将泛型参数与 impl 中的关联类型匹配

问题描述

我有一个带有关联类型和泛型结构的特征::

I have a trait with an associated type and a generic struct::

trait Generator {
    type Foo;
    fn generate(&self) -> Self::Foo;
}

struct Baz<A, B>
where
    A: Generator,
{
    generator: A, // will be some struct implementing Generator, but the exact type will vary
    vec: Vec<B>,  // Each element will be A::Foo
}

我想生成并将其放入我的向量中:

I want to generate and put it into my vector:

impl<A: Generator, B> Baz<A, B> {
    fn addFoo(&mut self) {
        self.vec.push(self.generator.generate());
    }
}

呃-哦！编译错误:

error[E0308]: mismatched types
  --> src/main.rs:16:27
   |
16 |             self.vec.push(self.generator.generate());
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter, found associated type
   |
   = note: expected type `B`
              found type `<A as Generator>::Foo`

公平，我必须向编译器解释 B 与 A::Foo 相同；让我们试试 where:

Fair enough, I must explain to the compiler that B is the same as A::Foo; let's try with where:

impl<A: Generator, B> Baz<A, B>
where
    A::Foo = B,
{

这没有帮助:

error: equality constraints are not yet supported in where clauses (#20041)
  --> src/main.rs:16:5
   |
16 |     A::Foo = B,
   |     ^^^^^^^^^^

嗯，没有平等.也许我可以用冒号运算符来代替?

Hmm, no equals. Maybe I can do this with the colon operator instead?

impl<A: Generator, B> Baz<A, B>
where
    B: A::Foo,
{

error[E0405]: cannot find trait `Foo` in `A`
  --> src/main.rs:16:11
   |
16 |     B: A::Foo,
   |           ^^^ not found in `A`

不，现在它在抱怨 A.也许我应该说Generator?

Nope, now it's complaining about A. Maybe I should say Generator?

impl<A: Generator, B> Baz<A, B>
where
    B: Generator::Foo,
{

error[E0404]: expected trait, found associated type `Generator::Foo`
  --> src/main.rs:16:8
   |
16 |     B: Generator::Foo,
   |        ^^^^^^^^^^^^^^ not a trait

干得好，编译器——它不是一个特性；它是一个关联类型，但这并没有告诉我如何编写匹配它的 where 子句.

Well good work, compiler — it's not a trait; it's an associated type, but that doesn't tell me how to write a where clause that matches it.

推荐答案

它有一个特殊的语法:

impl<A, B> Baz<A, B>
where
    A: Generator<Foo = B>,
{
    fn add_foo(&mut self) {
        self.vec.push(self.generator.generate());
    }
}

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