Function fail 

pub fn fail<I, O, E>(i: I) -> Result<(I, O), Err<E>>
where
    E: ParseError<I>,

A parser which always fails.

use nom::combinator::fail;

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