will allow default implementations?

[djdisodo]

default implementation on functions

trait Foo {
    f: T, mut g: U, h: U = g;
}

i can't explain well enough

in this case using : to set both type of field and implementing will be a bit confusing

[SpekalsG3]

In my opinion, It looks like it's better to forbid them. Fields will be used only on instances in methods that use self (borrow or move). If not using self than we already have static variables. And to access self methods we have to either init struct explicitly and so we can require user to provide values, or we can set them in builder function (e.g. new) and default values can be set there.