Open Radar

 
Summary:
The more characters I type after a comment on the same line as a case statement like this: // the more characters...

...causes SourceKitService to spin up to 200% CPU and then typing slows, beach balls happen, and then Xcode hangs.

Steps to Reproduce:
I have an enum like this:
enum ImageShape: String {
    static let name = "shape"

    case tall = "tall"
    case thumb = "thumb"
    case square = "square"
}

I was trying to type: // Crop gravity: north, ratio: 1:2.25 at the end of the line for case tall.

The farther along I get in typing those characters, the slower input to Xcode is received until the application hangs. If I leave that line before the comment gets too long, I can continue to use the application fine. If I return and add more text to that comment, Xcode goes back to hanging immediately on the first keystroke.

Expected Results:
Able to type comments anywhere in the code without hanging

Observed Results:
Xcode hangs.

Version:
Xcode 9, beta 4, OS X 10.12.6

Notes:
I can reproduce this in a Playground without any other impacting code. Playground and video showing the experience attached.