Can't implement a trait I don't own for al

问题:

pub trait AllValues {
    fn all_values() -> Vec<Self> where Self: std::marker::Sized;
}

use rand::Rand;
use rand::Rng;
impl<T: AllValues + Sized> Rand for T {
    fn rand<R: Rng, T>(rng: &mut R) -> T {
        let values = T::all_values();

        let len = values.len();

        if len == 0 {
            panic!("Cannot pick a random value because T::all_values() returned an empty vector!")
        } else {
            let i = rng.gen_range(0, len);

            values[i]
        }
    }
}

The preceding code produces the following compile-time error:

error[E0210]: type parameter `T` 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
   --> src/lib.rs:137:1
    |
137 | impl<T: AllValues + Sized> Rand for T {
    | ^

According to the restrictions on implementing traits mentioned here I should be able to implement Rand for AllValues since AllValues is defined in my crate. Is this actually allowed by the coherence/orphan impls rules? And if so, what is the right way to implement Rand for things that implement AllValues?

回答1:

I should be able to implement Rand for AllValues since AllValues is defined in my crate.

No, you are only allowed to implement your own trait AllValues for types you didn't define. You can't make the logical jump to implementing an unrelated trait that you also didn't define.

There are two considerations to remember:

• If your trait is public (which it is based on the code you've provided), you aren't the only one that can implement the trait. Consumers of your crate might be able to implement it for their own types, where they might also decide to implement Rand!
• The rand crate might decide to implement Rand for T some time in the future.

What is the right way to implement Rand for things that implement AllValues?

I don't believe there is one. I'd just introduce a wrapper type that holds a value or a reference to a value that implements your trait and implement Rand for that.

See also:

• How do I implement a trait I don't own for a type I don't own?

回答2:

I see now where my mistake in interpretation was. Quoting from the the traits section of the book:

There’s one more restriction on implementing traits: either the trait or the type you’re implementing it for must be defined by you. Or more precisely, one of them must be defined in the same crate as the impl you're writing.

(Emphasis added.)

Since I was trying to implement a trait I must have read that as "either the trait or the trait you’re implementing it for". This discussion about an eventually implemented rfc specifically mentions a similar case to the one I presented : impl<T: Copy> Clone for T as something that would not be allowed.

Creating a wrapper type as suggested elsewhere is one solution for this problem. Assuming the ownership of the type implementations allows it, implementing the trait explicitly for each concrete instance, (optionally condensing the code with a macro,) as suggested here is another.