$ cargo test
   Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
error[E0596]: cannot borrow `self.sent_messages` as mutable, as it is behind a `&` reference
  --> src/lib.rs:58:13
   |
57 |         fn send(&self, message: &str) {
   |                 ----- help: consider changing this to be a mutable reference: `&mut self`
58 |             self.sent_messages.push(String::from(message));
   |             ^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.
error: could not compile `limit-tracker`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed