Make tidy check for lang gate tests
Add gate tests to the checks that tidy performs. Excerpt from the commit message of the main commit:
Require compile-fail tests for new lang features
Its non trivial to test lang feature gates, and people
forget to add such tests. So we extend the features lint
of the tidy tool to ensure that all new lang features
contain a new compile-fail test.
Of course, one could drop this requirement and just
grep all tests in run-pass for #![feature(abc)] and
then run this test again, removing the mention,
requiring that it fails.
But this only tests for the existence of a compilation
failure. Manual tests ensure that also the correct lines
spawn the error, and also test the actual error message.
For library features, it makes no sense to require such
a test, as here code is used that is generic for all
library features.
The tidy lint extension now checks the compile-fail test suite for occurences of "gate-test-X" where X is a feature. Alternatively, it also accepts file names with the form "feature-gate-X.rs". If a lang feature is found that has no such check, we emit a tidy error.
I've applied the markings to all tests I could find in the test suite. I left a small (20 elements) whitelist of features that right now have no gate test, or where I couldn't find one. Once this PR gets merged, I'd like to close issue #22820 and open a new one on suggestion of @nikomatsakis to track the removal of all elements from that whitelist (already have a draft). Writing such a small test can be a good opportunity for a first contribution, so I won't touch it (let others have the fun xD).
cc @brson , @pnkfelix (they both discussed about this in the issue linked above).
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