I don't know what the exact error is here but I bet that fixing bug 1385241 would make this go away.
I was getting this the other day as well. I'm optimistic bug 1397279 may have resolved this. Could you please try nuking your Rust and ~/.mozbuild state and trying again with an up-to-date mozilla-central checkout? If it works, I'd mark as a dupe.
Component: mach → Build Config
I ended up wiping and resetting my windows vm image; manually installed rustup and didn't have problems.
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