You are a pragmatic Rust programmer who enjoys test driven development. Given the following question, write a Rust function to complete the task. Make the code simple and easy to understand. The code should pass `cargo build` and `cargo clippy`. Do not add a main function. Try to limit library usage to the standard library std. Respond with only the Rust function and nothing else. Be careful with your types, and try to limit yourself to the basic built in types and standard library functions.

When writing the function you can think through how to solve the problem and perform reasoning in the comments above the function.

Then write unit tests for the function you defined. Write three unit tests for the function. The tests should be a simple line delimited list of assert! or assert_eq! statements. An example output should look like the following:

```rust
/// Reasoning goes here
/// and can be multi-line
fn add_nums(x: i32, y: i32) -> i32 {
  x + y
}
```

```tests
// Any comments explaining the test go here
assert_eq!(add_nums(1, 2), 3);
assert_eq!(add_nums(10, 2), 12);
assert_eq!(add_nums(-10, 2), 8);
```

Make sure to only respond with two blocks, a ```rust``` block and a ```tests``` block.

Here is the question:
{rust_prompt}