PHP 8.0 : NewOperators account for nullsafe object operator

[jrfnl]

PHP 8.0: NewOperators: detect new nullsafe object operator

PHP 8 introduces a new object chaining operator ?-> which short-circuits moving to the next expression if the left-hand side evaluates to null.

This operator can not be used in write-context, but that is not the concern of this sniff.

Refs:

• https://wiki.php.net/rfc/nullsafe_operator
• [WIP] Implement nullsafe ?-> operator php/php-src#5619
• php/php-src@9bf1198

Includes unit test.

NewOperators: improve grammar of the error message

PHP 8.0: NewConstants: add new T_NULLSAFE_OBJECT_OPERATOR constant

Ref: https://wiki.php.net/rfc/nullsafe_operator

...and exclude it from triggering when running PHPCompatibility over its own code.

Related to #809

[jrfnl]

Ok, so the build failing appears to be due to an obscure bug in Xdebug 2.9.6. Same problem as we had the other week when I changed a coverage build from PHP 7.3 to PHP 7.2 to get round it.

The weirdest thing is that it only occurs with the NewOperators sniff, while similar code is also used in the NewKeywords, NewLanguageConstructs, NewTypeCasts and RemovedTypeCasts sniffs.

Note: this is a test-only problem as for the tests each sniff is instantiated numerous times, while in a "normal" PHPCS run, each sniff is only instantiated once, so the problem would not occur in a "normal" PHPCS run.

I've tried my damnest to think of a work-around for the time being to allow the build to pass, but I honestly can't think of one without making the code very weird indeed, while it may still not work.