Ref: https://github.com/reactjs/rfcs/pull/147#issuecomment-623740218

React version: 0.0.0-experimental-e5d06e34b

## Steps To Reproduce

1. createMutableSource that contains functions
2. getSnapshot returns one of the functions

Link to code example:

https://codesandbox.io/s/jovial-dew-1eg4t?file=/src/App.js

## The current behavior

If getSnapshot returns a function, it will run the function in render. This results in various errors.

My workaround: return an object wrapping a function, and destruct on caller.

> For now, you can work around this by returning a function that returns your function

I don't know how this works as workaround.

## The expected behavior

It would be nice if returning a function just works.

Note: this is not a rare use case in my library https://github.com/dai-shi/use-context-selector
because we often pass `[state, setState]` in a context value.
}
  }, [getSnapshot, source, subscribe]);
  
  dispatcher.useEffect(() => {
    const handleChange = () => {
      const latestGetSnapshot = refs.getSnapshot;
      const latestSetSnapshot = refs.setSnapshot;
      try {
        latestSetSnapshot(latestGetSnapshot(source._source));
        
        const suspenseConfig = requestCurrentSuspenseConfig();
        const lane = requestUpdateLane(fiber, suspenseConfig);
        markRootMutableRead(root, lane);
      } catch (error) {
        
        
        
        
        latestSetSnapshot(
          (() => {
            throw error;
          }: any),
        );
      }
    };
    const unsubscribe = subscribe(source._source, handleChange);
    if (__DEV__) {
      if (typeof unsubscribe !== 'function') {
        console.error(
          'Mutable source subscribe function must return an unsubscribe function.',