Ok, question number two 
How to mark a target as “done”?
Suppose a run-pass target fails on my machine due to a rustc/test/whatever bug (e.g. this).
With the old build system I can touch tmp/x86-64-blah-blah-blah-rpass.ok and then build remaining targets, what is the rustbuild equivalent? Spurious (a la “can’t write directory”) or persistent failures almost always happen on Windows, so this is a blocker.
Also, why are targets named “steps”? And how to know which targets depend on which? E.g. how do I know if check runs tidy or not? With makefiles it was obvious, with rustbuild the dependencies must be somewhere in the source code, but I haven’t read the bootstrap code carefully yet. (Also, what is the equivalent of make check-notidy?)