Stabilize 2021 edition - rebased - please ignore#9860

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m-ou-se wants to merge 3 commits intorust-lang:masterfrom m-ou-se:stabilize-2021

+57-60

Commits on Aug 31, 2021

Stabilize 2021 edition

ehuss
authored and
m-ou-se
committed
Make --force-warn support auto-detect.

ehuss
authored and
m-ou-se
committed
Use SUPPORTS_FORCE_WARN instead of config.nightly_features_allowed.
m-ou-se
committed