Idea: Concretizing Function `impl` Return Types

[scottmcm]

@SichangHe

    // Concretize return values and put them into enums to avoid boxing.
    enum ConcreteTrait {
        Fn0(return_type_of!(make_trait_0)),
        Fn1(return_type_of!(make_trait_1)),
    }

There's also been discussion of having a way for the compiler to do this for you in expressions, often called "enum impl trait":

• Add trait objects with static dispatch - #10 by CAD97
• pre-RFC: anonymous enums - #4 by Ixrec

The idea being then that you'd just be able to write something like

fn pick_impl(b: bool) -> impl Trait {
    enum if b {
        make_trait_0()
    } else {
        make_trait_1()
    }
}

And the compiler would generate an enum automatically that delegated the trait.

(No marker in the signature, because it's no different from if you'd returned Either internally, but a marker in the expression to not regress the usual case where it's better to get an error for mismatched things -- with that error mentioning enum if as a way out, probably.)

---

The other thing you can do today, if you don't need to return it, is something like this:

let temp_0;
let temp_1;

let my_fn: &dyn Trait = 
    if predicate { temp_0 = make_trait_0(); &temp_0 }
    else{ temp_1 = make_trait_1(); &temp_1 };

to type-erase without needing to Box things.