Introduce SingleQuoteRule

src/Rules/String/SingleQuoteRule.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TwigCsFixer\Rules\String;
+
+use TwigCsFixer\Rules\AbstractFixableRule;
+use TwigCsFixer\Token\Token;
+
+/**
+ * Ensures that string use single quotes when possible.
+ */
+final class SingleQuoteRule extends AbstractFixableRule
+{
+    protected function process(int $tokenPosition, array $tokens): void
+    {
+        $token = $tokens[$tokenPosition];
+        if (!$this->isTokenMatching($token, Token::STRING_TYPE)) {
+            return;
+        }
+
+        $content = $token->getValue();
+        if (
+            '"' !== $content[0]
+//            && (true === $this->configuration['strings_containing_single_quote_chars'] || !str_contains($content, "'"))
+            // regex: odd number of backslashes, not followed by double quote or dollar
+        ) {
+            return;
+        }
+
+        $fixer = $this->addFixableError('String should be defined with single quotes.', $token);
+        if (null === $fixer) {
+            return;
+        }
+
+        $content = substr($content, 1, -1);
+        $content = str_replace(
+            ['\\"', '\\#{', '#\\{', '\\\'', '\''],
+            ['"', '#{', '#{', '\'', '\\\''],
+            $content);
+        $fixer->replaceToken($tokenPosition, '\''.$content.'\'');
+    }
+}

tests/Rules/String/SimpleQuote/SingleQuoteRuleTest.fixed.twig

@@ -0,0 +1,11 @@
+'foo'
+"foo"
+
+{% set foo = 'foo' %}
+{% set foo2 = 'foo' %}
+{% set foo3 = '\'foo\'' %}
+{% set foo4 = '\'foo\'' %}
+
+{% set foo5 = "#{p.first}" %}
+{% set foo6 = '#{p.first}' %}
+{% set foo7 = '#{p.first}' %}

tests/Rules/String/SimpleQuote/SingleQuoteRuleTest.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace TwigCsFixer\Tests\Rules\String\SimpleQuote;
+
+use TwigCsFixer\Rules\String\SingleQuoteRule;
+use TwigCsFixer\Tests\Rules\AbstractRuleTestCase;
+
+class SingleQuoteRuleTest extends AbstractRuleTestCase
+{
+    public function testRule(): void
+    {
+        $this->checkRule(new SingleQuoteRule(), [
+            'SingleQuote.Error:5:15' => 'String should be defined with single quotes.',
+            'SingleQuote.Error:6:15' => 'String should be defined with single quotes.',
+            'SingleQuote.Error:7:15' => 'String should be defined with single quotes.',
+            'SingleQuote.Error:10:15' => 'String should be defined with single quotes.',
+            'SingleQuote.Error:11:15' => 'String should be defined with single quotes.',
+        ]);
+    }
+}

tests/Rules/String/SimpleQuote/SingleQuoteRuleTest.twig

@@ -0,0 +1,11 @@
+'foo'
+"foo"
+
+{% set foo = 'foo' %}
+{% set foo2 = "foo" %}
+{% set foo3 = "'foo'" %}
+{% set foo4 = "\'foo\'" %}
+
+{% set foo5 = "#{p.first}" %}
+{% set foo6 = "\#{p.first}" %}
+{% set foo7 = "#\{p.first}" %}