6

当试图回答这个问题时，我写了这段代码：

trait MyTrait {
    type A;
    type B;
}

trait WithFoo: MyTrait {
    fn foo(a: Self::A) -> Self::B;
}

impl<T, U: MyTrait<A = T, B = T>> WithFoo for U {
    fn foo(a: T) -> T {
        a
    }
}

struct S1;

impl MyTrait for S1 {
    type A = u32;
    type B = f32;
}

impl WithFoo for S1 {
    fn foo<T>(a: Self::A) -> Self::B {
        a as f32
    }
}

struct S2;

impl MyTrait for S2 {
    type A = u32;
    type B = u32;
}

fn main() {
    S1::foo(42);
    S2::foo(42);
}

操场

这无法编译并出现以下错误：

error[E0119]: conflicting implementations of trait `WithFoo` for type `S1`:
  --> src/main.rs:23:1
   |
10 | impl<T, U: MyTrait<A = T, B = T>> WithFoo for U {
   | ----------------------------------------------- first implementation here
...
23 | impl WithFoo for S1 {
   | ^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S1`

根据这些 答案，即使类型S1不符合impl特征边界，当编译器无法确定某些未来的毯子impl会导致S1突然匹配边界时，也会发生此错误。

但是在这种情况下，不可能S1实现MyTraitwith ，A == B因为它已经实现MyTrait了 with A != B。该错误是当前编译器实现的限制，可以想象在以后的某个时间点会被解除，还是我还缺少其他东西？

4

0 回答 0