Before we push too hard on a new language feature like this, @cmr could you add a more extensive suite of regression tests? I'd be curious to see what happens for cases like:

• a string with multiple escape sequences
• a string like \\{20} where the initial backslash is escaped
• a string with a non-matching brace
• an escape sequence for a surrogate hex specifier
• an escape for a too-large hex specifier
• an escape which isn't hex

@alexcrichton all excellent testcases, I'll add these.

Thanks @cmr! Forgot to say, but r=me with those added.