error for functions which cannot return without recurring

[andrewrk]

rust has this:

fn thing(x: i32) -> i32 {
    return thing(x) + thing(x);
}

fn main() {
    thing(0);
}

warning: function cannot return without recurring
 --> test.rs:1:1
  |
1 | / fn thing(x: i32) -> i32 {
2 | |     return thing(x) + thing(x);
3 | | }
  | |_^
  |
  = note: #[warn(unconditional_recursion)] on by default
note: recursive call site
 --> test.rs:2:12
  |
2 |     return thing(x) + thing(x);
  |            ^^^^^^^^
  = help: a `loop` may express intention better if this is on purpose

It's a pretty nice compiler error. We should have it too.