Short list vs short array

[jrfnl]

Repost from squizlabs/PHP_CodeSniffer#1984:

PHP 7.1 introduced the "short list syntax", i.e. using the short array syntax instead of calling list().
Ref: http://php.net/manual/en/migration71.new-features.php#migration71.new-features.symmetric-array-destructuring

Now I can imagine that in some situations it would be useful to be able to make the distinction whether something is used as a short list vs a short array.

There are two ways this can be done:

1. Introduce T_OPEN_SHORT_LIST/T_CLOSE_SHORT_LIST tokens and let the Tokenizer handle it.
   The downside of this first solution is that in a lot of cases - especially when sniffing for code style -, I imagine that short list/array should be treated the same, so those sniff would then need to add two additional tokens to sniff for.
2. Introduce a utility function which can make the distinction and can be called by select sniffs if/when needed.

For the PHPCompatibility standard, I have already created the utility function (solution 2), including extensive unit tests for it.
See: PHPCompatibility/PHPCompatibility#635

If there would be interest in having such a utility function in PHPCS itself, I'd be happy to pull it here.

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See the original issue for an extensive discussion on the topic.

My current line of thinking is to add a nested_brackets index to the token array. This would allow for sniffs to figure out if they are dealing with a (nested) short list or a short array much more easily.

I'm tentatively earmarking this ticket for 4.0, but it may well be moved to a later release.