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.
do {
      didScheduleRenderPhaseUpdateDuringThisPass = false;
      invariant(
        numberOfReRenders < RE_RENDER_LIMIT,
        'Too many re-renders. React limits the number of renders to prevent ' +
          'an infinite loop.',
      );
      numberOfReRenders += 1;
      if (__DEV__) {
        
        
        ignorePreviousDependencies = false;
      }
       Start over from the beginning of the list
      currentHook = null;
      workInProgressHook = null;
      workInProgress.updateQueue = null;
      if (__DEV__) {
        
        hookTypesUpdateIndexDev = -1;
      }
      ReactCurrentDispatcher.current = __DEV__
        ? HooksDispatcherOnRerenderInDEV
        : HooksDispatcherOnRerender;
      children = Component(props, secondArg);
    } while (didScheduleRenderPhaseUpdateDuringThisPass);
  }
  
  
  ReactCurrentDispatcher.current = ContextOnlyDispatcher;