Any small change to the tests introduces a compile time of at least 30s (on my machine, at least). This makes developing and testing the driver a much slower experience than it needs to be. Based on initial profiling, it appears most of the time spent is in the evaluate_obligation function in rustc, which indicates that conditional compilation may have something to do with this. We should investigate our extensive usage of macros in the tests and see if we can cut down on the compile time.