I think I've noticed something related to Servo's "spurious rebuilds" issue! They seem to be triggered by Cargo deciding that one of the paths-overrided dependencies needs to be rebuilt. So, we see it a LOT when working on a rustup (or things like my android changes touching a lot of stuff), but still see it a bit on Servo itself because we have a paths override even in our default build (https://github.com/servo/servo/blob/master/components/servo/.cargo/config#L1 ).

Basically, without touching my machine I can just re-reun a build and it will seemingly randomly occasionally have a "good rebuild" log, but much more often have a "bad rebuild" log. The bad ones always start with things that are being overridden - in this case, I'm overriding skia, angle, gleam, rust-offscreen-rendering-context, and glutin.

Fingerprint log of a good rebuild:
https://drive.google.com/file/d/0B9-EBhw4XiWEZlVRZUpGcjFqYVU/view?usp=sharing

Fingerprint log of a bad rebuild:
https://drive.google.com/file/d/0B9-EBhw4XiWEWU1TRGFaZVJfR2s/view?usp=sharing

Hope this is enough info to debug it!