Rustlings 'traits3.rs' question

[torrance]

Hi all, apologies for the noob question, but I'm working my way through the rustlings course and I'm stuck at the traits3.rs question, where we are expected to provide a default implementation of a trait to allow the the tests to pass.

For reference, the question is:

/ traits3.rs
//
// Your task is to implement the Licensed trait for
// both structures and have them return the same
// information without writing the same function twice.
//
// Consider what you can add to the Licensed trait.
// Execute `rustlings hint traits3` or use the `hint` watch subcommand for a hint.

// I AM NOT DONE

pub trait Licensed {
    fn licensing_info(&self) -> String
}

struct SomeSoftware {
    version_number: i32,
}

struct OtherSoftware {
    version_number: String,
}

impl Licensed for SomeSoftware {} // Don't edit this line
impl Licensed for OtherSoftware {} // Don't edit this line

My naive fix was to provide a default implementation that accessed the version_number, e.g.:

pub trait Licensed {
    fn licensing_info(&self) -> String {
        format!("{}", self.version_number)
    }
}

But of course this doesn't work since there's no guarantee that Self has such a member.

I'm at a loss how to to solve this without providing getters for the version number as part of each structs implementation.

What am I missing?

[Neutron3529]

I have no idea what the true answer is,but derive macro could help.

perhaps write a derive macro could solve your problem.

If derive is not allow, maybe you could firstly use Debug trait to convert structs into "SomeSoftware(version_number)" and then discard useless chars.

[torrance]

Appreciate the suggestions but the solutions here are usually straightforward one or two liners (and we haven't even got to the use of derive yet).

[torrance]

Oops, my bad - apparently the tests only expected a constant, placeholder string. Solved!

[Michael-F-Bryan]

If the goal is to provide a default information, and your trait has no other methods or super-traits which you can use to find out more about self (e.g. getters), your only real choice is to return a hard-coded value.

pub trait Licensed {
    fn licensing_info(&self) -> String {
        "unknown".into()
    }
}

[H2CO3]

It likely is. Judging from the test:

fn is_licensing_info_the_same() {
    let licensing_info = String::from("Some information");
    let some_software = SomeSoftware { version_number: 1 };
    let other_software = OtherSoftware {
        version_number: "v2.0.0".to_string(),
    };
    assert_eq!(some_software.licensing_info(), licensing_info);
    assert_eq!(other_software.licensing_info(), licensing_info);
}