$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
error[E0277]: `Rectangle` doesn't implement `Debug`
  --> src/main.rs:12:31
   |
12 |     println!("rect1 is {:?}", rect1);
   |                        ----   ^^^^^ `Rectangle` cannot be formatted using `{:?}` because it doesn't implement `Debug`
   |                        |
   |                        required by this formatting parameter
   |
   = help: the trait `Debug` is not implemented for `Rectangle`
   = note: add `#[derive(Debug)]` to `Rectangle` or manually `impl Debug for Rectangle`
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider annotating `Rectangle` with `#[derive(Debug)]`
   |
 1 + #[derive(Debug)]
 2 | struct Rectangle {
   |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rectangles` (bin "rectangles") due to 1 previous error
