Provide a syntax extension to derive traits for a newtype

[aochagavia]

It would be nice to be able to do something like:

#[newtype_deriving(Add, Sub, Mul, Div)]
struct Centimeters(i32);

The current solution to this problem (implementing the traits manually), causes a lot of boilerplate and makes usage of newtypes less attractive.

If there is enough interest, I would love to submit a PR for this

cc @aturon

[ghost]

cc #14047

[huonw]

I believe this is impossible as a syntax extension, since they have no access to the methods and method signatures of the trait to be able to create the impls. I'm willing to be proved wrong, but I also think this requires an RFC.

[huonw]

(At least, it is impossible to do for arbitrary traits.)

[aturon]

Yes, this definitely would need an RFC. That said, I would love functionality like this.

I'd suggest this issue be closed in favor of an issue in the RFC tracker.

[kmcallister]

I think this is a good feature for a version 2 deriving system that isn't purely syntax-based.

[steveklabnik]

Closing in favor of rust-lang/rfcs#261