Make msg! macro to return value

[onsails]

msg! macro returning value is useful for constructing async interfaces to actors:

struct AsyncFoo {
    child: ChildRef,
}

impl AsyncFoo {
    pub async fn open_bar(&self, open_bar: &OpenBar) -> Result<BarId, Error> {
        let answer = self
            .child
            .ask(open_bar)
            .expect("Failed to send message");

        msg! { answer.await.expect("Failed to receive answer"),
            msg: Result<BarId, Error> => {
                msg;
            };
            _: _ => Err(format_err!("wrong answer"));
        }
    }
}

Currently, this code leads to compilation error, compiler sees () return type but expects Result<BarId, Error>.

Same problem happens with explicit return inside a macro:

struct AsyncFoo {
    child: ChildRef,
}

impl AsyncFoo {
    pub async fn open_bar(&self, open_bar: &OpenBar) -> Result<BarId, Error> {
        let answer = self
            .child
            .ask(open_bar)
            .expect("Failed to send message");

        msg! { answer.await.expect("Failed to receive answer"),
            msg: Result<BarId, Error> => {
                return msg;
            };
            _: _ => return Err(format_err!("wrong answer"));
        }
    }
}