Find configuration based on CWD when formatting from stdin

Sources/swift-format/Frontend/ConfigurationLoader.swift

@@ -19,13 +19,13 @@ struct ConfigurationLoader {
   /// The cache of previously loaded configurations.
   private var cache = [String: Configuration]()
 
-  /// Returns the configuration found by searching in the directory (and ancestor directories)
-  /// containing the given `.swift` source file.
+  /// Returns the configuration found by walking up the file tree from `url`.
+  /// This function works for both files and directories.
   ///
   /// If no configuration file was found during the search, this method returns nil.
   ///
   /// - Throws: If a configuration file was found but an error occurred loading it.
-  mutating func configuration(forSwiftFileAt url: URL) throws -> Configuration? {
+  mutating func configuration(forPath url: URL) throws -> Configuration? {
     guard let configurationFileURL = Configuration.url(forConfigurationFileApplyingTo: url)
     else {
       return nil

Sources/swift-format/Frontend/Frontend.swift

@@ -213,7 +213,7 @@ class Frontend {
     // then try to load the configuration by inferring it based on the source file path.
     if let swiftFileURL = swiftFileURL {
       do {
-        if let configuration = try configurationLoader.configuration(forSwiftFileAt: swiftFileURL) {
+        if let configuration = try configurationLoader.configuration(forPath: swiftFileURL) {
           self.checkForUnrecognizedRules(in: configuration)
           return configuration
         }
@@ -223,11 +223,26 @@ class Frontend {
           "Unable to read configuration for \(swiftFileURL.path): \(error.localizedDescription)")
         return nil
       }
+    } else {
+      // If reading from stdin and no explicit configuration file was given,
+      // walk up the file tree from the cwd to find a config.
+
+      let cwd = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)
+      // Definitely a Swift file. Definitely not a directory. Shhhhhh.
+      do {
+        if let configuration = try configurationLoader.configuration(forPath: cwd) {
+          self.checkForUnrecognizedRules(in: configuration)
+          return configuration
+        }
+      } catch {
+        diagnosticsEngine.emitError(
+          "Unable to read configuration for \(cwd): \(error.localizedDescription)")
+        return nil
+      }
     }
 
-    // If neither path was given (for example, formatting standard input with no assumed filename)
-    // or if there was no configuration found by inferring it from the source file path, return the
-    // default configuration.
+    // An explicit configuration has not been given, and one cannot be found.
+    // Return the default configuration.
     return Configuration()
   }