@Oli I think your instance printing fix was incomplete. With https://github.com/RalfJung/rust/commit/22b1fae859d3afc5caa108239f36ead893f0132b, I still get ICEs when building libcore.
weird, I'm sure I tried it out. Maybe I built repros and only tried those... I don't remember
I am now rebuilding after
rm build/*/stage* -rf, just to be sure
(but the symptoms are different than the incremental issue I have now run into twice)
the issue reproduces with a clean build
@Oli can I assign that issue to you? :)