nom::take!

macro_rules! take(
  ($i:expr, $count:expr) => (
    {
      let cnt = $count as usize;
      if $i.len() < cnt {
        $crate::IResult::Incomplete($crate::Needed::Size(cnt))
      } else {
        $crate::IResult::Done(&$i[cnt..],&$i[0..cnt])
      }
    }
  );
);

take!(nb) => &[T] -> IResult<&[T], &[T]> generates a parser consuming the specified number of bytes

 // Desmond parser
 named!(take5, take!( 5 ) );

 let a = b"abcdefgh";

 assert_eq!(take5(&a[..]), Done(&b"fgh"[..], &b"abcde"[..]));