Memoizing DocBlock#requirements() so that repeated calls do not lea…

…d to overhead

Parsing requirements on docblocks seems to be repeated multiple times,
so we need to prevent this from being performed more than necessary.
The regular expressions around this are quite complex, and therefore there
is a performance impact (~2% total runtime on an IO-free test suite).

src/Annotation/DocBlock.php

@@ -23,7 +23,7 @@
  * @internal This class is part of PHPUnit internals, an not intended
  *           for downstream usage
  *
- * @psalm-immutable
+ * @psalm-mutation-free
  */
 final class DocBlock
 {
@@ -52,6 +52,20 @@ final class DocBlock
     /** @var array<string, array<int, string>> pre-parsed annotations indexed by name and occurrence index */
     private $symbolAnnotations;
 
+    /**
+     * @var array<string, mixed>|null
+     *
+     * @psalm-var null|(array{
+     *   __OFFSET: array<string, int>&array{__FILE: string},
+     *   setting?: array<string, string>,
+     *   extension_versions?: array<string, array{version: string, operator: string}>
+     * }&array<
+     *   string,
+     *   string|array{version: string, operator: string}|array{constraint: string}|array<int|string, string>
+     * >)
+     */
+    private $parsedRequirements;
+
     /** @var int */
     private $startLine;
 
@@ -137,9 +151,15 @@ private function __construct(
      *   string,
      *   string|array{version: string, operator: string}|array{constraint: string}|array<int|string, string>
      * >
+     *
+     * @throws Warning if the requirements version constraint is not well-formed
      */
     public function requirements(): array
     {
+        if ($this->parsedRequirements !== null) {
+            return $this->parsedRequirements;
+        }
+
         $offset            = $this->startLine;
         $requires          = [];
         $recordedSettings  = [];
@@ -212,7 +232,7 @@ public function requirements(): array
             $offset++;
         }
 
-        return \array_merge(
+        return $this->parsedRequirements = \array_merge(
             $requires,
             ['__OFFSET' => $recordedOffsets],
             \array_filter([