recognize(multispace0) broken if multispace0 consumes all

[chrjabs]

In updating one of my projects to nom 8, I noticed one of my tests started failing. I narrowed it down to the combination of nom::combinator::recognize and nom::character::complete::multispace0 being broken if multispace0 consumes the entire input.

In more detail, when adding the following test to src/combinator/tests.rs, cases A-C pass, but case D fails. This seems to have worked correctly in nom 7.

#[test]
fn recognize_issue() {
  use crate::character::complete::{multispace0, multispace1};

  let input = "\na";
  // Case A
  assert_eq!(
    recognize::<_, crate::error::Error<_>, _>(multispace1).parse(input),
    Ok(("a", "\n"))
  );
  // Case B
  assert_eq!(
    recognize::<_, crate::error::Error<_>, _>(multispace0).parse(input),
    Ok(("a", "\n"))
  );

  let input = "\n";
  // Case C
  assert_eq!(
    recognize::<_, crate::error::Error<_>, _>(multispace1).parse(input),
    Ok(("", "\n"))
  );
  // Case D
  assert_eq!(
    recognize::<_, crate::error::Error<_>, _>(multispace0).parse(input),
    Ok(("", "\n"))
  );
}

[marcdejonge]

Thanks for the test, this was a nice project for me to get used to debugging Rust. I've found the problem and opened a PR to fix it: #1811

[mbbutler]

I have run into a similar bug in 8.0.0 when using recognize and consumed with a digit0 parser that consumes all:

use nom::{Parser, character::complete::digit0, combinator::recognize};

#[test]
fn digit_parser() {
    let mut p = recognize(digit0::<&str, ()>);

    // If `digit0` doesn't consume the entire input then everything works as expected
    let s = "1234A";
    let (rem, res) = p.parse(s).unwrap();
    assert_eq!(rem, "A");
    assert_eq!(res, "1234");

    // But if `digit0` does consume the entire input then the result is incorrect
    let s = "1234";
    let (rem, res) = p.parse(s).unwrap();
    assert_eq!(rem, "");
    assert_eq!(res, "1234");
}

That final assert_eq! panics with:

assertion `left == right` failed
  left: ""
 right: "1234"