将通用参数与impl中的关联类型进行匹配 [英] Matching a generic parameter to an associated type in an impl

问题描述

我有一个关联类型和一个泛型结构的特征::

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 generate 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：
$ b

Uh-oh! Compile error:

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`

很公平，我必须向编译器解释 B 与 A :: Foo ;让我们用来尝试：

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,
{

这不会有帮助：

which doesn't help:

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

嗯，不等于。也许我可以用冒号操作符来做到这一点吗？

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

好的工作，编译器 - 它不是特质;它是一个关联类型，但是它并没有告诉我如何编写一个匹配它的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

我必须向编译器解释 B 与 A :: Foo

它有一个特殊的语法：

There is a special syntax for it:

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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