The Windows buildbots have been broken by PR 12073. Problem: AppVeyor didn't catch the bug before the change has been merged. Why? Because AppVeyor builds Python in release mode, whereas Windows buildbots build Python in debug mode.

https://bugs.python.org/issue36139#msg337320

IMHO AppVeyor should also build Python in debug mode to catch most bugs.

We should compare build time in debug mode and in release mode to see the cost of debug mode.

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Somehow related issue: AppVeyor and Travis CI tests passed on PR 10497 "bpo-35224: PEP 572 Implementation" but buildbots failed. The problem is that buildbots pass "-u all" to "python -m test", whereas pre-commit CIs use "-uall,-cpu" (.travis.yml) or "-uall -u-cpu -u-largefile" (.github/appveyor.yml).

https://github.com/python/cpython/pull/10497#issuecomment-457409029

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It's a trade-off between preventing bugs and CI performance...