[FR] Arrow functions: sniff to enforce spacing after the `fn` keyword

[rodrigoprimo]

Setting the scene

PHP 7.4 introduced arrow functions (short closures) via the fn() => expression syntax. At this moment, neither PHPCS itself nor PHPCSExtra contain a sniff to handle the spacing after the fn keyword.

The PER Coding Standard from FIG, since PER 2.0, outlines a set of rules for short closures in section 7.1.

Proposed new sniff: Universal.WhiteSpace.FnKeywordSpacing

To address this rule from PER:

The fn keyword MUST NOT be succeeded by a space.

Notes for the implementation

The fn keyword is tokenized as T_FN, so registering [T_FN] is a good entry point. The SpacesFixer from PHPCSUtils handles the check for the spacing after the keyword.

Suggested error code:

• SpacingAfterKeyword: incorrect spacing after the fn keyword.

The sniff should be configurable and use the PER-CS value as default:

• public $spacingAfterKeyword = 0;

Describe the solution you'd like

A new sniff as outlined above.

The sniff should be able to flag and auto-fix the following:

// OK.
$fn = fn($x) => $x + 1;

// Error: space after the `fn` keyword.
$fn = fn ($x) => $x + 1;

Also see the examples outlined in the PER documents (rules + migration guide).

Additional context (optional)

This ticket is part of a series of tickets that is the result of a detailed analysis of the rules as outlined in PER-CS 2.0, as well as a critical look at what's still missing rule-wise.

• I intend to create a pull request to implement this feature.