How to add a Sized supertrait to a Rust trait?

Question

At this issue page for Rust , it gives the following example code for core::num::bignum::FullOps :

pub trait FullOps {
    ...
    fn full_mul(self, other: Self, carry: Self) -> (Self /*carry*/, Self);
    ...
}

Then it says that:

Here the function full_mul returns a (Self, Self) tuple, which is only well-formed when the Self -type is Sized - for that and other reasons, the trait only makes sense when Self is Sized . The solution in this case and most others is to add the missing Sized supertrait.

How does one add the missing Sized supertrait?

Answer 1

A "super trait" is just a bound, really.

You can place a bound either at trait level or method level. Here, you are advised to place it at trait level:

pub trait FullOps: Sized {
    fn full_mul(self, other: Self, carry: Self) -> (Self, Self);
}

The other solution would be to place it at method level:

pub trait FullOps {
    fn full_mul(self, other: Self, carry: Self) -> (Self, Self)
        where Self: Sized;
}

Answer 2

It's quite simple: change the first line to:

pub trait FullOps : Sized {

Playground link