Define lifetime for associative type without lifetime bound

Question

I am implementing FromStr for a type and for the error I want to return the passed &str inside a wrapper struct. However, this requires the the struct to have a lifetime bound for the &str but that will cause errors inside FromStr since its associative type Err does not have any lifetime bounds and cannot be modified either. Lastly, it's not possible to add a lifetime bound to the impl since the lifetime does not get constrained by anything, at least in the ways I have tried. So, how can I squeeze in my wrapper into the associative type?

For more context, here are the definitions:

pub enum RarityTag {
    // ...
}

pub struct ParseRarityTagError<'a>(&'a str);

impl FromStr for RarityTag {
    type Err = ParseRarityTagError<'a>;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            // ...
            rest => Err(ParseRarityTagError(rest)),
        }
    }
}

Answer 1

You cannot return an error with a lifetime bound to the string that FromStr::from_str gives you. The trait was not designed for it. It would have to define Err a generic associated type with a lifetime parameter in order to support that, but it doesn't.

You need either a different error scheme that doesn't rely on the lifetime or use an inherent method / free function and not the FromStr trait.