Improve the exception_dir exception message with more details

phpstan-baseline.neon

@@ -294,12 +294,6 @@ parameters:
 			count: 1
 			path: src/Platform/InstalledPiePackages.php
 
-		-
-			message: '#^Call to function array_key_exists\(\) with 1 and array\{non\-falsy\-string, non\-empty\-string, non\-empty\-string\} will always evaluate to true\.$#'
-			identifier: function.alreadyNarrowedType
-			count: 1
-			path: src/Platform/TargetPhp/PhpBinaryPath.php
-
 		-
 			message: '#^Call to function array_key_exists\(\) with 2 and array\{non\-falsy\-string, string, string\} will always evaluate to true\.$#'
 			identifier: function.alreadyNarrowedType

src/Platform/TargetPhp/Exception/ExtensionPathProblem.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Php\Pie\Platform\TargetPhp\Exception;
+
+use Php\Pie\Platform\TargetPhp\PhpBinaryPath;
+use RuntimeException;
+
+use function file_exists;
+use function is_dir;
+
+class ExtensionPathProblem extends RuntimeException
+{
+    public static function new(PhpBinaryPath $php, string|null $extensionPath): self
+    {
+        $message = 'Could not determine extension path for ' . $php->phpBinaryPath;
+
+        if ($extensionPath === null) {
+            $message .= '; extension_dir => not set';
+        } else {
+            $message .= '; extension_dir => ' . $extensionPath;
+
+            if (file_exists($extensionPath)) {
+                $message .= '; exists';
+
+                if (is_dir($extensionPath)) {
+                    $message .= ', is a directory';
+                } else {
+                    $message .= ', not a directory';
+                }
+            } else {
+                $message .= '; does not exist';
+            }
+        }
+
+        return new self($message);
+    }
+}

src/Platform/TargetPhp/PhpBinaryPath.php

@@ -12,6 +12,7 @@
 use Php\Pie\Platform\DebugBuild;
 use Php\Pie\Platform\OperatingSystem;
 use Php\Pie\Platform\OperatingSystemFamily;
+use Php\Pie\Platform\TargetPhp\Exception\ExtensionPathProblem;
 use Php\Pie\Util\Process;
 use RuntimeException;
 use Symfony\Component\Process\PhpExecutableFinder;
@@ -109,9 +110,10 @@ public function extensionPath(string|null $prefixInstallRoot = null): string
     {
         $phpinfo = $this->phpinfo();
 
+        $extensionPath = null;
+
         if (
             preg_match('#^extension_dir\s+=>\s+([^=]+)\s+=>\s+([^=]+)$#m', $phpinfo, $matches)
-            && array_key_exists(1, $matches)
             && trim($matches[1]) !== ''
             && trim($matches[1]) !== 'no value'
         ) {
@@ -142,7 +144,7 @@ public function extensionPath(string|null $prefixInstallRoot = null): string
             }
         }
 
-        throw new RuntimeException('Could not determine extension path for ' . $this->phpBinaryPath);
+        throw ExtensionPathProblem::new($this, $extensionPath);
     }
 
     public function assertExtensionIsLoadedInRuntime(ExtensionName $extension, IOInterface|null $io = null): void

test/unit/Platform/TargetPhp/Exception/ExtensionPathProblemTest.php

@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Php\PieUnitTest\Platform\TargetPhp\Exception;
+
+use Php\Pie\Platform\TargetPhp\Exception\ExtensionPathProblem;
+use Php\Pie\Platform\TargetPhp\PhpBinaryPath;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(ExtensionPathProblem::class)]
+final class ExtensionPathProblemTest extends TestCase
+{
+    public function testExtensionPathNotSet(): void
+    {
+        $php = PhpBinaryPath::fromCurrentProcess();
+
+        self::assertSame(
+            'Could not determine extension path for ' . $php->phpBinaryPath . '; extension_dir => not set',
+            ExtensionPathProblem::new($php, null)->getMessage(),
+        );
+    }
+
+    public function testExtensionPathDoesNotExist(): void
+    {
+        $php = PhpBinaryPath::fromCurrentProcess();
+
+        self::assertSame(
+            'Could not determine extension path for ' . $php->phpBinaryPath . '; extension_dir => /path/does/not/exist; does not exist',
+            ExtensionPathProblem::new($php, '/path/does/not/exist')->getMessage(),
+        );
+    }
+
+    public function testExtensionPathIsAFile(): void
+    {
+        $php = PhpBinaryPath::fromCurrentProcess();
+
+        self::assertSame(
+            'Could not determine extension path for ' . $php->phpBinaryPath . '; extension_dir => ' . __FILE__ . '; exists, not a directory',
+            ExtensionPathProblem::new($php, __FILE__)->getMessage(),
+        );
+    }
+
+    public function testExtensionPathIsADir(): void
+    {
+        $php = PhpBinaryPath::fromCurrentProcess();
+
+        self::assertSame(
+            'Could not determine extension path for ' . $php->phpBinaryPath . '; extension_dir => ' . __DIR__ . '; exists, is a directory',
+            ExtensionPathProblem::new($php, __DIR__)->getMessage(),
+        );
+    }
+}