Fix simple relative path helper stripping first character when working directory is a slash

Author: liamduckett

src/File/SimpleRelativePathHelper.php

@@ -23,7 +23,13 @@ public function __construct(
 	public function getRelativePath(string $filename): string
 	{
 		if ($this->currentWorkingDirectory !== '' && str_starts_with($filename, $this->currentWorkingDirectory)) {
-			return str_replace('\\', '/', substr($filename, strlen($this->currentWorkingDirectory) + 1));
+			$length = strlen($this->currentWorkingDirectory);
+
+			if ($this->currentWorkingDirectory !== '/') {
+				$length += 1;
+			}
+
+			return str_replace('\\', '/', substr($filename, $length));
 		}
 
 		return str_replace('\\', '/', $filename);

tests/PHPStan/File/SimpleRelativePathHelperTest.php

@@ -0,0 +1,46 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\File;
+
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+class SimpleRelativePathHelperTest extends TestCase
+{
+
+	public static function dataGetRelativePath(): array
+	{
+		return [
+			[
+				'/project',
+				'/project/src/HelloWorld.php',
+				'src/HelloWorld.php',
+			],
+			[
+				'',
+				'/project/src/HelloWorld.php',
+				'/project/src/HelloWorld.php',
+			],
+			[
+				'/',
+				'/project/src/HelloWorld.php',
+				'project/src/HelloWorld.php',
+			],
+		];
+	}
+
+	#[DataProvider('dataGetRelativePath')]
+	public function testGetRelativePath(
+		string $currentWorkingDirectory,
+		string $filename,
+		string $expectedRelativePath,
+	): void
+	{
+		$helper = new SimpleRelativePathHelper($currentWorkingDirectory);
+		$this->assertSame(
+			$expectedRelativePath,
+			$helper->getRelativePath($filename),
+		);
+	}
+
+}