[valid-expect] include fixer for adding missing await

[jayu]

How do you feel about making jest/valid-expect rule fixable for cases where await is missing ?

So adding await in front of expect in such cases:

it('should', async () => {
   expect(someAsyncThing).rejects.toBe(whatever)
})

//or 

it('should', async () => {
   expect(someAsyncThing).resolves.toBe(whatever)
})

Fixed code then would look like

it('should', async () => {
   await expect(someAsyncThing).rejects.toBe(whatever)
})

//or 

it('should', async () => {
   await expect(someAsyncThing).resolves.toBe(whatever)
})

I'm happy to give it a try if there is a chance it will be merged :)

[G-Rath]

Yup sounds good - we probably should have it as a suggestion if the function isn't already async.

[y-hsgw]

@G-Rath As for the remaining tasks for this issue, would it be appropriate to suggest that if the target for validation is not async?

e.g:

it('should', async () => {
  const someAsyncThing = () => { ... }
  expect(someAsyncThing).rejects.toBe(whatever)
})

[G-Rath]

@y-hsgw if you mean suggesting adding await regardless of if async is present, then yeah that's probably fine; though I think we should be able to successfully add async to at least some patterns.

Broadly speaking the logic should be something like:

1. find the closest function expression upwards in the tree (I'm pretty sure that's what findFirstAsyncFunction does exactly except it also checks if its async; but worth double checking)
2. if it is not async, include making a fixer for making it async
3. if the line has a return and ... etc (all the current fixer behaviour)
4. include a fixer for adding await

While it might seem complicated on the face, I'm actually pretty sure we should be able to always accurately know and fix async/await.

I'm happy to do incremental improvement PRs too, so we can aim for addressing the simple cases and then a followup PR for handling more complex ones