Spanning algorithm assumes all codepoints have width equal to 1

[nagisa]

/tmp/test.rs:2:23: 2:23 error: invalid unicode character escape
/tmp/test.rs:2     let a = "きききききき\u{}";
                                     ^
/tmp/test.rs:3:37: 3:37 error: invalid unicode character escape
/tmp/test.rs:3     let a = "ōōōōōōōōōō\u{}";
                                                   ^
error: aborting due to 2 previous errors

the first example has several codepoints that occupy space of more than one regular ASCII character and 2nd example has several codepoints that occupy no space (combining characters).

[huonw]

This is essentially a dupe of #8706. In particular, this issue (and that one) was fixed in #21499, but #24428 reverted it when moving the unicode functionality out.