nom::dbg!

macro_rules! dbg {
    ($i: expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($i:expr, $f:ident) => { ... };
}

Prints a message if the parser fails

The message prints the Error or Incomplete and the parser's calling code

   named!(f, dbg!( tag!( "abcd" ) ) );

   let a = &b"efgh"[..];

   // Will print the following message:
   // Error(Position(0, [101, 102, 103, 104])) at l.5 by ' tag ! ( "abcd" ) '
   f(a);