[3] = y;
+    $[4] = t0;
+  } else {
+    t0 = $[4];
+  }
+  return t0;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.js b/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.js
new file mode 100644
index 0000000000..47cba2aa68
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/compiler/destructuring-property-inference.js
@@ -0,0 +1,7 @@
+function Component(props) {
+  const x = [];
+  x.push(props.value);
+  const { length: y } = x;
+  foo(y);
+  return [x, y];
+}