Function str_literal_to_char_literal 

pub fn str_literal_to_char_literal<'sm>(
    sm: impl HasSourceMap<'sm>,
    expr: &Expr<'_>,
    applicability: &mut Applicability,
    ascii_only: bool,
) -> Option<String>

Converts expr to a char literal if it’s a str literal containing a single character (or a single byte with ascii_only)