fix #4471 and #5138

[ahl]

Fixes #4471 and #5138

I now see that there have been several efforts to fix these (#4524 and #5483), but this takes a pretty different approach. I'm submitting this because I think there may be advantages to this approach. In particular, it may be preferable to not split a \u{123} escape sequence at either brace.

I got here because I hit a variant of this when formatting strings of serialized JSON and rustfmt was splitting the line in the middle of a \" escape.

[ytmimi]

@ahl I still haven't had a chance to review this, but I saw the GitHub notification about the PR being updated. I wanted to mention that we strongly discourage merge commits and prefer if you'd rebase your branch to bring it up to date

[ahl]

I can certainly rebase it; when do you imagine it might be reviewed? I'd rather not rebase it multiple times if I can avoid it. Thanks! I had forgotten that this PR was still out there!

[ytmimi]

Fair enough. To be completely honest it'll probably a while before I'm able to review this so you can hold off on rebasing until then, but I wanted to give you a head up

[ahl]

Sure thing. It's already been quite a while so I didn't have any expectations of imminent review.