After looking at everyone else's solutions, I realised that the test for the num crate is just pattern matching on "num", which means you can't use that as part of a variable name, e.g. num_string, digits_of_number.

It should be possible to pattern match just on the use statement instead.

Best of luck, everyone!

It should also work if you remove the special case for matrices of size 2 :)

Fixed. Please don't open duplicates of existing issues.

Rust random tests still broken :(

I've got a Rust solution which failed the "num" crate check, since I used the div_floor function from there. Thought only num::BigInt was disallowed. No matter, I removed the dependency, and it still failed the crate check, so I removed all dependencies (itertools) and it still fails the crate check. I reset the environment with the button at the bottom, still fails. I don't know what to do now. Any ideas? I'm guessing the crate is still installed somewhere so the test is failing.

Beautiful!

To be clear, there is an unwrap here: 0 inside fold is the default value. Same you could write as .max().unwrap_or(0) , which is also correct error handling. .unwrap() is what you should never use...