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.
}
let isUpdatingOpaqueValueInRenderPhase = false;
export function getIsUpdatingOpaqueValueInRenderPhaseInDEV(): boolean | void {
  if (__DEV__) {
    return isUpdatingOpaqueValueInRenderPhase;
  }
}
function warnOnOpaqueIdentifierAccessInDEV(fiber) {
  if (__DEV__) {
    
    const name = getComponentName(fiber.type) || 'Unknown';
    if (getIsRendering() && !didWarnAboutUseOpaqueIdentifier[name]) {
      console.error(
        'The object passed back from useOpaqueIdentifier is meant to be ' +
          'passed through to attributes only. Do not read the ' +
          'value directly.',
      );
      didWarnAboutUseOpaqueIdentifier[name] = true;
    }
  }
}
function mountOpaqueIdentifier(): OpaqueIDType | void {
  const makeId = __DEV__
    ? makeClientIdInDEV.bind(
        null,
        warnOnOpaqueIdentifierAccessInDEV.bind(null, currentlyRenderingFiber),
      )
    : makeClientId;
  if (getIsHydrating()) {
    let didUpgrade = false;