Partial support for update expressions

Adds limited support for UpdateExpressions (`x++`). We now support the postfix 
form (`x++` ok, `++x` is a todo) and only when the argument is an identifier. We 
can relax these restrictions with more work, but this PR should be sufficient 
for the examples we've seen so far.

compiler/forget/src/HIR/BuildHIR.ts

@@ -1405,6 +1405,55 @@ function lowerExpression(
         loc: exprLoc,
       };
     }
+    case "UpdateExpression": {
+      let expr = exprPath as NodePath<t.UpdateExpression>;
+      const argument = expr.get("argument");
+      if (!argument.isIdentifier()) {
+        builder.errors.push({
+          reason: `(BuildHIR::lowerExpression) Handle UpdateExpression with ${argument.type} argument`,
+          severity: ErrorSeverity.Todo,
+          nodePath: exprPath,
+        });
+        return { kind: "UnsupportedNode", node: exprNode, loc: exprLoc };
+      }
+      if (expr.node.prefix) {
+        builder.errors.push({
+          reason: `(BuildHIR::lowerExpression) Handle prefix UpdateExpression`,
+          severity: ErrorSeverity.Todo,
+          nodePath: exprPath,
+        });
+        return { kind: "UnsupportedNode", node: exprNode, loc: exprLoc };
+      }
+      const temp = buildTemporaryPlace(
+        builder,
+        expr.node.loc ?? GeneratedSource
+      );
+      builder.push({
+        id: makeInstructionId(0),
+        lvalue: { place: { ...temp }, kind: InstructionKind.Const },
+        value: {
+          kind: "Primitive",
+          value: 1,
+          loc: expr.node.loc ?? GeneratedSource,
+        },
+        loc: expr.node.loc ?? GeneratedSource,
+      });
+      const identifier = argument as NodePath<t.Identifier>;
+      const place = lowerExpressionToPlace(builder, identifier);
+      builder.push({
+        id: makeInstructionId(0),
+        lvalue: { place: { ...place }, kind: InstructionKind.Reassign },
+        value: {
+          kind: "BinaryExpression",
+          operator: expr.node.operator === "++" ? "+" : "-",
+          left: { ...place },
+          right: { ...temp },
+          loc: exprLoc,
+        },
+        loc: exprLoc,
+      });
+      return place;
+    }
     default: {
       builder.errors.push({
         reason: `(BuildHIR::lowerExpression) Handle ${exprPath.type} expressions`,

compiler/forget/src/__tests__/fixtures/hir/error.todo-kitchensink.expect.md

@@ -52,6 +52,12 @@ function foo([a, b], { c, d, e = "e" }, f = "f", ...args) {
   }
 
   do {} while (i < 3);
+
+  let updateIdentifier = 0;
+  --updateIdentifier;
+  ++updateIdentifier;
+  updateIdentifier.y++;
+  updateIdentifier.y--;
 }
 
 ```
@@ -327,8 +333,44 @@ function foo([a, b], { c, d, e = "e" }, f = "f", ...args) {
   49 |
 > 50 |   do {} while (i < 3);
      |   ^^^^^^^^^^^^^^^^^^^^
-  51 | }
-  52 |
+  51 |
+  52 |   let updateIdentifier = 0;
+  53 |   --updateIdentifier;
+
+[ReactForget] TodoError: (BuildHIR::lowerExpression) Handle prefix UpdateExpression
+  51 |
+  52 |   let updateIdentifier = 0;
+> 53 |   --updateIdentifier;
+     |   ^^^^^^^^^^^^^^^^^^
+  54 |   ++updateIdentifier;
+  55 |   updateIdentifier.y++;
+  56 |   updateIdentifier.y--;
+
+[ReactForget] TodoError: (BuildHIR::lowerExpression) Handle prefix UpdateExpression
+  52 |   let updateIdentifier = 0;
+  53 |   --updateIdentifier;
+> 54 |   ++updateIdentifier;
+     |   ^^^^^^^^^^^^^^^^^^
+  55 |   updateIdentifier.y++;
+  56 |   updateIdentifier.y--;
+  57 | }
+
+[ReactForget] TodoError: (BuildHIR::lowerExpression) Handle UpdateExpression with MemberExpression argument
+  53 |   --updateIdentifier;
+  54 |   ++updateIdentifier;
+> 55 |   updateIdentifier.y++;
+     |   ^^^^^^^^^^^^^^^^^^^^
+  56 |   updateIdentifier.y--;
+  57 | }
+  58 |
+
+[ReactForget] TodoError: (BuildHIR::lowerExpression) Handle UpdateExpression with MemberExpression argument
+  54 |   ++updateIdentifier;
+  55 |   updateIdentifier.y++;
+> 56 |   updateIdentifier.y--;
+     |   ^^^^^^^^^^^^^^^^^^^^
+  57 | }
+  58 |
 ```
           
       

compiler/forget/src/__tests__/fixtures/hir/error.todo-kitchensink.js

@@ -48,4 +48,10 @@ function foo([a, b], { c, d, e = "e" }, f = "f", ...args) {
   }
 
   do {} while (i < 3);
+
+  let updateIdentifier = 0;
+  --updateIdentifier;
+  ++updateIdentifier;
+  updateIdentifier.y++;
+  updateIdentifier.y--;
 }

compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md

@@ -4,7 +4,7 @@
 ```javascript
 function foo() {
   let x = 1;
-  for (let i = 0; i < 10; i += 1) {
+  for (let i = 0; i < 10; i++) {
     x += 1;
   }
   return x;

compiler/forget/src/__tests__/fixtures/hir/ssa-for.js

@@ -1,6 +1,6 @@
 function foo() {
   let x = 1;
-  for (let i = 0; i < 10; i += 1) {
+  for (let i = 0; i < 10; i++) {
     x += 1;
   }
   return x;

compiler/forget/src/__tests__/fixtures/hir/update-expression.expect.md

@@ -0,0 +1,35 @@
+
+## Input
+
+```javascript
+function foo(props) {
+  let x = props.x;
+  let y = x++;
+  let z = x--;
+  return { x, y, z };
+}
+
+```
+
+## Code
+
+```javascript
+function foo(props) {
+  const $ = React.useMemoCache();
+  const x = props.x;
+  const x$0 = x + 1;
+  const y = x$0;
+  const x$1 = x$0 - 1;
+  const z = x$1;
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = { x: x$1, y: y, z: z };
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/update-expression.js

@@ -0,0 +1,6 @@
+function foo(props) {
+  let x = props.x;
+  let y = x++;
+  let z = x--;
+  return { x, y, z };
+}