Matching a generic parameter to an associated type in an impl

Question

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 } 

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());     } } 

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` 

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, { 

which doesn't help:

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

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` 

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 

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.

Answer 1

I must explain to the compiler that B is the same as A::Foo

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());     } }