为具有嵌套边界的类型实现特征

Question

I'm trying to implement a trait for a type implementing a generic trait with a bound on the inner type. The compiler shows me an error. Are there any known workarounds for this?

Code

trait MyTrait1 {}
trait MyTrait2 {}
trait MyTrait3<T> {}

impl <T: MyTrait1, U: MyTrait3<T>> MyTrait2 for Vec<U> {}

Playground

Error:

error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:5:7
  |
5 | impl <T: MyTrait1, U: MyTrait3<T>> MyTrait2 for Vec<U> {}
  |       ^ unconstrained type parameter

For more information about this error, try `rustc --explain E0207`.
error: could not compile `playground` due to previous error

Answer 1

Not quite sure what you are trying to do but as @Sven Marnach said you can use associated types. That way it will not be unconstrained because only one type can be associated with the trait implementation.

trait MyTrait1 {}
trait MyTrait2 {}
trait MyTrait3 {
    type AssociatedType;
}

impl<T, U> MyTrait2 for Vec<U>
where
    U: MyTrait3<AssociatedType = T>,
    T: MyTrait1,
{
}