$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
  --> src/main.rs:12:24
   |
12 |     println!("rect1 is {rect1}");
   |                        ^^^^^^^ `Rectangle` cannot be formatted with the default formatter
   |
help: the trait `std::fmt::Display` is not implemented for `Rectangle`
  --> src/main.rs:1:1
   |
 1 | struct Rectangle {
   | ^^^^^^^^^^^^^^^^
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead

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