Solution

1. ```
   /// Determine the length of the collatz sequence beginning at `n`.
   ```
2. ```
   fn collatz_length(mut n: i32) -> u32 {
   ```
3. ```
       let mut len = 1;
   ```
4. ```
       while n > 1 {
   ```
5. ```
           n = if n % 2 == 0 { n / 2 } else { 3 * n + 1 };
   ```
6. ```
           len += 1;
   ```
7. ```
       }
   ```
8. ```
       len
   ```
9. ```
   }
   ```

11. ```
    #[test]
    ```
12. ```
    fn test_collatz_length() {
    ```
13. ```
        assert_eq!(collatz_length(11), 15);
    ```
14. ```
    }
    ```

16. ```
    fn main() {
    ```
17. ```
        println!("Length: {}", collatz_length(11));
    ```
18. ```
    }
    ```