Escape that is not a Unicode scalar value

Built from the repository at commit a8614dc.

\u{...} names a Unicode scalar value: a code point from 0 to 10FFFF, except the surrogates D800 to DFFF. Any other value is an error, reported over the escape.

Why

A surrogate is half of a pair in UTF-16 and is not a character. A value above 10FFFF is outside Unicode. Neither can be encoded as UTF-8, which text always is.

Broken

fn main() {
    let c = '\u{D800}'
    print(c)
}

What sisu says

error[E0091]: `\u{D800}` is not a Unicode scalar value
 --> E0091.sisu:2:14
  |
2 |     let c = '\u{D800}'
  |              ^^^^^^^^
  |
  = help: scalar values run from 0 to 10FFFF, without D800 to DFFF

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0091

Fixed

fn main() {
    let c = '\u{E4}'
    print(c)
}