Add where clause on an impl

[nventuro]

Problem

I want to write an impl on some generic type T which I use in some functions that require that T implements some trait. This requires restricting each impl function with where T: Trait, which leads to a lot of repetition.

Happy Case

Ideally I'd simply do impl<T> MyStruct<T> where T: Trait { ... } instead of having to restrict T in each function. This is of course less granular, but I imagine in most scenarios this coarse restriction would be fine.

This happens in many places in aztec-nr, where most if not all functions need the clause.

Ultimately this is just syntax sugar though.

Project Impact

Nice-to-have

[nventuro]

Apparently Rust also allows the above to be written as impl<T: Trait> ....

[nventuro]

The error you get if you don't add the where clause in all the correct places can also be a bit confusing. It reads (assuming the code samples above):

No matching impl found for `T: Trait`

I found this weird - how could there be a matching impl if T is generic?

[jfecher]

@nventuro there can still be a matching impl for a generic T if there is a generic impl in scope implemented for any type:

impl<T> Trait for T {
    ...
}