[λ] Parse and codegen concise arrow expressions

compiler/forget/src/HIR/BuildHIR.ts

@@ -81,11 +81,14 @@ export function lower(
 
   const body = func.get("body");
   if (body.isExpression()) {
-    builder.pushError({
-      reason: "Support arrow functions",
-      severity: ErrorSeverity.Todo,
-      nodePath: body,
-    });
+    const fallthrough = builder.reserve();
+    const terminal: ReturnTerminal = {
+      kind: "return",
+      loc: GeneratedSource,
+      value: lowerExpressionToPlace(builder, body),
+      id: makeInstructionId(0),
+    };
+    builder.terminateWithContinuation("block", terminal, fallthrough);
   } else if (body.isBlockStatement()) {
     lowerStatement(builder, body);
   } else {

compiler/forget/src/__tests__/fixtures/hir/concise-arrow-expr.expect.md

@@ -0,0 +1,49 @@
+
+## Input
+
+```javascript
+function component() {
+  let [x, setX] = useState(0);
+  const handler = (v) => setX(v);
+  return <Foo handler={handler}></Foo>;
+}
+
+```
+
+## Code
+
+```javascript
+function component() {
+  const $ = React.useMemoCache();
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = useState(0);
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  const x = t0[0];
+  const setX = t0[1];
+  const c_1 = $[1] !== setX;
+  let handler;
+  if (c_1) {
+    handler = (v) => setX(v);
+    $[1] = setX;
+    $[2] = handler;
+  } else {
+    handler = $[2];
+  }
+  const c_3 = $[3] !== handler;
+  let t4;
+  if (c_3) {
+    t4 = <Foo handler={handler}></Foo>;
+    $[3] = handler;
+    $[4] = t4;
+  } else {
+    t4 = $[4];
+  }
+  return t4;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/concise-arrow-expr.js

@@ -0,0 +1,5 @@
+function component() {
+  let [x, setX] = useState(0);
+  const handler = (v) => setX(v);
+  return <Foo handler={handler}></Foo>;
+}