Derive for newtypes with generics

[adiba]

#[derive(Float)]
struct A<T:Float>(T);

Here, float will try to implement Float for T, but T is a parameter. Instead, it should:

impl<T:Float> Float for A<T>{...}

[cuviper]

Here, float will try to implement Float for T, but T is a parameter.

It is implementing for A, but it's missing the generic parameter. The expanded code looks like:

        impl _num_traits::Float for A {
            fn nan() -> Self { A(<T as _num_traits::Float>::nan()) }
...

error[E0412]: cannot find type `T` in this scope
 --> src/lib.rs:8:19
  |
8 | struct A<T:Float>(T);
  |                   ^ did you mean `A`?

error[E0243]: wrong number of type arguments: expected 1, found 0
 --> src/lib.rs:8:8
  |
8 | struct A<T:Float>(T);
  |        ^ expected 1 type argument

error: aborting due to 2 previous errors

Instead, it should:

impl<T:Float> Float for A<T>{...}

Yes, that should work.

We should see if there are other derive crates we can use as an example of how to produce generic args and constraints properly.

[adiba]

https://crates.io/crates/derive_more This one performs well on this example for a handful of std-traits.

[asfishman]

Any updates on this issue?