Function fail

pub fn fail<I, O, E>() -> impl Parser<I, Output = O, Error = E>
where
    E: ParseError<I>,

A parser which always fails.

use nom::combinator::fail;

let s = "string";
assert_eq!(fail::<_, &str, _>().parse(s), Err(Err::Error((s, ErrorKind::Fail))));