fix(state): allow local updater parameter reassignment (#1872)

Author: aidenybai

.changeset/fp104-local-updater-parameter.md

@@ -0,0 +1,7 @@
+---
+"oxlint-plugin-react-doctor": patch
+"eslint-plugin-react-doctor": patch
+"react-doctor": patch
+---
+
+Allow state updater callbacks to reassign their local parameter while still reporting writes to prior-state properties and captured values.

packages/fuzz/corpus/regressions/no-impure-state-updater--local-parameter.tsx

@@ -0,0 +1,5 @@
+import { useState } from "react";
+export const Counter = () => {
+  const [count, setCount] = useState(0);
+  return <button onClick={() => setCount((previous) => --previous)}>{count}</button>;
+};

packages/oxlint-plugin-react-doctor/src/plugin/rules/state-and-effects/no-impure-state-updater-parameters.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vite-plus/test";
+import { runRule } from "../../../test-utils/run-rule.js";
+import { noImpureStateUpdater } from "./no-impure-state-updater.js";
+
+describe("updater parameter reassignment", () => {
+  it.each(["++previous", "--previous", "previous++", "previous += 1", "previous = 2"])(
+    "accepts local parameter operation %s",
+    (operation) => {
+      const result = runRule(
+        noImpureStateUpdater,
+        `
+        import { useState } from 'react';
+        const Counter = () => {
+          const [count, setCount] = useState(0);
+          return <button onClick={() => setCount(previous => ${operation})}>{count}</button>;
+        };
+      `,
+      );
+      expect(result.parseErrors).toEqual([]);
+      expect(result.diagnostics).toHaveLength(0);
+    },
+  );
+
+  it.each(["++previous.count", "previous.count = 2", "++captured", "captured = 2"])(
+    "still reports external mutation %s",
+    (operation) => {
+      const result = runRule(
+        noImpureStateUpdater,
+        `
+        import { useState } from 'react';
+        let captured = 0;
+        const Counter = () => {
+          const [value, setValue] = useState({ count: 0 });
+          return <button onClick={() => setValue(previous => { ${operation}; return previous; })}>Change</button>;
+        };
+      `,
+      );
+      expect(result.diagnostics).toHaveLength(1);
+    },
+  );
+});

packages/oxlint-plugin-react-doctor/src/plugin/rules/state-and-effects/no-impure-state-updater.ts

@@ -209,6 +209,7 @@ const getExternalAssignmentDescription = (
   const symbol = scopes.symbolFor(rootIdentifier);
   if (!symbol) return `the external value "${rootIdentifier.name}"`;
   if (symbol.kind === "parameter" && symbol.scope === updaterScope) {
+    if (isNodeOfType(stripParenExpression(assignmentTarget), "Identifier")) return null;
     return `the updater argument "${rootIdentifier.name}"`;
   }
   return isDescendantScope(symbol.scope, updaterScope)