Strange `unreachable` warning in `Future`

[sbwtw]

Code:

use futures::future;
use futures::Future;

fn f() -> impl Future<Item = (), Error = ()> {
    future::ok(unimplemented!())
}

fn main() {
    let s = f();

    tokio::run(s);
}

And when build with cargo build, It will says the future::ok(unimplemented!()) is unreachable expression.
but when cargo run, I got a panic: thread 'main' panicked at 'not yet implemented', src\main.rs:5:16. the unimplemented!() macro is worked.

[Centril]

What happens here is that the function call future::ok(...) is unreachable. The macro "call" itself isn't.
We can see this with:

use futures::{future, Future};

fn f() -> impl Future<Item = (), Error = ()> {
    let _u = unimplemented!();
    future::ok(_u)
}

fn main() {
    tokio::run(f());
}

---

cc @estebank Can we highlight the issue like so perhaps?

warning: unreachable expression
 --> src/main.rs:4:5
  |
4 |     future::ok(unimplemented!())
  |     ^^^^^^^^^^^----------------^

[estebank]

I think this will be a side effect of a long standing paper cut, where we point at the method or entire statement instead of the argument that caused a problem. I can take a look, but I don't think this will be fixed in the near term.