The lack of upcasting in Rust. Other than that I fucking love it.

package main

// go is very frustrating. in their efforts to keep the language simple, they've broken its consistency :(

// A A is just some arbitrary interface
type A interface {
Foo()
}

// B is an interface requiring a function that returns an A
type B interface {
Bar() A
}

// Aimpl implements A
type Aimpl struct{}

// Foo is Aimpl's implementation of A
func (a Aimpl) Foo() {}

// Bimpl attempts to implement B
type Bimpl struct{}

// Bar is Bimpl's attempt at implementing B.
// problem is, if Bar returns an Aimpl instead of A, the interface is not satisfied
// this is because Go doesn't support implicit upcasting on returns from interfaced objects.
// if we were to simply change the declared return type of Bar() to 'A', without changing
// the returned value, Bimpl will satisfy B.
func (b Bimpl) Bar() Aimpl {
return Aimpl{}
}

var _ B = Bimpl{}

func main() {

}