Currently, when you run the rust-test function it's pretty much equivalent to running cargo test in the terminal, and this is very useful!
However sometimes you need the print output from tests, which in the terminal you can get by running cargo test -- --show-output. As far as I can tell, rust-mode does not support this functionality out of the box at the moment- so it would be nice to see it added. Maybe with a function called rust-test-with-output or something similar.