I want to write a macro to automatically generate implementation of a trait for all types that can implement it. But I encountered a problem.

Usage

#[impl_trait_alias]
pub trait FutureIter<Err>
where
    Self: Future<Output = Result<Self::FutOut, Err>>,
    Self::FutOut: Iterator<Item: Display>,
{
    type FutOut;
}

it generates

// ... original trait item

impl<
    Err,
    #[allow(non_camel_case_types)] __ImplTraitAlias_Self,
    #[allow(non_camel_case_types)] __ImplTraitAlias_FutOut,
> FutureIter<Err> for __ImplTraitAlias_Self
where
    Self: Future<Output = Result<__ImplTraitAlias_FutOut, Err>>,
    __ImplTraitAlias_FutOut: Iterator<Item: Display>,
{
    type FutOut = __ImplTraitAlias_FutOut;
}

And you can express a bundle of trait bounds with only typing a name!

fn get() -> impl FutureIter<String> {
    async { Ok([123].into_iter()) }
}

Problem

As you can see, I replaced all Self::FutOut to a custom generic __ImplTraitAlias_FutOut. Of course the code works fine, but the shortcoming is obvious: The user can only write Self::FutOut and any other style is unacceptable, like <Self>::FutOut, <Self as FutureIter<Err>>::FutOut, macros, etc.

So the problem is: is there a way to realize the auto-implementation while remain all where-predicates unchanged? Thanks.

Note: Directly keep the origin where-predicates unchanged will trigger an compile error.

The code:

impl<
    Err,
    #[allow(non_camel_case_types)] __ImplTraitAlias_Self,
    #[allow(non_camel_case_types)] __ImplTraitAlias_FutOut,
> FutureIter<Err> for __ImplTraitAlias_Self
where
    Self: Future<Output = Result<Self::FutOut, Err>>,
    Self::FutOut: Iterator<Item: Display>,
{
    type FutOut = __ImplTraitAlias_FutOut;
}

The error:

error[E0275]: overflow evaluating the requirement `<__ImplTraitAlias_Self as FutureIter<Err>>::FutOut == _`

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