default methods for the cmp traits

[thestinger]

(blocked on working default methods)

Implementing TotalOrd should implement Ord with default methods, and implementing TotalEq should provide a default Eq. Types with only a partial ordering can implement Eq/Ord, and types with two different ordering semantics (floats) can implement both.

A TotalOrd implementation can also provide TotalEq via a default, so most types would only implement one trait directly or just derive everything.

[emberian]

@thestinger @msullivan do default methods work well enough for this yet?

[thestinger]

Not really.

[omasanori]

Default methods for Ord and Eq are provided. Is it ready now?

[alexcrichton]

I don't think this should be closed until all the implementations of the now-default methods have been deleted (just to ensure that the default methods are working).

[huonw]

Also, the current default method implementation doesn't allow one to automatically get an Ord impl from a TotalOrd impl without having Ord: TotalOrd and that is definitely wrong (similarly, TotalEq: TotalOrd is wrong).

[msullivan]

Yeah, default methods don't quite work for what you want, and are not likely to. I think I"m going to close this.

[gifnksm]

I think the following code works fine for this purpose.

impl<T: TotalEq> Eq for T {
    fn eq(&self, other: &T) -> bool { self.equals(other) }
}

impl<T: TotalOrd> Ord for T {
    fn lt(&self, other: &T) -> bool { self.cmp(other) == Less }
}

[huonw]

@gifnksm that will mean that no other types can implement Eq or Ord. (It requires some sort of precedence/specialisation, i.e. #7059.)

[catamorphism]

@alexcrichton thinks this is done; closing

[alexcrichton]

If there are specific instances which need to be switched over, there should be individual bugs for them at this point. I believe that the bulk of types which can benefit from default methods are currently benefiting from default methods.