If Rector rules have bugs, the produced downgrade code may also have bugs. The tests must fail whenever this happens

[leoloso]

Downgrading code to PHP 7.1 with Rector is currently producing code with a few bugs:

• A chained rule is not being executed, so a self return type is not being removed: [Downgrade] A 2nd rule should modify the output from a previous rule in the set, but it doesn't happen rectorphp/rector#5962
• A method is being changed its signature, when it should not: Incorrect behavior of DowngradeParameterTypeWideningRector rectorphp/rector#5994

We need to make sure that, after downgrading the code, no buggy code has been produced.

For that, in downgrade_php_tests.yml, run the Rector process a 2nd time, using --dry-run:

• If buggy code was produced, Rector will very likely fail
• If not all chained rules were executed, a new modification will take place, and using --dry-run will fail

[leoloso]

@TomasVotruba This issue may be relevant for you too, for when generating rector-prefixed.

I hope you'll agree with my proposed solution. If there's anything better, please let me know.