Rust

But you’re also missing one use of the impl keyword: fn func() -> impl Trait.

[...] So dropping the impl in [return position] might not be completely impossible like the other uses of impl.

But the impl markes that it is a trait to the programmers.
Take the following functions:

func1()->A{...}
func2()->A{...}

Does the following snippet compile?

let mut thing = func1();
thing = func2();

Under the current rules we know it will. But if A could be a trait, the functions could return different types. We currently mark that with the impl.

Why? What value does -> () provide? Why not elide that?

What value is provided by keeping it?

What value does cluttering up your code with -> () provide?

Why a syntactic special-case for exactly that type and not any other random type?

Because the unit type is special, just like the never ! type. () also has the special importance of being the return value of an empty statement and some other stuff.

languages w/o [semicolons] feel awkward since you’re generally limited to one statement per line

Then fixing that might make sense. :-)

It's fixed with semicolons ;-)