Function parser_combinators::not_followed_by

pub fn not_followed_by<P>(parser: P) -> NotFollowedBy<P> where
    P: Parser,
    <P as Parser>::Output: Display, 

Succeeds only if parser fails. Never consumes any input.

 let result = string("let")
     .skip(not_followed_by(alpha_num()))
     .parse("letx")
     .map(|x| x.0);
 assert!(result.is_err());