[FrameworkBundle] encourage installing intl when String is available

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -196,9 +196,18 @@ class FrameworkExtension extends Extension
         }
 
         // If the slugger is used but the String component is not available, we should throw an error
-        if (!class_exists(SluggerInterface::class)) {
+        if (!interface_exists(SluggerInterface::class)) {
             $container->register('slugger', 'stdClass')
-                ->addError('You cannot use the "slugger" since the String component is not installed. Try running "composer require symfony/string".');
+                ->addError('You cannot use the "slugger" service since the String component is not installed. Try running "composer require symfony/string".');
+        } else {
+            if (!interface_exists(LocaleAwareInterface::class)) {
+                $container->register('slugger', 'stdClass')
+                    ->addError('You cannot use the "slugger" service since the Translation contracts are not installed. Try running "composer require symfony/translation".');
+            }
+
+            if (!\extension_loaded('intl')) {
+                @trigger_error('Please install the "intl" PHP extension for best performance.', E_USER_DEPRECATED);
+            }
         }
 
         if (isset($config['secret'])) {

src/Symfony/Component/String/Slugger/AsciiSlugger.php

@@ -107,7 +107,7 @@ class AsciiSlugger implements SluggerInterface, LocaleAwareInterface
 
     private function createTransliterator(string $locale): ?\Transliterator
     {
-        if (isset($this->transliterators[$locale])) {
+        if (\array_key_exists($locale, $this->transliterators)) {
             return $this->transliterators[$locale];
         }
 
@@ -118,7 +118,7 @@ class AsciiSlugger implements SluggerInterface, LocaleAwareInterface
 
         // Locale not supported and no parent, fallback to any-latin
         if (false === $str = strrchr($locale, '_')) {
-            return null;
+            return $this->transliterators[$locale] = null;
         }
 
         // Try to use the parent locale (ie. try "de" for "de_AT") and cache both locales
@@ -126,11 +126,8 @@ class AsciiSlugger implements SluggerInterface, LocaleAwareInterface
 
         if ($id = self::LOCALE_TO_TRANSLITERATOR_ID[$parent] ?? null) {
             $transliterator = \Transliterator::create($id.'/BGN') ?? \Transliterator::create($id);
-            $this->transliterators[$locale] = $this->transliterators[$parent] = $transliterator;
-
-            return $transliterator;
         }
 
-        return null;
+        return $this->transliterators[$locale] = $this->transliterators[$parent] = $transliterator ?? null;
     }
 }

src/Symfony/Component/String/composer.json

@@ -19,7 +19,8 @@
         "php": "^7.2.9",
         "symfony/polyfill-intl-grapheme": "~1.0",
         "symfony/polyfill-intl-normalizer": "~1.0",
-        "symfony/polyfill-mbstring": "~1.0"
+        "symfony/polyfill-mbstring": "~1.0",
+        "symfony/translation-contracts": "^1.1|^2.0"
     },
     "autoload": {
         "psr-4": { "Symfony\\Component\\String\\": "" },