How to implement a trait for a parameterized trait

Question

I have a design issue, when using something like :

trait MyTrait<K: OtherTrait> { ... }

impl<K: OtherTrait, M: MyTrait<K>> AnyTrait for M { ... }

I cannot implement trait for this trait due to E207 error ("the type parameter K is not constrained by the impl trait, self type, or predicates").

Finding no way to get rid of this error, I apply this not-so-good-looking workaround (verbose and struct with no intrinsic value):

use std::fmt;
use std::marker::PhantomData;

pub trait MyTrait<K: fmt::Display> {
    fn get_some_k(&self) -> Option<K>;
}

/* // This is my target impl but results in E207 due to K not constrained
impl<K: fmt::Display, S: MyTrait<K>> fmt::Display for S {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.get_some_k().unwrap())
    }
} */
pub struct Ugly<'a, K: fmt::Display, S: 'a + MyTrait<K>>(&'a S, PhantomData<K>);
impl<'a, K: fmt::Display, S: MyTrait<K>> fmt::Display for Ugly<'a, K, S> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0.get_some_k().unwrap())
    }
}

fn main() { }

I think there should be some nicer way to implement a trait for this kind of parameterized trait.

I did not find good example in std (for instance no Display implementation in traits with associated type like Iterator)?

Answer 1

Here’s an implementation using associated types (which means that you can only implement MyTrait for one K per type):

use std::fmt;

pub trait MyTrait {
    type K: fmt::Display;
    fn get_some_k(&self) -> Option<Self::K>;
}

impl<S: MyTrait> fmt::Display for S {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.get_some_k().unwrap())
    }
}

fn main() { }

However, when clarified like this it becomes clear that this approach won’t work either, because you’re implementing Display for all types that implement MyTrait—types that could have their own Display implementation. This is forbidden, and so you get E0210:

error: type parameter S must be used as the type parameter for some local type (e.g. MyStruct<T>); only traits defined in the current crate can be implemented for a type parameter [E0210]

Wrapping it in something—like your Ugly did—is the only way to allow such an implementation. Or implement a trait in your own crate rather than one in someone else’s (like Display is).