Introduce --map-folder switch

And create PathMapper based on its value

src/Psalm/Internal/Cli/LanguageServer.php

@@ -10,6 +10,7 @@ use Psalm\Internal\Fork\PsalmRestarter;
 use Psalm\Internal\IncludeCollector;
 use Psalm\Internal\LanguageServer\ClientConfiguration;
 use Psalm\Internal\LanguageServer\LanguageServer as LanguageServerLanguageServer;
+use Psalm\Internal\LanguageServer\PathMapper;
 use Psalm\Report;
 
 use function array_key_exists;
@@ -75,6 +76,7 @@ final class LanguageServer
             'find-dead-code',
             'help',
             'root:',
+            'map-folder::',
             'use-ini-defaults',
             'version',
             'tcp:',
@@ -127,6 +129,14 @@ final class LanguageServer
 
         // get options from command line
         $options = getopt(implode('', $valid_short_options), $valid_long_options);
+        if ($options === false) {
+            // shouldn't really happen, but just in case
+            fwrite(
+                STDERR,
+                'Failed to get CLI args' . PHP_EOL,
+            );
+            exit(1);
+        }
 
         if (!array_key_exists('use-ini-defaults', $options)) {
             ini_set('display_errors', '1');
@@ -169,6 +179,14 @@ final class LanguageServer
                 -r, --root
                     If running Psalm globally you'll need to specify a project root. Defaults to cwd
 
+                --map-folder[=SERVER_FOLDER:CLIENT_FOLDER]
+                    Specify folder to map between the client and the server. Use this when the client
+                    and server have different views of the filesystem (e.g. in a docker container).
+                    Defaults to mapping the rootUri provided by the client to the server's cwd,
+                    or `-r` if provided.
+
+                    No mapping is done when this option is not specified.
+
                 --find-dead-code
                     Look for dead code
 
@@ -254,8 +272,6 @@ final class LanguageServer
             $current_dir = $root_path . DIRECTORY_SEPARATOR;
         }
 
-        $server_start_dir = $current_dir;
-
         $vendor_dir = CliUtils::getVendorDir($current_dir);
 
         $include_collector = new IncludeCollector();
@@ -293,6 +309,8 @@ final class LanguageServer
 
         setlocale(LC_CTYPE, 'C');
 
+        $path_mapper = self::createPathMapper($options, $current_dir);
+
         $path_to_config = CliUtils::getPathToConfig($options);
 
         if (isset($options['tcp'])) {
@@ -396,6 +414,49 @@ final class LanguageServer
         $clientConfiguration->TCPServerAddress = $options['tcp'] ?? null;
         $clientConfiguration->TCPServerMode = isset($options['tcp-server']);
 
-        LanguageServerLanguageServer::run($config, $clientConfiguration, $current_dir, $server_start_dir, $inMemory);
+        LanguageServerLanguageServer::run($config, $clientConfiguration, $current_dir, $path_mapper, $inMemory);
+    }
+
+    /** @param array<string,string|false|list<string|false>> $options */
+    private static function createPathMapper(array $options, string $server_start_dir): PathMapper
+    {
+        if (!isset($options['map-folder'])) {
+            // dummy no-op mapper
+            return new PathMapper('/', '/');
+        }
+
+        $map_folder = $options['map-folder'];
+
+        if ($map_folder === false) {
+            // autoconfigured mapper
+            return new PathMapper($server_start_dir, null);
+        }
+
+        if (is_string($map_folder)) {
+            if (strpos($map_folder, ':') === false) {
+                fwrite(
+                    STDERR,
+                    'invalid format for --map-folder option' . PHP_EOL,
+                );
+                exit(1);
+            }
+            /** @psalm-suppress PossiblyUndefinedArrayOffset we just checked that we have the separator*/
+            [$server_dir, $client_dir] = explode(':', $map_folder, 2);
+            if (!strlen($server_dir) || !strlen($client_dir)) {
+                fwrite(
+                    STDERR,
+                    'invalid format for --map-folder option, '
+                    . 'neither SERVER_FOLDER nor CLIENT_FOLDER can be empty' . PHP_EOL,
+                );
+                exit(1);
+            }
+            return new PathMapper($server_dir, $client_dir);
+        }
+
+        fwrite(
+            STDERR,
+            '--map-folder option can only be specified once' . PHP_EOL,
+        );
+        exit(1);
     }
 }

src/Psalm/Internal/LanguageServer/LanguageServer.php

@@ -153,7 +153,7 @@ class LanguageServer extends Dispatcher
         Codebase $codebase,
         ClientConfiguration $clientConfiguration,
         Progress $progress,
-        string $server_start_dir
+        PathMapper $path_mapper
     ) {
         parent::__construct($this, '/');
 
@@ -163,6 +163,8 @@ class LanguageServer extends Dispatcher
 
         $this->codebase = $codebase;
 
+        $this->path_mapper = $path_mapper;
+
         $this->protocolWriter = $writer;
 
         $this->protocolReader = $reader;
@@ -245,7 +247,6 @@ class LanguageServer extends Dispatcher
 
         $this->client = new LanguageClient($reader, $writer, $this, $clientConfiguration);
 
-        $this->path_mapper = new PathMapper($server_start_dir, null);
 
         $this->logInfo("Psalm Language Server ".PSALM_VERSION." has started.");
     }
@@ -257,7 +258,7 @@ class LanguageServer extends Dispatcher
         Config $config,
         ClientConfiguration $clientConfiguration,
         string $base_dir,
-        string $server_start_dir,
+        PathMapper $path_mapper,
         bool $inMemory = false
     ): void {
         $progress = new Progress();
@@ -330,7 +331,7 @@ class LanguageServer extends Dispatcher
                 $codebase,
                 $clientConfiguration,
                 $progress,
-                $server_start_dir,
+                $path_mapper,
             );
             Loop::run();
         } elseif ($clientConfiguration->TCPServerMode && $clientConfiguration->TCPServerAddress) {
@@ -354,7 +355,7 @@ class LanguageServer extends Dispatcher
                     $codebase,
                     $clientConfiguration,
                     $progress,
-                    $server_start_dir,
+                    $path_mapper,
                 );
                 Loop::run();
             }
@@ -368,7 +369,7 @@ class LanguageServer extends Dispatcher
                 $codebase,
                 $clientConfiguration,
                 $progress,
-                $server_start_dir,
+                $path_mapper,
             );
             Loop::run();
         }

tests/LanguageServer/DiagnosticTest.php

@@ -9,6 +9,7 @@ use Psalm\Internal\Analyzer\ProjectAnalyzer;
 use Psalm\Internal\LanguageServer\ClientConfiguration;
 use Psalm\Internal\LanguageServer\LanguageServer;
 use Psalm\Internal\LanguageServer\Message;
+use Psalm\Internal\LanguageServer\PathMapper;
 use Psalm\Internal\LanguageServer\Progress;
 use Psalm\Internal\Provider\FakeFileProvider;
 use Psalm\Internal\Provider\Providers;
@@ -85,7 +86,7 @@ class DiagnosticTest extends AsyncTestCase
             $this->codebase,
             $clientConfiguration,
             new Progress,
-            getcwd(),
+            new PathMapper(getcwd(), getcwd()),
         );
 
         $write->on('message', function (Message $message) use ($deferred, $server): void {