Performance: remove memory allocations in useReducer

[romgrk]

Summary

Take 2 for #27383. I'm once again investigating a high-performance situation, and having a state hook that doesn't needlessly allocate memory on every render would be invaluable.

This time, I've added an Object.freeze() in dev mode to ensure nothing unholy is happening to the memoizedState.

How did you test this change?

yarn test and local build of React.

[react-sizebot]

Comparing: 3820740...9c3343e

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	528.09 kB	528.09 kB	=	93.17 kB	93.17 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	646.40 kB	646.40 kB	=	113.71 kB	113.71 kB
facebook-www/ReactDOM-prod.classic.js	=	674.15 kB	674.15 kB	=	118.41 kB	118.41 kB
facebook-www/ReactDOM-prod.modern.js	=	664.43 kB	664.43 kB	=	116.81 kB	116.81 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 9c3343e

[romgrk]

I thought about .memoizedState[0] = newState, but that would make the return value of useState stable across renders, even when there are changes, and that would be conceptually wrong for this API.

---

Tbh my true aim was to have a state hook with a stable value (and absolutely no memory allocations beyond initialization), but it would have an API like this:

const enabled = useStableState(false);
// => { get: () => boolean, set: (v: boolean) => void }

return (
  <button onClick=(() => enabled.set(!enabled.get())>
    {enabled.get() ? 'enabled' : 'disabled'}
  </button>
)

However, I fear it won't be possible without adding it directly in React as I need access to the internals to implement it properly.

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[github-actions]

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