Tests are good but moving left by lifting invariants into the type system is better. Compare fn send_email(addr: &str, subject: &str, body: &str) -> Result

Tests are good but moving left by lifting invariants into the type system is better. Compare fn send_email(addr: &str, subject: &str, body: &str) -> Result<()> to fn send_email(add: &EmailAddr, subject: &str, body: &str) -> Result<()> In the second case, the edge cases of an empty or invalid email address don't need to be tested, they are statically impossible.