warning: `println!` writes to the service output channel. If you meant to log debugging information, use `eprintln!` or `dbg!`.
  --> $DIR/println_warn.rs:6:9
   |
6  |         println!("hello, world!");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
...
11 |         print!("sender: {:?}", ctx.sender());
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^