Emulating generic associated types

help
1

I've tried to emulate GAT by using an intermediate builder trait like this:

trait Foo: Sized {
    type AssocBuilder: BuildAssoc<'static, Foo = Self>; // removing the bound allows this to be compiled
    
    fn build_assoc<'a>(&'a self) -> <Self::AssocBuilder as BuildAssoc<'a>>::Assoc
    where
        Self::AssocBuilder: BuildAssoc<'a, Foo = Self>
    {
        <Self::AssocBuilder as BuildAssoc<'a>>::build_assoc(self)
    }
}

trait BuildAssoc<'a> {
    type Foo: Foo;
    type Assoc: 'a;
    
    fn build_assoc(foo: &'a Self::Foo) -> Self::Assoc;
}

I've required a trait bound for Foo::AssocBuilder to improve ergonomics in case the trait is used for global Foos (common use case), but this fails to compile with:

error[E0284]: type annotations needed
 --> src/lib.rs:4:5
  |
4 | /     fn build_assoc<'a>(&'a self) -> <Self::AssocBuilder as BuildAssoc<'a>>::Assoc
5 | |     where
6 | |         Self::AssocBuilder: BuildAssoc<'a, Foo = Self>
7 | |     {
8 | |         <Self::AssocBuilder as BuildAssoc<'a>>::build_assoc(self)
9 | |     }
  | |_____^ cannot infer type for associated type `<Self as Foo>::AssocBuilder`
  |
  = note: cannot resolve `<<Self as Foo>::AssocBuilder as BuildAssoc<'a>>::Assoc == _`

When I remove the trait bound for Foo::AssocBuilder the code compiles, but I don't understand why the type can supposedly not be inferred otherwise, since I very explicitly state all involved types and lifetimes.

2

For higher-kinded drop the where and use;
type AssocBuilder: for<'a> BuildAssoc<'a, Foo = Self>;

3

Thanks, this is extremely useful and simplifies things significantly!

I still wonder why my original code fails to compile, to me it appears to be only valid bounds and the compiler error is at least misleading.

4

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