I'm struggling to see the point in this. Your roll example using Winnow. Not really that much (un)wrapping logic:

    use winnow::{Result, ascii::digit1, combinator::separated_pair, prelude::*};

    #[derive(Debug, PartialEq)]
    struct Roll {
        count: u32,
        sides: u32,
    }

    fn number(input: &mut &str) -> Result<u32> {
        digit1.try_map(str::parse).parse_next(input)
    }

    fn roll(input: &mut &str) -> Result<Roll> {
        let (count, sides) = separated_pair(number, 'd', number).parse_next(input)?;
        Ok(Roll { count, sides })
    }

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn test_roll() {
            let input = &mut "3d6";
            assert_eq!(roll(input), Ok(Roll { count: 3, sides: 6 }));
        }
    }