Synced to Apple’s issue tracker as rdar://139287738

@ahoppen How would you like OnTypeFormatting to be implemented?

I quite like the Xcode formatting behavior, so not sure if any of your colleagues working on Xcode can provide some tips about this.
Also not sure that swift-format will have the ability to do those formatting anyway, even if we knew Xcode's algorithm.

What I did for the vknabel formatter extensions was to perform a format of the whole file after hitting "return" on non-empty lines. But that formatted the whole file, not just some certain lines (this was before the selection-formatting PR being merged into swift-format).

I guess we could start with that "perform a format of the whole file after hitting "return" on non-empty lines" although I'd like to leverage the selection-formatting capabilities.

What do you think of running swift-format on the line that we receive the textDocument/onTypeFormatting request for if ch is a newline? I am a little doubtful how well this will work though because swift-format can’t format if you have syntax errors and I assume there will likely be syntax errors during typing.

Anyway, I think we would need to get a feel for this, so I’d implement it behind an experimental feature and then use it for a while to see how it feels and if we have improvement ideas.