Almost did the same thing...but I changed the trait bounds on array_diff to also require that T implement the Copy trait.
Then I used iter and copied instead of just using into_iter like I should have...because I initially misspelled into_iter while trying this exact thing :P

Yes, but array sizes are quite small in test cases, so creating additional structures can be less productive in this specific case.

This code has O(n*m) runtime as for each element we have to re-scan in b from scratch.

nvm it was just a typo.

This compiles and runs perfectly. Could you copy your code here?

i tried this and kept getting an error when a[] was empty

Genius!

Interestingly there's no test for this. My code was essentially your code without rev() and it still passed.