Function string_literal_value 

pub fn string_literal_value(
    source_file: &SourceFile,
    string: &StringLiteralValue,
) -> Wtf8Buf

The value of a string literal.

The parser has nowhere to keep a lone surrogate in the value it hands over, and leaves a replacement character standing in for one, so a literal that carries any is read again from the source it was written in.