Auto-insert `await` for property accesses on Promise

[DanielRosenwasser]

async function foo(x: Promise<string>) {
  x./**/
}

Go to the /**/ marker and request completions.

The proposal here is that all the completions for string should show up. When the user selects a string member, we should automatically await the value, so selecting toLowerCase changes the code to (await x).toLowerCase.

[fatcerberus]

This seems like a good idea but should only be done inside async functions if possible. In the example above, the function is not async so adding an await there would make the code invalid.

[DanielRosenwasser]

Yes, thank you, that was one of the subtler points I forgot to add to this issue. It would be incredibly annoying/unsafe otherwise.

[fatcerberus]

Something else I just thought of: since this would be done via autocompletion, the feature probably shouldn't get any ideas about automatically converting the function to async - doing so is a breaking change in most cases since it forces the function into returning a promise.

[DanielRosenwasser]

Spoke with @littledan today, and once top-level await lands, we should consider doing this at the top level based on target and module options.

[andrewbranch]

Related: #30646