$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
  --> src/main.rs:12:29
   |
12 |     println!("rect1 is {}", rect1);
   |                             ^^^^^ `Rectangle` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `Rectangle`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = 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)

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