perf(javascript): defer ancestry for member identifier roles

src/domain/javascript/javascriptSemanticProjection.test.ts

@@ -0,0 +1,75 @@
+import * as t from "@babel/types";
+import { expect, it } from "vitest";
+
+import { semanticIdentifierRole } from "./javascriptSemanticProjection.js";
+import { traverseJavaScriptAst } from "./javascriptSemanticTraversal.js";
+import { parseJavaScriptSource } from "./javascriptSourceParser.js";
+
+const rolesFor = (source: string) => {
+  const parsed = parseJavaScriptSource(source);
+  if (parsed === null) throw new Error("Expected parsed source");
+  const references: { name: string; role: string }[] = [];
+  let snapshots = 0;
+  traverseJavaScriptAst(parsed.program, {
+    enter(node, parent, readAncestors) {
+      if (!t.isIdentifier(node) || parent === null) return;
+      const role = semanticIdentifierRole(node, parent, () => {
+        snapshots++;
+        return readAncestors();
+      });
+      if (role !== null) references.push({ name: node.name, role });
+    },
+  });
+  return { references, snapshots };
+};
+
+it.each([256, 512])(
+  "classifies a %s-member chain without copying or scanning its ancestry",
+  (depth) => {
+    expect(rolesFor(`object${".property".repeat(depth)};`)).toEqual({
+      references: [{ name: "object", role: "read" }],
+      snapshots: 0,
+    });
+  },
+);
+
+it("retains optional and computed member reads without ancestor snapshots", () => {
+  expect(
+    rolesFor("object.property; object[key]; object?.property; object?.[key];"),
+  ).toEqual({
+    references: [
+      { name: "object", role: "read" },
+      { name: "object", role: "read" },
+      { name: "key", role: "read" },
+      { name: "object", role: "read" },
+      { name: "object", role: "read" },
+      { name: "key", role: "read" },
+    ],
+    snapshots: 0,
+  });
+});
+
+it("still reads ancestry to distinguish bound patterns from default-value reads", () => {
+  const result = rolesFor(
+    "const {key: value = compute()} = object; ({key: target} = source);",
+  );
+  expect(result.references).toEqual([
+    { name: "compute", role: "read" },
+    { name: "object", role: "read" },
+    { name: "target", role: "write" },
+    { name: "source", role: "read" },
+  ]);
+  expect(result.snapshots).toBeGreaterThan(0);
+});
+
+it("keeps member receivers and computed keys as reads in assignment patterns and updates", () => {
+  expect(
+    rolesFor("({field: object[key]} = source); object[key]++;").references,
+  ).toEqual([
+    { name: "object", role: "read" },
+    { name: "key", role: "read" },
+    { name: "source", role: "read" },
+    { name: "object", role: "read" },
+    { name: "key", role: "read" },
+  ]);
+});