Bug: StrictMode is not preventing side effects

React version: any with StrictMode ## Steps To Reproduce 1. Create a few components with improper or absent memoization 2. Create `useEffect` with unstable dependency 3. Observe how `StrictMode` is not able to help here - Incorrect code ```tsx const {propA, propB, ...rest} = propsC; useEffect(() => { // some effect not expected executed on every render }, [rest]); // ^^ rest is "unstable" ``` ```tsx <Component onSomething={() => {}} // we dont know what Component will do with unstable callback. May be something, may be not /> ``` - Correct code ```tsx const {propA, propB, rest} = useMemo(() => { const {propA, propB, ...rest} = propsC; return {propA, propB, rest}; }, [propC]); useEffect(() => { // some effect not expected executed on every render }, [rest]); // ^^ rest is "stable" ``` ```tsx <Component onSomething={useCallback(() => {},[])} /> ``` ## The current behavior >If a function is pure, running it twice does not change its behavior because a pure function produces the same result every time. However, if a function is impure (for example, it mutates the data it receives), running it twice tends to be noticeable (that’s what makes it impure!) This helps you spot and fix the bug early. https://react.dev/reference/react/StrictMode I personally never found this behaviour any helpful. It never helped me find a bug, especially a bug related to useEffect. ## The expected behavior I would assume that `StrictMode` should not try to execute `useEffect` twice - it should render Component twice and ensure no `useEffect` or `useMemo` is invalidated. Ideally, it should cause full application re-render to detect memoization issues spanning across multiple components, like using `children` in effect dependencies or passing unstable prop to a component with `useEffect` as such case cannot be detected by isolated re-render. ---- React Forget is going to change the game rules and automatically fix the problems from above, but how one can prove it without having…