Universal/FnKeywordSpacing: new sniff to enforce no space after the fn keyword

Add the `Universal.WhiteSpace.FnKeywordSpacing` sniff which enforces that the `fn` keyword of an arrow function is not succeeded by a space, as required by PER Coding Style 2.0 (section 7.1). The amount of spacing is configurable via the `spacingAfterKeyword` property (default 0).

Fixes 441.

Ref: https://github.com/php-fig/per-coding-style/blob/2.0.0/spec.md#71-short-closures

Author: rodrigoprimo

Universal/Docs/WhiteSpace/FnKeywordSpacingStandard.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<documentation xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://phpcsstandards.github.io/PHPCSDevTools/phpcsdocs.xsd"
+    title="Fn Keyword Spacing"
+    >
+    <standard>
+    <![CDATA[
+    The `fn` keyword of an arrow function should not be succeeded by a space.
+    ]]>
+    </standard>
+    <code_comparison>
+        <code title="Valid: No space after the `fn` keyword.">
+        <![CDATA[
+$fn = fn<em></em>($x) => $x + 1;
+        ]]>
+        </code>
+        <code title="Invalid: Space after the `fn` keyword.">
+        <![CDATA[
+$fn = fn<em> </em>($x) => $x + 1;
+        ]]>
+        </code>
+    </code_comparison>
+</documentation>

Universal/Sniffs/WhiteSpace/FnKeywordSpacingSniff.php

@@ -0,0 +1,73 @@
+<?php
+/**
+ * PHPCSExtra, a collection of sniffs and standards for use with PHP_CodeSniffer.
+ *
+ * @package   PHPCSExtra
+ * @copyright 2020 PHPCSExtra Contributors
+ * @license   https://opensource.org/licenses/LGPL-3.0 LGPL3
+ * @link      https://github.com/PHPCSStandards/PHPCSExtra
+ */
+
+namespace PHPCSExtra\Universal\Sniffs\WhiteSpace;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+use PHP_CodeSniffer\Util\Tokens;
+use PHPCSUtils\Fixers\SpacesFixer;
+
+/**
+ * Enforce the spacing after the "fn" keyword of arrow functions.
+ *
+ * @since 1.6.0
+ */
+final class FnKeywordSpacingSniff implements Sniff
+{
+
+    /**
+     * The number of spaces to demand after the "fn" keyword.
+     *
+     * @since 1.6.0
+     *
+     * @var int
+     */
+    public $spacingAfterKeyword = 0;
+
+    /**
+     * Registers the tokens that this sniff wants to listen for.
+     *
+     * @since 1.6.0
+     *
+     * @return array<int|string>
+     */
+    public function register()
+    {
+        return [\T_FN];
+    }
+
+    /**
+     * Processes this sniff, when one of its tokens is encountered.
+     *
+     * @since 1.6.0
+     *
+     * @param \PHP_CodeSniffer\Files\File $phpcsFile The file being scanned.
+     * @param int                         $stackPtr  The position of the current token in the stack passed in $tokens.
+     *
+     * @return void
+     */
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        $nextNonEmpty = $phpcsFile->findNext(Tokens::$emptyTokens, ($stackPtr + 1), null, true);
+
+        SpacesFixer::checkAndFix(
+            $phpcsFile,
+            $stackPtr,
+            $nextNonEmpty,
+            (int) $this->spacingAfterKeyword,
+            'Expected %s after the "fn" keyword. Found: %s.',
+            'Incorrect',
+            'error',
+            0,
+            'Spacing after the arrow function "fn" keyword'
+        );
+    }
+}

Universal/Tests/WhiteSpace/FnKeywordSpacingUnitTest.1.inc

@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * Valid spacing.
+ */
+
+$fn = fn($x) => $x + 1;
+$fn = fn&() => doSomething();
+$fn = static fn($x) => fn($y) => $x + $y;
+
+/*
+ * Invalid spacing.
+ */
+
+$fn = fn ($x) => $x + 1;
+$fn = fn  &($x) => $x + 1;
+$fn = static fn
+    ($x) => $x + 1;
+$fn = fn () => doSomething();
+
+$fn = fn  ($x) => fn   ($y) => $x + $y;
+
+$fn = fn
+    /* comment */
+    ($x) => $x + 1;
+
+// phpcs:set Universal.WhiteSpace.FnKeywordSpacing spacingAfterKeyword 1
+$fn = fn($x) => $x + 1; // Error.
+$fn = fn ($x) => $x + 1; // OK.
+// Resetting to the default value.
+// phpcs:set Universal.WhiteSpace.FnKeywordSpacing spacingAfterKeyword 0

Universal/Tests/WhiteSpace/FnKeywordSpacingUnitTest.1.inc.fixed

@@ -0,0 +1,30 @@
+<?php
+
+/*
+ * Valid spacing.
+ */
+
+$fn = fn($x) => $x + 1;
+$fn = fn&() => doSomething();
+$fn = static fn($x) => fn($y) => $x + $y;
+
+/*
+ * Invalid spacing.
+ */
+
+$fn = fn($x) => $x + 1;
+$fn = fn&($x) => $x + 1;
+$fn = static fn($x) => $x + 1;
+$fn = fn() => doSomething();
+
+$fn = fn($x) => fn($y) => $x + $y;
+
+$fn = fn
+    /* comment */
+    ($x) => $x + 1;
+
+// phpcs:set Universal.WhiteSpace.FnKeywordSpacing spacingAfterKeyword 1
+$fn = fn ($x) => $x + 1; // Error.
+$fn = fn ($x) => $x + 1; // OK.
+// Resetting to the default value.
+// phpcs:set Universal.WhiteSpace.FnKeywordSpacing spacingAfterKeyword 0

Universal/Tests/WhiteSpace/FnKeywordSpacingUnitTest.php

@@ -0,0 +1,52 @@
+<?php
+/**
+ * PHPCSExtra, a collection of sniffs and standards for use with PHP_CodeSniffer.
+ *
+ * @package   PHPCSExtra
+ * @copyright 2020 PHPCSExtra Contributors
+ * @license   https://opensource.org/licenses/LGPL-3.0 LGPL3
+ * @link      https://github.com/PHPCSStandards/PHPCSExtra
+ */
+
+namespace PHPCSExtra\Universal\Tests\WhiteSpace;
+
+use PHP_CodeSniffer\Tests\Standards\AbstractSniffTestCase;
+
+/**
+ * Unit test class for the FnKeywordSpacing sniff.
+ *
+ * @covers PHPCSExtra\Universal\Sniffs\WhiteSpace\FnKeywordSpacingSniff
+ *
+ * @since 1.6.0
+ */
+final class FnKeywordSpacingUnitTest extends AbstractSniffTestCase
+{
+
+    /**
+     * Returns the lines where errors should occur.
+     *
+     * @return array<int, int> Key is the line number, value is the number of expected errors.
+     */
+    public function getErrorList()
+    {
+        return [
+            15 => 1,
+            16 => 1,
+            17 => 1,
+            19 => 1,
+            21 => 2,
+            23 => 1,
+            28 => 1,
+        ];
+    }
+
+    /**
+     * Returns the lines where warnings should occur.
+     *
+     * @return array<int, int> Key is the line number, value is the number of expected warnings.
+     */
+    public function getWarningList()
+    {
+        return [];
+    }
+}