$ rustc cast_bad.rs 
error[E0277]: cannot multiply `f64` to `{integer}`
 --> cast_bad.rs:5:17
  |
5 |     let cir = 2 * PI * radius;
  |                 ^ no implementation for `{integer} * f64`
  |
  = help: the trait `std::ops::Mul<f64>` is not implemented for `{integer}`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.