- a `Box<dyn Trait>` is like a Java object referenced by an interface. Everything uses dynamic dispatch, and it is an actual concrete type (though the actual "underlying" type is type-erased).
- an `-> impl Trait` is an existential type which uses a trait as a bound, which should be equivalent to D's "Voldemort" types, except that it can still satisfy Trait requires for other functions. For example, if you `-> impl Iterator<Item = u32>`, you can pass that result to a function expecting an iterator of `u32`. However, the type is fully defined by the callee and can't be instantiated/inspected externally.
Not true in Rust, because the type returned by the function is a concrete type.
I think we may be overloading “name” here, a bit. If you return an “impl” in Rust, you have to name an interface, but you can’t actually create new instances of the concrete type returned by the function.
I think that's the distinction that really matters. The thing that's neat about Voldemort types, and gives them their interesting properties that are distinct from anonymous types, is that they have no name at all. Not even an interface or trait name.
I don't really intend to say whether they are better or worse than other ways of accomplishing similar things. Just that they're different.
Note that interfaces in Rust are not types, they do not name types. A value cannot have interface type. What happens when you return an “impl” type is that you return some unspecified type, but that type must implement the specified interface.
You could translate the D example to:
trait HasGetValue {
fn get_value(&self) -> i32;
}
fn create_voldemort_type(value: i32) -> impl HasGetValue {
...
}
That has some implications for how Voldemort types work that can't be replicated with anonymous interface implementations. The main one is, Voldemort types are very thoroughly sealed. There is no way to create an instance of one outside of the function where it's defined. With an interface, anyone else could come along and create their own implementation.