Having issues implementing FromStr for a struct

[djacu]

Background context

I am building my own parser to parse through Nix derivation (.drv) files. Source for the repository and the branch I'm currently working on can be found here djacu/nix-derivation-tools.

The exact file where I am testing things out is here: https://github.com/djacu/nix-derivation-tools/blob/switch-from-nom-to-minnow/packages/nix-derivation-parser/src/derivations/parsers.rs

Issue

I read through the tutorials and the Implementing FromStr topic and wanted to do the same for my project. Specifically on this Derivation struct.

I started with something similar to the topic like this:

impl std::str::FromStr for Derivation {
    type Err = String;

    fn from_str(input: &str) -> Result<Self, Self::Err> {
        parse_derivation::<Derivation>.parse(input).map_err(|e| e.to_string())
    }
}

but was getting error like this

the method to_string exists for struct ParseError<&str, Derivation>, but its trait bounds were not satisfied the following trait bounds were not satisfied: ParseError<&str, Derivation>: std::fmt::Display which is required by ParseError<&str, Derivation>: ToString

I tried implementing a dummy Display to see if that would satisfy the trait requirements like so:

impl std::fmt::Display for Derivation {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
       write!(f, "{}", "") 
    }
}

but that produced an even more inscrutable error.

the trait bound Derivation: ParserError<&str> is not satisfied the following other types implement trait ParserError<I>: () ContextError ErrMode InputError TreeError<I, C> winnow::error::ErrorKind

What am I doing wrong?

[epage]

parse_derivation::<Derivation>.parse(input)

Derivation is your Output type but you are passing it to your parser as the Error type. Instead, select a different error, like ContextError.

impl std::str::FromStr for Derivation {
    type Err = String;

    fn from_str(input: &str) -> Result<Self, Self::Err> {
        parse_derivation::<ContextError>.parse(input).map_err(|e| e.to_string())
    }
}

Unless you are building reusable parsers for others, I'd recommend hard coding the Error type to the type that you prefer, much like you hard code the Input type. Note that ContextError is the default for PResult