rust - 如何消除关联类型的歧义？

Question

我当前的代码如下所示:

pub trait A {}
pub trait HasA {
    type A: A;
    fn gimme_a() -> Self::A;
}

pub trait RichA: A {}
pub trait RichHasA: HasA {
    type A: RichA;
    fn gimme_a() -> Self::A;
    // ... more things go here ...
}

pub fn main() {}

我的目标是能够将 RichHasA 用作 HasA。上面的代码无法编译:

error[E0221]: ambiguous associated type `A` in bounds of `Self`
  --> src/main.rs:10:21
   |
3  |     type A: A;
   |     ---------- ambiguous `A` from `HasA`
...
9  |     type A: RichA;
   |     -------------- ambiguous `A` from `RichHasA`
10 |     fn gimme_a() -> Self::A;
   |                     ^^^^^^^ ambiguous associated type `A`

这是有道理的。如何消除关联类型的歧义？

Answer 1

你可以使用所谓的Fully Qualified Syntax (FQS):

fn gimme_a() -> <Self as HasA>::A;
fn gimme_a() -> <Self as RichHasA>::A;

另见:

• How to call a method when a trait and struct use the same method name?