From 6e488242a5d5dfd0b6402d4caf4fd8d64173ad53 Mon Sep 17 00:00:00 2001
From: Andre Richter <andre_richter@apple.com>
Date: Thu, 6 Feb 2025 15:11:51 +0100
Subject: [PATCH 1/4] Add dump-effective-configuration subcommand

closes #667

Implement the subcommand `dump-effective-configuration`, which dumps
the configuration that would be used if `swift-format` was executed
from the current working directory (cwd), incorporating configuration
files found in the cwd or its parents, or input from the
`--configuration` option.

This helps when composing a configuration or with configuration
debugging/verification activities.
---
 README.md                                     | 16 ++++--
 .../SwiftFormat/API/Configuration+Dump.swift  | 39 ++++++++++++++
 .../SwiftFormat/API/SwiftFormatError.swift    |  5 ++
 Sources/SwiftFormat/CMakeLists.txt            |  3 +-
 Sources/swift-format/CMakeLists.txt           |  6 ++-
 .../DumpEffectiveConfigurationFrontend.swift  | 25 +++++++++
 .../Frontend/FormatFrontend.swift             |  6 +--
 Sources/swift-format/Frontend/Frontend.swift  | 12 +++--
 .../Subcommands/ConfigurationOptions.swift    | 28 ++++++++++
 .../Subcommands/DumpConfiguration.swift       | 21 +-------
 .../DumpEffectiveConfiguration.swift          | 54 +++++++++++++++++++
 Sources/swift-format/Subcommands/Format.swift | 11 +++-
 Sources/swift-format/Subcommands/Lint.swift   |  7 ++-
 .../Subcommands/LintFormatOptions.swift       | 14 +----
 Sources/swift-format/SwiftFormatCommand.swift |  3 +-
 .../swift-format/Utilities/FormatError.swift  | 23 --------
 16 files changed, 198 insertions(+), 75 deletions(-)
 create mode 100644 Sources/SwiftFormat/API/Configuration+Dump.swift
 create mode 100644 Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift
 create mode 100644 Sources/swift-format/Subcommands/ConfigurationOptions.swift
 create mode 100644 Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift
 delete mode 100644 Sources/swift-format/Utilities/FormatError.swift

diff --git a/README.md b/README.md
index db2e090a9..94aace284 100644
--- a/README.md
+++ b/README.md
@@ -210,13 +210,19 @@ settings in the default configuration can be viewed by running
 `swift-format dump-configuration`, which will dump it to standard
 output.
 
-If the `--configuration <file>` option is passed to `swift-format`, then that
-configuration will be used unconditionally and the file system will not be
-searched.
+If the `--configuration <configuration>` option is passed to `swift-format`,
+then that configuration will be used unconditionally and the file system will
+not be searched.
 
 See [Documentation/Configuration.md](Documentation/Configuration.md) for a
-description of the configuration file format and the settings that are
-available.
+description of the configuration format and the settings that are available.
+
+#### Viewing the Effective Configuration
+
+The `dump-effective-configuration` subcommand dumps the configuration that
+would be used if `swift-format` was executed from the current working directory,
+and accounts for `.swift-format` files or `--configuration` options as outlined
+above.
 
 ### Miscellaneous
 
diff --git a/Sources/SwiftFormat/API/Configuration+Dump.swift b/Sources/SwiftFormat/API/Configuration+Dump.swift
new file mode 100644
index 000000000..a9a68fb7e
--- /dev/null
+++ b/Sources/SwiftFormat/API/Configuration+Dump.swift
@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+extension Configuration {
+  /// Return the configuration as a JSON string.
+  public func asJsonString() throws -> String {
+    let data: Data
+
+    do {
+      let encoder = JSONEncoder()
+      encoder.outputFormatting = [.prettyPrinted]
+      if #available(macOS 10.13, *) {
+        encoder.outputFormatting.insert(.sortedKeys)
+      }
+
+      data = try encoder.encode(self)
+    } catch {
+      throw SwiftFormatError.configurationDumpFailed("\(error)")
+    }
+
+    guard let jsonString = String(data: data, encoding: .utf8) else {
+      // This should never happen, but let's make sure we fail more gracefully than crashing, just in case.
+      throw SwiftFormatError.configurationDumpFailed("The JSON was not valid UTF-8")
+    }
+
+    return jsonString
+  }
+}
diff --git a/Sources/SwiftFormat/API/SwiftFormatError.swift b/Sources/SwiftFormat/API/SwiftFormatError.swift
index 6e4183162..cb35a82d0 100644
--- a/Sources/SwiftFormat/API/SwiftFormatError.swift
+++ b/Sources/SwiftFormat/API/SwiftFormatError.swift
@@ -28,6 +28,9 @@ public enum SwiftFormatError: LocalizedError {
   /// The requested experimental feature name was not recognized by the parser.
   case unrecognizedExperimentalFeature(String)
 
+  /// An error happened while dumping the tool's configuration.
+  case configurationDumpFailed(String)
+
   public var errorDescription: String? {
     switch self {
     case .fileNotReadable:
@@ -38,6 +41,8 @@ public enum SwiftFormatError: LocalizedError {
       return "file contains invalid Swift syntax"
     case .unrecognizedExperimentalFeature(let name):
       return "experimental feature '\(name)' was not recognized by the Swift parser"
+    case .configurationDumpFailed(let message):
+      return "dumping configuration failed: \(message)"
     }
   }
 }
diff --git a/Sources/SwiftFormat/CMakeLists.txt b/Sources/SwiftFormat/CMakeLists.txt
index 46937f713..9306c49f3 100644
--- a/Sources/SwiftFormat/CMakeLists.txt
+++ b/Sources/SwiftFormat/CMakeLists.txt
@@ -1,7 +1,7 @@
 #[[
 This source file is part of the swift-format open source project
 
-Copyright (c) 2024 Apple Inc. and the swift-format project authors
+Copyright (c) 2024 - 2025 Apple Inc. and the swift-format project authors
 Licensed under Apache License v2.0 with Runtime Library Exception
 
 See https://swift.org/LICENSE.txt for license information
@@ -9,6 +9,7 @@ See https://swift.org/LICENSE.txt for license information
 
 add_library(SwiftFormat
   API/Configuration+Default.swift
+  API/Configuration+Dump.swift
   API/Configuration.swift
   API/DebugOptions.swift
   API/Finding.swift
diff --git a/Sources/swift-format/CMakeLists.txt b/Sources/swift-format/CMakeLists.txt
index 9ae9603e1..b563a4463 100644
--- a/Sources/swift-format/CMakeLists.txt
+++ b/Sources/swift-format/CMakeLists.txt
@@ -1,7 +1,7 @@
 #[[
 This source file is part of the swift-format open source project
 
-Copyright (c) 2024 Apple Inc. and the swift-format project authors
+Copyright (c) 2024 - 2025 Apple Inc. and the swift-format project authors
 Licensed under Apache License v2.0 with Runtime Library Exception
 
 See https://swift.org/LICENSE.txt for license information
@@ -12,10 +12,13 @@ add_executable(swift-format
   SwiftFormatCommand.swift
   VersionOptions.swift
   Frontend/ConfigurationLoader.swift
+  Frontend/DumpEffectiveConfigurationFrontend.swift
   Frontend/FormatFrontend.swift
   Frontend/Frontend.swift
   Frontend/LintFrontend.swift
+  Subcommands/ConfigurationOptions.swift
   Subcommands/DumpConfiguration.swift
+  Subcommands/DumpEffectiveConfiguration.swift
   Subcommands/Format.swift
   Subcommands/Lint.swift
   Subcommands/LintFormatOptions.swift
@@ -23,7 +26,6 @@ add_executable(swift-format
   Utilities/Diagnostic.swift
   Utilities/DiagnosticsEngine.swift
   Utilities/FileHandleTextOutputStream.swift
-  Utilities/FormatError.swift
   Utilities/StderrDiagnosticPrinter.swift
   Utilities/TTY.swift)
 target_link_libraries(swift-format PRIVATE
diff --git a/Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift b/Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift
new file mode 100644
index 000000000..1608c9aa8
--- /dev/null
+++ b/Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import SwiftFormat
+
+/// The frontend for dumping the effective configuration.
+class DumpEffectiveConfigurationFrontend: Frontend {
+  private(set) var dumpResult: Result<String, Error> = .failure(
+    SwiftFormatError.configurationDumpFailed("Configuration not resolved yet")
+  )
+
+  override func processFile(_ fileToProcess: FileToProcess) {
+    dumpResult = Result.init(catching: fileToProcess.configuration.asJsonString)
+  }
+}
diff --git a/Sources/swift-format/Frontend/FormatFrontend.swift b/Sources/swift-format/Frontend/FormatFrontend.swift
index 23d127719..a205b6405 100644
--- a/Sources/swift-format/Frontend/FormatFrontend.swift
+++ b/Sources/swift-format/Frontend/FormatFrontend.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -20,9 +20,9 @@ class FormatFrontend: Frontend {
   /// Whether or not to format the Swift file in-place.
   private let inPlace: Bool
 
-  init(lintFormatOptions: LintFormatOptions, inPlace: Bool) {
+  init(configurationOptions: ConfigurationOptions, lintFormatOptions: LintFormatOptions, inPlace: Bool) {
     self.inPlace = inPlace
-    super.init(lintFormatOptions: lintFormatOptions)
+    super.init(configurationOptions: configurationOptions, lintFormatOptions: lintFormatOptions)
   }
 
   override func processFile(_ fileToProcess: FileToProcess) {
diff --git a/Sources/swift-format/Frontend/Frontend.swift b/Sources/swift-format/Frontend/Frontend.swift
index b0e262a94..d7bb5cbe5 100644
--- a/Sources/swift-format/Frontend/Frontend.swift
+++ b/Sources/swift-format/Frontend/Frontend.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -67,6 +67,9 @@ class Frontend {
   /// The diagnostic engine to which warnings and errors will be emitted.
   final let diagnosticsEngine: DiagnosticsEngine
 
+  /// Options that control the tool's configuration.
+  final let configurationOptions: ConfigurationOptions
+
   /// Options that apply during formatting or linting.
   final let lintFormatOptions: LintFormatOptions
 
@@ -85,7 +88,8 @@ class Frontend {
   /// Creates a new frontend with the given options.
   ///
   /// - Parameter lintFormatOptions: Options that apply during formatting or linting.
-  init(lintFormatOptions: LintFormatOptions) {
+  init(configurationOptions: ConfigurationOptions, lintFormatOptions: LintFormatOptions) {
+    self.configurationOptions = configurationOptions
     self.lintFormatOptions = lintFormatOptions
 
     self.diagnosticPrinter = StderrDiagnosticPrinter(
@@ -139,7 +143,7 @@ class Frontend {
 
     guard
       let configuration = configuration(
-        fromPathOrString: lintFormatOptions.configuration,
+        fromPathOrString: configurationOptions.configuration,
         orInferredFromSwiftFileAt: assumedUrl
       )
     else {
@@ -190,7 +194,7 @@ class Frontend {
 
     guard
       let configuration = configuration(
-        fromPathOrString: lintFormatOptions.configuration,
+        fromPathOrString: configurationOptions.configuration,
         orInferredFromSwiftFileAt: url
       )
     else {
diff --git a/Sources/swift-format/Subcommands/ConfigurationOptions.swift b/Sources/swift-format/Subcommands/ConfigurationOptions.swift
new file mode 100644
index 000000000..9c018a111
--- /dev/null
+++ b/Sources/swift-format/Subcommands/ConfigurationOptions.swift
@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+
+/// Common arguments used by the `lint`, `format` and `dump-effective-configuration` subcommands.
+struct ConfigurationOptions: ParsableArguments {
+  /// The path to the JSON configuration file that should be loaded.
+  ///
+  /// If not specified, the default configuration will be used.
+  @Option(
+    name: .customLong("configuration"),
+    help: """
+      The path to a JSON file containing the configuration of the linter/formatter or a JSON string containing the \
+      configuration directly.
+      """
+  )
+  var configuration: String?
+}
diff --git a/Sources/swift-format/Subcommands/DumpConfiguration.swift b/Sources/swift-format/Subcommands/DumpConfiguration.swift
index ff41c8554..66964e9c7 100644
--- a/Sources/swift-format/Subcommands/DumpConfiguration.swift
+++ b/Sources/swift-format/Subcommands/DumpConfiguration.swift
@@ -22,26 +22,9 @@ extension SwiftFormatCommand {
     )
 
     func run() throws {
-      let configuration = Configuration()
-      do {
-        let encoder = JSONEncoder()
-        encoder.outputFormatting = [.prettyPrinted]
-        if #available(macOS 10.13, *) {
-          encoder.outputFormatting.insert(.sortedKeys)
-        }
+      let configuration = try Configuration().asJsonString()
 
-        let data = try encoder.encode(configuration)
-        guard let jsonString = String(data: data, encoding: .utf8) else {
-          // This should never happen, but let's make sure we fail more gracefully than crashing, just
-          // in case.
-          throw FormatError(
-            message: "Could not dump the default configuration: the JSON was not valid UTF-8"
-          )
-        }
-        print(jsonString)
-      } catch {
-        throw FormatError(message: "Could not dump the default configuration: \(error)")
-      }
+      print(configuration)
     }
   }
 }
diff --git a/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift b/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift
new file mode 100644
index 000000000..277823ca0
--- /dev/null
+++ b/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift
@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import Foundation
+import SwiftFormat
+
+extension SwiftFormatCommand {
+  /// Dumps the tool's effective configuration in JSON format to standard output.
+  struct DumpEffectiveConfiguration: ParsableCommand {
+    static var configuration = CommandConfiguration(
+      abstract: "Dump the effective configuration in JSON format to standard output",
+      discussion: """
+        Dumps the configuration that would be used if swift-format was executed from the current working \
+        directory (cwd), incorporating configuration files found in the cwd or its parents, or input from the \
+        --configuration option.
+        """
+    )
+
+    @OptionGroup()
+    var configurationOptions: ConfigurationOptions
+
+    func run() throws {
+      // Pretend to use stdin, so that the configuration loading machinery in the Frontend base class can be used in the
+      // next step. This produces the same results as if "format" or "lint" subcommands were called.
+      let lintFormatOptions = try LintFormatOptions.parse(["-"])
+
+      let frontend = DumpEffectiveConfigurationFrontend(
+        configurationOptions: configurationOptions,
+        lintFormatOptions: lintFormatOptions
+      )
+      frontend.run()
+      if frontend.diagnosticsEngine.hasErrors {
+        throw ExitCode.failure
+      }
+
+      switch frontend.dumpResult {
+      case .success(let configuration):
+        print(configuration)
+      case .failure(let error):
+        throw error
+      }
+    }
+  }
+}
diff --git a/Sources/swift-format/Subcommands/Format.swift b/Sources/swift-format/Subcommands/Format.swift
index 42c2da165..59da36ffb 100644
--- a/Sources/swift-format/Subcommands/Format.swift
+++ b/Sources/swift-format/Subcommands/Format.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -29,6 +29,9 @@ extension SwiftFormatCommand {
     )
     var inPlace: Bool = false
 
+    @OptionGroup()
+    var configurationOptions: ConfigurationOptions
+
     @OptionGroup()
     var formatOptions: LintFormatOptions
 
@@ -43,7 +46,11 @@ extension SwiftFormatCommand {
 
     func run() throws {
       try performanceMeasurementOptions.printingInstructionCountIfRequested() {
-        let frontend = FormatFrontend(lintFormatOptions: formatOptions, inPlace: inPlace)
+        let frontend = FormatFrontend(
+          configurationOptions: configurationOptions,
+          lintFormatOptions: formatOptions,
+          inPlace: inPlace
+        )
         frontend.run()
         if frontend.diagnosticsEngine.hasErrors { throw ExitCode.failure }
       }
diff --git a/Sources/swift-format/Subcommands/Lint.swift b/Sources/swift-format/Subcommands/Lint.swift
index 3002c5912..cee4cee41 100644
--- a/Sources/swift-format/Subcommands/Lint.swift
+++ b/Sources/swift-format/Subcommands/Lint.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -20,6 +20,9 @@ extension SwiftFormatCommand {
       discussion: "When no files are specified, it expects the source from standard input."
     )
 
+    @OptionGroup()
+    var configurationOptions: ConfigurationOptions
+
     @OptionGroup()
     var lintOptions: LintFormatOptions
 
@@ -34,7 +37,7 @@ extension SwiftFormatCommand {
 
     func run() throws {
       try performanceMeasurementOptions.printingInstructionCountIfRequested {
-        let frontend = LintFrontend(lintFormatOptions: lintOptions)
+        let frontend = LintFrontend(configurationOptions: configurationOptions, lintFormatOptions: lintOptions)
         frontend.run()
 
         if frontend.diagnosticsEngine.hasErrors || strict && frontend.diagnosticsEngine.hasWarnings {
diff --git a/Sources/swift-format/Subcommands/LintFormatOptions.swift b/Sources/swift-format/Subcommands/LintFormatOptions.swift
index 737de42fc..ca1232912 100644
--- a/Sources/swift-format/Subcommands/LintFormatOptions.swift
+++ b/Sources/swift-format/Subcommands/LintFormatOptions.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -15,18 +15,6 @@ import Foundation
 
 /// Common arguments used by the `lint` and `format` subcommands.
 struct LintFormatOptions: ParsableArguments {
-  /// The path to the JSON configuration file that should be loaded.
-  ///
-  /// If not specified, the default configuration will be used.
-  @Option(
-    name: .customLong("configuration"),
-    help: """
-      The path to a JSON file containing the configuration of the linter/formatter or a JSON \
-      string containing the configuration directly.
-      """
-  )
-  var configuration: String?
-
   /// A list of comma-separated "start:end" pairs specifying UTF-8 offsets of the ranges to format.
   ///
   /// If not specified, the whole file will be formatted.
diff --git a/Sources/swift-format/SwiftFormatCommand.swift b/Sources/swift-format/SwiftFormatCommand.swift
index 5b814a159..cdcb77fa0 100644
--- a/Sources/swift-format/SwiftFormatCommand.swift
+++ b/Sources/swift-format/SwiftFormatCommand.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -21,6 +21,7 @@ struct SwiftFormatCommand: ParsableCommand {
     abstract: "Format or lint Swift source code",
     subcommands: [
       DumpConfiguration.self,
+      DumpEffectiveConfiguration.self,
       Format.self,
       Lint.self,
     ],
diff --git a/Sources/swift-format/Utilities/FormatError.swift b/Sources/swift-format/Utilities/FormatError.swift
deleted file mode 100644
index b922038ee..000000000
--- a/Sources/swift-format/Utilities/FormatError.swift
+++ /dev/null
@@ -1,23 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-import Foundation
-
-struct FormatError: LocalizedError {
-  var message: String
-  var errorDescription: String? { message }
-
-  static var exitWithDiagnosticErrors: FormatError {
-    // The diagnostics engine has already printed errors to stderr.
-    FormatError(message: "")
-  }
-}

From 8172c06f9391213b932ada7f428a6496cc8c92a7 Mon Sep 17 00:00:00 2001
From: Andre Richter <andre_richter@apple.com>
Date: Thu, 6 Feb 2025 19:59:41 +0100
Subject: [PATCH 2/4] Merge into single subcommand

---
 README.md                                     |  8 +--
 Sources/swift-format/CMakeLists.txt           |  3 +-
 ....swift => DumpConfigurationFrontend.swift} | 10 ++--
 .../Subcommands/ConfigurationOptions.swift    |  2 +-
 .../Subcommands/DumpConfiguration.swift       | 50 +++++++++++++++--
 .../DumpEffectiveConfiguration.swift          | 54 -------------------
 Sources/swift-format/SwiftFormatCommand.swift |  3 +-
 7 files changed, 57 insertions(+), 73 deletions(-)
 rename Sources/swift-format/Frontend/{DumpEffectiveConfigurationFrontend.swift => DumpConfigurationFrontend.swift} (71%)
 delete mode 100644 Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift

diff --git a/README.md b/README.md
index 94aace284..77342ff5c 100644
--- a/README.md
+++ b/README.md
@@ -219,10 +219,10 @@ description of the configuration format and the settings that are available.
 
 #### Viewing the Effective Configuration
 
-The `dump-effective-configuration` subcommand dumps the configuration that
-would be used if `swift-format` was executed from the current working directory,
-and accounts for `.swift-format` files or `--configuration` options as outlined
-above.
+The `dump-configuration` subcommand accepts a `--effective` flag. If set, it
+dumps the configuration that would be used if `swift-format` was executed from
+the current working directory, and accounts for `.swift-format` files or
+ `--configuration` options as outlined above.
 
 ### Miscellaneous
 
diff --git a/Sources/swift-format/CMakeLists.txt b/Sources/swift-format/CMakeLists.txt
index b563a4463..ef77a26b0 100644
--- a/Sources/swift-format/CMakeLists.txt
+++ b/Sources/swift-format/CMakeLists.txt
@@ -12,13 +12,12 @@ add_executable(swift-format
   SwiftFormatCommand.swift
   VersionOptions.swift
   Frontend/ConfigurationLoader.swift
-  Frontend/DumpEffectiveConfigurationFrontend.swift
+  Frontend/DumpConfigurationFrontend.swift
   Frontend/FormatFrontend.swift
   Frontend/Frontend.swift
   Frontend/LintFrontend.swift
   Subcommands/ConfigurationOptions.swift
   Subcommands/DumpConfiguration.swift
-  Subcommands/DumpEffectiveConfiguration.swift
   Subcommands/Format.swift
   Subcommands/Lint.swift
   Subcommands/LintFormatOptions.swift
diff --git a/Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift b/Sources/swift-format/Frontend/DumpConfigurationFrontend.swift
similarity index 71%
rename from Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift
rename to Sources/swift-format/Frontend/DumpConfigurationFrontend.swift
index 1608c9aa8..5bae2451f 100644
--- a/Sources/swift-format/Frontend/DumpEffectiveConfigurationFrontend.swift
+++ b/Sources/swift-format/Frontend/DumpConfigurationFrontend.swift
@@ -13,13 +13,13 @@
 import Foundation
 import SwiftFormat
 
-/// The frontend for dumping the effective configuration.
-class DumpEffectiveConfigurationFrontend: Frontend {
-  private(set) var dumpResult: Result<String, Error> = .failure(
-    SwiftFormatError.configurationDumpFailed("Configuration not resolved yet")
+/// The frontend for dumping the configuration.
+class DumpConfigurationFrontend: Frontend {
+  private(set) var dumpedConfiguration: Result<String, Error> = .failure(
+    SwiftFormatError.configurationDumpFailed("Configuration not dumped yet")
   )
 
   override func processFile(_ fileToProcess: FileToProcess) {
-    dumpResult = Result.init(catching: fileToProcess.configuration.asJsonString)
+    dumpedConfiguration = Result.init(catching: fileToProcess.configuration.asJsonString)
   }
 }
diff --git a/Sources/swift-format/Subcommands/ConfigurationOptions.swift b/Sources/swift-format/Subcommands/ConfigurationOptions.swift
index 9c018a111..1025715e9 100644
--- a/Sources/swift-format/Subcommands/ConfigurationOptions.swift
+++ b/Sources/swift-format/Subcommands/ConfigurationOptions.swift
@@ -12,7 +12,7 @@
 
 import ArgumentParser
 
-/// Common arguments used by the `lint`, `format` and `dump-effective-configuration` subcommands.
+/// Common arguments used by the `lint`, `format` and `dump-configuration` subcommands.
 struct ConfigurationOptions: ParsableArguments {
   /// The path to the JSON configuration file that should be loaded.
   ///
diff --git a/Sources/swift-format/Subcommands/DumpConfiguration.swift b/Sources/swift-format/Subcommands/DumpConfiguration.swift
index 66964e9c7..753d4879e 100644
--- a/Sources/swift-format/Subcommands/DumpConfiguration.swift
+++ b/Sources/swift-format/Subcommands/DumpConfiguration.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -15,16 +15,56 @@ import Foundation
 import SwiftFormat
 
 extension SwiftFormatCommand {
-  /// Dumps the tool's default configuration in JSON format to standard output.
+  /// Dumps the tool's configuration in JSON format to standard output.
   struct DumpConfiguration: ParsableCommand {
     static var configuration = CommandConfiguration(
-      abstract: "Dump the default configuration in JSON format to standard output"
+      abstract: "Dump the configuration in JSON format to standard output",
+      discussion: """
+        Without any options, dumps the default configuration. When '--effective' is set, dumps the configuration that \
+        would be used if swift-format was executed from the current working directory (cwd), incorporating \
+        configuration files found in the cwd or its parents, or input from the '--configuration' option.
+        """
     )
 
+    /// Whether or not to dump the effective configuration.
+    @Flag(name: .shortAndLong, help: "Dump the effective instead of the default configuration.")
+    var effective: Bool = false
+
+    @OptionGroup()
+    var configurationOptions: ConfigurationOptions
+
+    func validate() throws {
+      if configurationOptions.configuration != nil && !effective {
+        throw ValidationError("'--configuration' is only valid in combination with '--effective'")
+      }
+    }
+
     func run() throws {
-      let configuration = try Configuration().asJsonString()
+      if !effective {
+        let configuration = try Configuration().asJsonString()
+        print(configuration)
+        return
+      }
+
+      // Pretend to use stdin, so that the configuration loading machinery in the Frontend base class can be used in the
+      // next step. This produces the same results as if "format" or "lint" subcommands were called.
+      let lintFormatOptions = try LintFormatOptions.parse(["-"])
+
+      let frontend = DumpConfigurationFrontend(
+        configurationOptions: configurationOptions,
+        lintFormatOptions: lintFormatOptions
+      )
+      frontend.run()
+      if frontend.diagnosticsEngine.hasErrors {
+        throw ExitCode.failure
+      }
 
-      print(configuration)
+      switch frontend.dumpedConfiguration {
+      case .success(let configuration):
+        print(configuration)
+      case .failure(let error):
+        throw error
+      }
     }
   }
 }
diff --git a/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift b/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift
deleted file mode 100644
index 277823ca0..000000000
--- a/Sources/swift-format/Subcommands/DumpEffectiveConfiguration.swift
+++ /dev/null
@@ -1,54 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2025 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-import ArgumentParser
-import Foundation
-import SwiftFormat
-
-extension SwiftFormatCommand {
-  /// Dumps the tool's effective configuration in JSON format to standard output.
-  struct DumpEffectiveConfiguration: ParsableCommand {
-    static var configuration = CommandConfiguration(
-      abstract: "Dump the effective configuration in JSON format to standard output",
-      discussion: """
-        Dumps the configuration that would be used if swift-format was executed from the current working \
-        directory (cwd), incorporating configuration files found in the cwd or its parents, or input from the \
-        --configuration option.
-        """
-    )
-
-    @OptionGroup()
-    var configurationOptions: ConfigurationOptions
-
-    func run() throws {
-      // Pretend to use stdin, so that the configuration loading machinery in the Frontend base class can be used in the
-      // next step. This produces the same results as if "format" or "lint" subcommands were called.
-      let lintFormatOptions = try LintFormatOptions.parse(["-"])
-
-      let frontend = DumpEffectiveConfigurationFrontend(
-        configurationOptions: configurationOptions,
-        lintFormatOptions: lintFormatOptions
-      )
-      frontend.run()
-      if frontend.diagnosticsEngine.hasErrors {
-        throw ExitCode.failure
-      }
-
-      switch frontend.dumpResult {
-      case .success(let configuration):
-        print(configuration)
-      case .failure(let error):
-        throw error
-      }
-    }
-  }
-}
diff --git a/Sources/swift-format/SwiftFormatCommand.swift b/Sources/swift-format/SwiftFormatCommand.swift
index cdcb77fa0..5b814a159 100644
--- a/Sources/swift-format/SwiftFormatCommand.swift
+++ b/Sources/swift-format/SwiftFormatCommand.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -21,7 +21,6 @@ struct SwiftFormatCommand: ParsableCommand {
     abstract: "Format or lint Swift source code",
     subcommands: [
       DumpConfiguration.self,
-      DumpEffectiveConfiguration.self,
       Format.self,
       Lint.self,
     ],

From 1e8c05ecb62ea0ddf198d7ff344dd908dbd0480d Mon Sep 17 00:00:00 2001
From: Andre Richter <andre_richter@apple.com>
Date: Tue, 11 Feb 2025 11:28:54 +0100
Subject: [PATCH 3/4] Do not use dummy frontend for effective config printing

---
 .../SwiftFormat/API/Configuration+Dump.swift  |   6 +-
 Sources/swift-format/CMakeLists.txt           |   1 -
 .../Frontend/DumpConfigurationFrontend.swift  |  25 --
 Sources/swift-format/Frontend/Frontend.swift  | 226 ++++++++++--------
 .../Subcommands/DumpConfiguration.swift       |  44 ++--
 5 files changed, 147 insertions(+), 155 deletions(-)
 delete mode 100644 Sources/swift-format/Frontend/DumpConfigurationFrontend.swift

diff --git a/Sources/SwiftFormat/API/Configuration+Dump.swift b/Sources/SwiftFormat/API/Configuration+Dump.swift
index a9a68fb7e..cd063b664 100644
--- a/Sources/SwiftFormat/API/Configuration+Dump.swift
+++ b/Sources/SwiftFormat/API/Configuration+Dump.swift
@@ -19,11 +19,7 @@ extension Configuration {
 
     do {
       let encoder = JSONEncoder()
-      encoder.outputFormatting = [.prettyPrinted]
-      if #available(macOS 10.13, *) {
-        encoder.outputFormatting.insert(.sortedKeys)
-      }
-
+      encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
       data = try encoder.encode(self)
     } catch {
       throw SwiftFormatError.configurationDumpFailed("\(error)")
diff --git a/Sources/swift-format/CMakeLists.txt b/Sources/swift-format/CMakeLists.txt
index ef77a26b0..09590be42 100644
--- a/Sources/swift-format/CMakeLists.txt
+++ b/Sources/swift-format/CMakeLists.txt
@@ -12,7 +12,6 @@ add_executable(swift-format
   SwiftFormatCommand.swift
   VersionOptions.swift
   Frontend/ConfigurationLoader.swift
-  Frontend/DumpConfigurationFrontend.swift
   Frontend/FormatFrontend.swift
   Frontend/Frontend.swift
   Frontend/LintFrontend.swift
diff --git a/Sources/swift-format/Frontend/DumpConfigurationFrontend.swift b/Sources/swift-format/Frontend/DumpConfigurationFrontend.swift
deleted file mode 100644
index 5bae2451f..000000000
--- a/Sources/swift-format/Frontend/DumpConfigurationFrontend.swift
+++ /dev/null
@@ -1,25 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2025 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-import Foundation
-import SwiftFormat
-
-/// The frontend for dumping the configuration.
-class DumpConfigurationFrontend: Frontend {
-  private(set) var dumpedConfiguration: Result<String, Error> = .failure(
-    SwiftFormatError.configurationDumpFailed("Configuration not dumped yet")
-  )
-
-  override func processFile(_ fileToProcess: FileToProcess) {
-    dumpedConfiguration = Result.init(catching: fileToProcess.configuration.asJsonString)
-  }
-}
diff --git a/Sources/swift-format/Frontend/Frontend.swift b/Sources/swift-format/Frontend/Frontend.swift
index d7bb5cbe5..6e24b7965 100644
--- a/Sources/swift-format/Frontend/Frontend.swift
+++ b/Sources/swift-format/Frontend/Frontend.swift
@@ -16,6 +16,118 @@ import SwiftParser
 import SwiftSyntax
 
 class Frontend {
+  /// Provides formatter configurations for given `.swift` source files, configuration files or configuration strings.
+  struct ConfigurationProvider {
+    /// Loads formatter configuration files and chaches them in memory.
+    private var configurationLoader: ConfigurationLoader = ConfigurationLoader()
+
+    /// The diagnostic engine to which warnings and errors will be emitted.
+    private let diagnosticsEngine: DiagnosticsEngine
+
+    /// Creates a new instance with the given options.
+    ///
+    /// - Parameter diagnosticsEngine: The diagnostic engine to which warnings and errors will be emitted.
+    init(diagnosticsEngine: DiagnosticsEngine) {
+      self.diagnosticsEngine = diagnosticsEngine
+    }
+
+    /// Checks if all the rules in the given configuration are supported by the registry.
+    ///
+    /// If there are any rules that are not supported, they are emitted as a warning.
+    private func checkForUnrecognizedRules(in configuration: Configuration) {
+      // If any rules in the decoded configuration are not supported by the registry,
+      // emit them into the diagnosticsEngine as warnings.
+      // That way they will be printed out, but we'll continue execution on the valid rules.
+      let invalidRules = configuration.rules.filter { !RuleRegistry.rules.keys.contains($0.key) }
+      for rule in invalidRules {
+        diagnosticsEngine.emitWarning("Configuration contains an unrecognized rule: \(rule.key)", location: nil)
+      }
+    }
+
+    /// Returns the configuration that applies to the given `.swift` source file, when an explicit
+    /// configuration path is also perhaps provided.
+    ///
+    /// This method also checks for unrecognized rules within the configuration.
+    ///
+    /// - Parameters:
+    ///   - pathOrString: A string containing either the path to a configuration file that will be
+    ///     loaded, JSON configuration data directly, or `nil` to try to infer it from
+    ///     `swiftFileURL`.
+    ///   - swiftFileURL: The path to a `.swift` file, which will be used to infer the path to the
+    ///     configuration file if `configurationFilePath` is nil.
+    ///
+    /// - Returns: If successful, the returned configuration is the one loaded from `pathOrString` if
+    ///   it was provided, or by searching in paths inferred by `swiftFileURL` if one exists, or the
+    ///   default configuration otherwise. If an error occurred when reading the configuration, a
+    ///   diagnostic is emitted and `nil` is returned. If neither `pathOrString` nor `swiftFileURL`
+    ///   were provided, a default `Configuration()` will be returned.
+    mutating func provide(
+      forConfigPathOrString pathOrString: String?,
+      orForSwiftFileAt swiftFileURL: URL?
+    ) -> Configuration? {
+      if let pathOrString = pathOrString {
+        // If an explicit configuration file path was given, try to load it and fail if it cannot be
+        // loaded. (Do not try to fall back to a path inferred from the source file path.)
+        let configurationFileURL = URL(fileURLWithPath: pathOrString)
+        do {
+          let configuration = try configurationLoader.configuration(at: configurationFileURL)
+          self.checkForUnrecognizedRules(in: configuration)
+          return configuration
+        } catch {
+          // If we failed to load this from the path, try interpreting the string as configuration
+          // data itself because the user might have written something like `--configuration '{...}'`,
+          let data = pathOrString.data(using: .utf8)!
+          if let configuration = try? Configuration(data: data) {
+            return configuration
+          }
+
+          // Fail if the configuration flag was neither a valid file path nor valid configuration
+          // data.
+          diagnosticsEngine.emitError("Unable to read configuration: \(error.localizedDescription)")
+          return nil
+        }
+      }
+
+      // If no explicit configuration file path was given but a `.swift` source file path was given,
+      // 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(forPath: swiftFileURL) {
+            self.checkForUnrecognizedRules(in: configuration)
+            return configuration
+          }
+          // Fall through to the default return at the end of the function.
+        } catch {
+          diagnosticsEngine.emitError(
+            "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
+        }
+      }
+
+      // An explicit configuration has not been given, and one cannot be found.
+      // Return the default configuration.
+      return Configuration()
+    }
+  }
+
   /// Represents a file to be processed by the frontend and any file-specific options associated
   /// with it.
   final class FileToProcess {
@@ -73,8 +185,8 @@ class Frontend {
   /// Options that apply during formatting or linting.
   final let lintFormatOptions: LintFormatOptions
 
-  /// Loads formatter configuration files.
-  final var configurationLoader = ConfigurationLoader()
+  /// The provider for formatter configurations.
+  final var configurationProvider: ConfigurationProvider
 
   /// Advanced options that are useful for developing/debugging but otherwise not meant for general
   /// use.
@@ -95,8 +207,8 @@ class Frontend {
     self.diagnosticPrinter = StderrDiagnosticPrinter(
       colorMode: lintFormatOptions.colorDiagnostics.map { $0 ? .on : .off } ?? .auto
     )
-    self.diagnosticsEngine =
-      DiagnosticsEngine(diagnosticsHandlers: [diagnosticPrinter.printDiagnostic])
+    self.diagnosticsEngine = DiagnosticsEngine(diagnosticsHandlers: [diagnosticPrinter.printDiagnostic])
+    self.configurationProvider = ConfigurationProvider(diagnosticsEngine: self.diagnosticsEngine)
   }
 
   /// Runs the linter or formatter over the inputs.
@@ -142,9 +254,9 @@ class Frontend {
     let assumedUrl = lintFormatOptions.assumeFilename.map(URL.init(fileURLWithPath:))
 
     guard
-      let configuration = configuration(
-        fromPathOrString: configurationOptions.configuration,
-        orInferredFromSwiftFileAt: assumedUrl
+      let configuration = configurationProvider.provide(
+        forConfigPathOrString: configurationOptions.configuration,
+        orForSwiftFileAt: assumedUrl
       )
     else {
       // Already diagnosed in the called method.
@@ -193,9 +305,9 @@ class Frontend {
     }
 
     guard
-      let configuration = configuration(
-        fromPathOrString: configurationOptions.configuration,
-        orInferredFromSwiftFileAt: url
+      let configuration = configurationProvider.provide(
+        forConfigPathOrString: configurationOptions.configuration,
+        orForSwiftFileAt: url
       )
     else {
       // Already diagnosed in the called method.
@@ -210,98 +322,4 @@ class Frontend {
     )
   }
 
-  /// Returns the configuration that applies to the given `.swift` source file, when an explicit
-  /// configuration path is also perhaps provided.
-  ///
-  /// This method also checks for unrecognized rules within the configuration.
-  ///
-  /// - Parameters:
-  ///   - pathOrString: A string containing either the path to a configuration file that will be
-  ///     loaded, JSON configuration data directly, or `nil` to try to infer it from
-  ///     `swiftFilePath`.
-  ///   - swiftFilePath: The path to a `.swift` file, which will be used to infer the path to the
-  ///     configuration file if `configurationFilePath` is nil.
-  ///
-  /// - Returns: If successful, the returned configuration is the one loaded from `pathOrString` if
-  ///   it was provided, or by searching in paths inferred by `swiftFilePath` if one exists, or the
-  ///   default configuration otherwise. If an error occurred when reading the configuration, a
-  ///   diagnostic is emitted and `nil` is returned. If neither `pathOrString` nor `swiftFilePath`
-  ///   were provided, a default `Configuration()` will be returned.
-  private func configuration(
-    fromPathOrString pathOrString: String?,
-    orInferredFromSwiftFileAt swiftFileURL: URL?
-  ) -> Configuration? {
-    if let pathOrString = pathOrString {
-      // If an explicit configuration file path was given, try to load it and fail if it cannot be
-      // loaded. (Do not try to fall back to a path inferred from the source file path.)
-      let configurationFileURL = URL(fileURLWithPath: pathOrString)
-      do {
-        let configuration = try configurationLoader.configuration(at: configurationFileURL)
-        self.checkForUnrecognizedRules(in: configuration)
-        return configuration
-      } catch {
-        // If we failed to load this from the path, try interpreting the string as configuration
-        // data itself because the user might have written something like `--configuration '{...}'`,
-        let data = pathOrString.data(using: .utf8)!
-        if let configuration = try? Configuration(data: data) {
-          return configuration
-        }
-
-        // Fail if the configuration flag was neither a valid file path nor valid configuration
-        // data.
-        diagnosticsEngine.emitError("Unable to read configuration: \(error.localizedDescription)")
-        return nil
-      }
-    }
-
-    // If no explicit configuration file path was given but a `.swift` source file path was given,
-    // 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(forPath: swiftFileURL) {
-          self.checkForUnrecognizedRules(in: configuration)
-          return configuration
-        }
-        // Fall through to the default return at the end of the function.
-      } catch {
-        diagnosticsEngine.emitError(
-          "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
-      }
-    }
-
-    // An explicit configuration has not been given, and one cannot be found.
-    // Return the default configuration.
-    return Configuration()
-  }
-
-  /// Checks if all the rules in the given configuration are supported by the registry.
-  /// If there are any rules that are not supported, they are emitted as a warning.
-  private func checkForUnrecognizedRules(in configuration: Configuration) {
-    // If any rules in the decoded configuration are not supported by the registry,
-    // emit them into the diagnosticsEngine as warnings.
-    // That way they will be printed out, but we'll continue execution on the valid rules.
-    let invalidRules = configuration.rules.filter { !RuleRegistry.rules.keys.contains($0.key) }
-    for rule in invalidRules {
-      diagnosticsEngine.emitWarning("Configuration contains an unrecognized rule: \(rule.key)", location: nil)
-    }
-  }
 }
diff --git a/Sources/swift-format/Subcommands/DumpConfiguration.swift b/Sources/swift-format/Subcommands/DumpConfiguration.swift
index 753d4879e..0014151a4 100644
--- a/Sources/swift-format/Subcommands/DumpConfiguration.swift
+++ b/Sources/swift-format/Subcommands/DumpConfiguration.swift
@@ -40,30 +40,34 @@ extension SwiftFormatCommand {
     }
 
     func run() throws {
-      if !effective {
-        let configuration = try Configuration().asJsonString()
-        print(configuration)
-        return
-      }
+      let diagnosticPrinter = StderrDiagnosticPrinter(colorMode: .auto)
+      let diagnosticsEngine = DiagnosticsEngine(diagnosticsHandlers: [diagnosticPrinter.printDiagnostic])
 
-      // Pretend to use stdin, so that the configuration loading machinery in the Frontend base class can be used in the
-      // next step. This produces the same results as if "format" or "lint" subcommands were called.
-      let lintFormatOptions = try LintFormatOptions.parse(["-"])
+      let configuration: Configuration
+      if effective {
+        var configurationProvider = Frontend.ConfigurationProvider(diagnosticsEngine: diagnosticsEngine)
 
-      let frontend = DumpConfigurationFrontend(
-        configurationOptions: configurationOptions,
-        lintFormatOptions: lintFormatOptions
-      )
-      frontend.run()
-      if frontend.diagnosticsEngine.hasErrors {
-        throw ExitCode.failure
+        guard
+          let effectiveConfiguration = configurationProvider.provide(
+            forConfigPathOrString: configurationOptions.configuration,
+            orForSwiftFileAt: nil
+          )
+        else {
+          // Already diagnosed in the called method through the diagnosticsEngine.
+          throw ExitCode.failure
+        }
+
+        configuration = effectiveConfiguration
+      } else {
+        configuration = Configuration()
       }
 
-      switch frontend.dumpedConfiguration {
-      case .success(let configuration):
-        print(configuration)
-      case .failure(let error):
-        throw error
+      do {
+        let configurationAsJson = try configuration.asJsonString()
+        print(configurationAsJson)
+      } catch {
+        diagnosticsEngine.emitError("\(error.localizedDescription)")
+        throw ExitCode.failure
       }
     }
   }

From 11d6de85e6eb3aefb597dddf62008466937a5e85 Mon Sep 17 00:00:00 2001
From: Andre Richter <andre_richter@apple.com>
Date: Fri, 14 Feb 2025 17:34:28 +0100
Subject: [PATCH 4/4] Incorporate review comments; Allow API breakage

---
 Sources/swift-format/Subcommands/DumpConfiguration.swift | 3 +--
 api-breakages.txt                                        | 1 +
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Sources/swift-format/Subcommands/DumpConfiguration.swift b/Sources/swift-format/Subcommands/DumpConfiguration.swift
index 0014151a4..3104ed588 100644
--- a/Sources/swift-format/Subcommands/DumpConfiguration.swift
+++ b/Sources/swift-format/Subcommands/DumpConfiguration.swift
@@ -63,8 +63,7 @@ extension SwiftFormatCommand {
       }
 
       do {
-        let configurationAsJson = try configuration.asJsonString()
-        print(configurationAsJson)
+        print(try configuration.asJsonString())
       } catch {
         diagnosticsEngine.emitError("\(error.localizedDescription)")
         throw ExitCode.failure
diff --git a/api-breakages.txt b/api-breakages.txt
index 987ca864c..07f9a3bc2 100644
--- a/api-breakages.txt
+++ b/api-breakages.txt
@@ -2,3 +2,4 @@
 ---
 
 API breakage: constructor FileIterator.init(urls:followSymlinks:) has been removed
+API breakage: enumelement SwiftFormatError.configurationDumpFailed has been added as a new enum case