I'm working on a tool for testing incremental compilation called cargo-incremental. The basic idea is that you should be able to do two things with it:
cargo incremental build: replaces cargo build, but does incremental builds. However, it also saves a git branch containing the status of your workdir at each build, so we can go back in time if a bug is uncovered and repeat precisely what you did.
cargo incremental replay: walks over the git history and checks out each commit in turn, attempting incremental builds at each point (and comparing to normal builds).
The replay command basically works today, but could be improved. The build command is not implemented at all. There are also a bunch of other possible improvements.
Anybody have some free time to pitch in? If so, leave some comments on the repository.