As seen in: http://build.servo.org/builders/mac-rel-wpt/builds/1966, despite sharing a target directory, runing ./mach build-cef -r after a ./mach build -r, it still takes 14 minutes and 43 seconds. This time is because the script crate believes that it has been changed and needs to re-run the build. Here's a sample fingerprint output:

INFO:cargo::ops::cargo_rustc::fingerprint: fingerprint error for script v0.0.1 (file:///Users/larsberg/servo/components/servo): precalculated components have changed: 1456675029.000000000s (/Users/larsberg/servo/components/script/parser.out) != 145644786\
6.000000000s (/Users/larsberg/servo/components/script/dom/element.rs)

Does not seem to be the same as #7936.

I'm not really sure what this file is (yet), but am looking into it.

CC: @jdm @Ms2ger @alexcrichton @metajack