Determine if a char is a valid identifier for a parser and/or lexer according to Unicode Standard Annex #31 rules.

Documentation

extern crate unicode_xid;
use unicode_xid::UnicodeXID;
fn main() {
    let ch = 'a';
    println!("Is {} a valid start of an identifier? {}", ch, UnicodeXID::is_xid_start(ch));
}

unicode-xid supports a no_std feature. This eliminates dependence on std, and instead uses equivalent functions from core.