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.
const prevSource = memoizedState.source;
  const prevSubscribe = memoizedState.subscribe;
  const fiber = currentlyRenderingFiber;
  hook.memoizedState = ({
    refs,
    source,
    subscribe,
  }: MutableSourceMemoizedState<Source, Snapshot>);
  
  dispatcher.useEffect(() => {
    refs.getSnapshot = getSnapshot;
    
    
    
    
    refs.setSnapshot = setSnapshot;
    
    const maybeNewVersion = getVersion(source._source);
    if (!is(version, maybeNewVersion)) {
      const maybeNewSnapshot = getSnapshot(source._source);
      if (!is(snapshot, maybeNewSnapshot)) {
        setSnapshot(maybeNewSnapshot);
        const suspenseConfig = requestCurrentSuspenseConfig();
        const lane = requestUpdateLane(fiber, suspenseConfig);
        markRootMutableRead(root, lane);
      }
      
      
      
      markRootEntangled(root, root.mutableReadLanes);