$ 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 `{:?}`
   |
   = help: the trait `Debug` is not implemented for `Rectangle`
   = note: add `#[derive(Debug)]` or manually implement `Debug`
   = note: required by `std::fmt::Debug::fmt`
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rectangles`

To learn more, run the command again with --verbose.