为通用特征实现特征

Question

In Rust, how to implement a Trait for a generic trait?

trait One<S> {}

trait Two {}

// fails because S isn't contrained
impl<S, T> Two for T where T: One<S> {}

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To clarify, I'm trying to provide the BitAnd trait for a generic Select trait.

struct Thing<S> {
    field: S,
}

trait Select<S> {
    fn select(&self, thing: &Thing<S>) -> bool;
}

struct SelectUnion<S> (Box<dyn Select<S>>, Box<dyn Select<S>>);

// fails because S isn't contrained
impl<S, T> std::ops::BitAnd for T where T: Select<S> {
    type Output = SelectUnion<S>;

    fn bitand(self, rhs: Self) -> Self::Output {
        SelectUnion(Box::new(self), Box::new(rhs))
    }
}

Answer 1

It can't be, and the reason is that it would be ambiguous.

Think of a situation like this:

struct A;

impl One<u16> for A {}

impl One<u32> for A {}

Which of the two One implementations would the Two be based on? Both satisfy the preconditions for the blanket Two implementation, but Two can only be implemented once for any type. It's like as if you'd provide two separate impl Two for A blocks.

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edit after the question clarification:

The above still holds, but you may want to try whether you can make Select into a type, possibly by turning it into a wrapper around a selectable.