My favorite tests are the ones I don't need to remember to write. I haven't needed to write a test for what happens when a function receives null in a while thanks to TS/mypy/C#'s nullable reference types/Rust's Option/etc. Similarly, I generally don't need to write tests for functions receiving the wrong type of values (strings vs numbers, for example), and with Rust, I generally don't even need to write tests for things like thread safety and sometimes even invalid states (since usually valid states can be represented by enum variants, and it's often impossible to have an invalid state because of that).

There is a point where it becomes too much, though. While I'd like it if the compiler ensured arbitrary preconditions like "x will always be between 2 and 4", I can't imagine what kinds of constraints that'd impose on actually writing the code in order to enforce that. Rust does have NonZero* types, but those are checked at runtime, not compile time.