implement no-mutating-reducer-state rule

[rayhanadev]

Why

Catches reducers that mutate the current state object and then return that same object.

React compares reducer state by reference. If a reducer changes the old object in place and returns it, React can treat the update as unchanged. That can skip rerenders or leave memoized consumers with stale data.

Before, this reducer mutates state.items and returns the same state object, so the rule reports it:

import { useReducer } from "react";

function reducer(state, action) {
  if (action.type === "add") {
    state.items.push(action.item);
    return state;
  }

  return state;
}

useReducer(reducer, { items: [] });

After, the reducer returns a new state object, so the rule does not report it:

import { useReducer } from "react";

function reducer(state, action) {
  if (action.type === "add") {
    return {
      ...state,
      items: [...state.items, action.item],
    };
  }

  return state;
}

useReducer(reducer, { items: [] });

What changed

• Added no-mutating-reducer-state under State & Effects.
• Detects real React useReducer calls and same-file reducer functions.
• Tracks aliases to the original state, like const next = state.
• Reports common mutations when the same state reference can be returned.
• Allows safe patterns like clone-first updates, no-op return state, Immer-style wrappers, and non-React useReducer calls.
• Added tests for valid bugs, false-positive guards, and eval-derived edge cases.

Test plan

pnpm exec vp test run packages/oxlint-plugin-react-doctor/src/plugin/rules/state-and-effects/no-mutating-reducer-state.test.ts
pnpm --filter oxlint-plugin-react-doctor typecheck
pnpm lint

[reactreview]

✅ No new issues

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[rayhanadev]

bugbot run

[rayhanadev]

bugbot run

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 4 additional findings in Devin Review.