Comment by EdiX

    fn main() {
      let a = "Hello,";
      let b = " world.";

      let c = a + b; 

      println!("{}", c);
    }

And the result is:

  rustc 1.15.0-beta.3 (a035041ba 2017-01-07)

error[E0369]: binary operation `+` cannot be applied to type `&str` | 5 | let c = a + b; // this is the only part that concats, really. | ^ | note: an implementation of `std::ops::Add` might be missing for `&str` | 5 | let c = a + b; // this is the only part that concats, really. | ^ timeout triggered!

What?

PS. looks like hacker news can't handle rustc error format, a pity...