[Yaml] Restore deprecated php/const: syntax in YAML key

CHANGELOG.md

@@ -5,6 +5,7 @@ CHANGELOG
 ---
 
  * Add support for `!php/enum` and `!php/enum *->value`
+ * Deprecate the `!php/const:` tag in key which will be replaced by the `!php/const` tag (without the colon) since 3.4
 
 6.1
 ---

Parser.php

@@ -198,9 +198,14 @@ private function doParse(string $value, int $flags)
                     array_pop($this->refsBeingParsed);
                 }
             } elseif (
-                self::preg_match('#^(?P<key>(?:![^\s]++\s++)?(?:'.Inline::REGEX_QUOTED_STRING.'|[^ \'"\[\{!].*?)) *\:(( |\t)++(?P<value>.+))?$#u', rtrim($this->currentLine), $values)
+                // @todo in 7.0 remove legacy "(?:!?!php/const:)?"
+                self::preg_match('#^(?P<key>(?:![^\s]++\s++)?(?:'.Inline::REGEX_QUOTED_STRING.'|(?:!?!php/const:)?[^ \'"\[\{!].*?)) *\:(( |\t)++(?P<value>.+))?$#u', rtrim($this->currentLine), $values)
                 && (!str_contains($values['key'], ' #') || \in_array($values['key'][0], ['"', "'"]))
             ) {
+                if (str_starts_with($values['key'], '!php/const:')) {
+                    trigger_deprecation('symfony/yaml', '6.2', 'YAML syntax for key "%s" is deprecated and replaced by "!php/const %s".', $values['key'], substr($values['key'], 11));
+                }
+
                 if ($context && 'sequence' == $context) {
                     throw new ParseException('You cannot define a mapping item when in a sequence.', $this->currentLineNb + 1, $this->currentLine, $this->filename);
                 }

Tests/ParserTest.php

@@ -12,15 +12,17 @@
 namespace Symfony\Component\Yaml\Tests;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Yaml\Exception\ParseException;
 use Symfony\Component\Yaml\Parser;
 use Symfony\Component\Yaml\Tag\TaggedValue;
 use Symfony\Component\Yaml\Yaml;
 
 class ParserTest extends TestCase
 {
-    /** @var Parser */
-    protected $parser;
+    use ExpectDeprecationTrait;
+
+    private ?Parser $parser;
 
     protected function setUp(): void
     {
@@ -1557,6 +1559,7 @@ public function testParseDateAsMappingValue()
     /**
      * @param $lineNumber
      * @param $yaml
+     *
      * @dataProvider parserThrowsExceptionWithCorrectLineNumberProvider
      */
     public function testParserThrowsExceptionWithCorrectLineNumber($lineNumber, $yaml)
@@ -2484,6 +2487,17 @@ public function testDeprecatedPhpConstantSyntax()
         $this->parser->parse('!php/const:App\Kernel::SEMART_VERSION', Yaml::PARSE_CUSTOM_TAGS | Yaml::PARSE_CONSTANT);
     }
 
+    /**
+     * @group legacy
+     */
+    public function testDeprecatedPhpConstantSyntaxAsScalarKey()
+    {
+        $this->expectDeprecation('Since symfony/yaml 6.2: YAML syntax for key "!php/const:Symfony\Component\Yaml\Tests\B::BAR" is deprecated and replaced by "!php/const Symfony\Component\Yaml\Tests\B::BAR".');
+        $actual = $this->parser->parse('!php/const:Symfony\Component\Yaml\Tests\B::BAR: value', Yaml::PARSE_CUSTOM_TAGS | Yaml::PARSE_CONSTANT);
+
+        $this->assertSame(['bar' => 'value'], $actual);
+    }
+
     public function testPhpConstantTagMappingAsScalarKey()
     {
         $yaml = <<<YAML