Unicode string literals formatted incorrectly

[marcusklaas]

Input:

fn main() {
    "☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃"
}

Output:

fn main() {
    "\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{260\⏎
     3}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2\⏎
     603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u\⏎
     {2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\⏎
     \u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{260\⏎
     3}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2\⏎
     603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u\⏎
     {2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\⏎
     \u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{260\⏎
     3}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2603}\u{2\⏎
     603}\u{2603}\u{2603}"
}

I believe there is already a FIXME for this in the code, but it may be good to have an issue and test case.

As an aside, why do we call escape_default on string literals?