Refactored file generation logic to be string-based and added validation tests

Package.swift

@@ -58,6 +58,10 @@ var targets: [Target] = [
         "SwiftOperators", "SwiftParser", "SwiftParserDiagnostics", "SwiftSyntax", "SwiftSyntaxBuilder",
       ])
   ),
+  .target(
+    name: "_GenerateSwiftFormat",
+    dependencies: ["SwiftFormat"]
+  ),
   .plugin(
     name: "Format Source Code",
     capability: .command(
@@ -86,9 +90,7 @@ var targets: [Target] = [
   ),
   .executableTarget(
     name: "generate-swift-format",
-    dependencies: [
-      "SwiftFormat"
-    ]
+    dependencies: ["_GenerateSwiftFormat"]
   ),
   .executableTarget(
     name: "swift-format",
@@ -113,6 +115,7 @@ var targets: [Target] = [
     dependencies: [
       "SwiftFormat",
       "_SwiftFormatTestSupport",
+      "_GenerateSwiftFormat",
       .product(name: "Markdown", package: "swift-markdown"),
     ] + swiftSyntaxDependencies(["SwiftOperators", "SwiftParser", "SwiftSyntax", "SwiftSyntaxBuilder"])
   ),

Sources/SwiftFormat/Core/Pipelines+Generated.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2019 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

Sources/SwiftFormat/Core/RuleNameCache+Generated.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2019 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

Sources/SwiftFormat/Core/RuleRegistry+Generated.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2019 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

Sources/generate-swift-format/FileGenerator.swift → Sources/_GenerateSwiftFormat/FileGenerator.swift

@@ -13,10 +13,9 @@
 import Foundation
 
 /// Common behavior used to generate source files.
-protocol FileGenerator {
-  /// Types conforming to this protocol must implement this method to write their content into the
-  /// given file handle.
-  func write(into handle: FileHandle) throws
+@_spi(Internal) public protocol FileGenerator {
+  /// Generates the file content as a String.
+  func generateContent() -> String
 }
 
 private struct FailedToCreateFileError: Error {
@@ -25,26 +24,12 @@ private struct FailedToCreateFileError: Error {
 
 extension FileGenerator {
   /// Generates a file at the given URL, overwriting it if it already exists.
-  func generateFile(at url: URL) throws {
+  public func generateFile(at url: URL) throws {
     let fm = FileManager.default
     if fm.fileExists(atPath: url.path) {
       try fm.removeItem(at: url)
     }
-
-    if !fm.createFile(atPath: url.path, contents: nil, attributes: nil) {
-      throw FailedToCreateFileError(url: url)
-    }
-    let handle = try FileHandle(forWritingTo: url)
-    defer { handle.closeFile() }
-
-    try write(into: handle)
-  }
-}
-
-extension FileHandle {
-  /// Writes the provided string as data to a file output stream.
-  public func write(_ string: String) {
-    guard let data = string.data(using: .utf8) else { return }
-    write(data)
+    let content = generateContent()
+    try content.write(to: url, atomically: true, encoding: .utf8)
   }
 }

Sources/_GenerateSwiftFormat/GenerateSwiftFormatPaths.swift

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+/// A namespace that provides paths for files generated by the `generate-swift-format` tool.
+@_spi(Internal) public enum GenerateSwiftFormatPaths {
+  private static let sourcesDirectory =
+    URL(fileURLWithPath: #file)
+    .deletingLastPathComponent()
+    .deletingLastPathComponent()
+  public static let rulesDirectory =
+    sourcesDirectory
+    .appendingPathComponent("SwiftFormat")
+    .appendingPathComponent("Rules")
+  public static let pipelineFile =
+    sourcesDirectory
+    .appendingPathComponent("SwiftFormat")
+    .appendingPathComponent("Core")
+    .appendingPathComponent("Pipelines+Generated.swift")
+  public static let ruleRegistryFile =
+    sourcesDirectory
+    .appendingPathComponent("SwiftFormat")
+    .appendingPathComponent("Core")
+    .appendingPathComponent("RuleRegistry+Generated.swift")
+  public static let ruleNameCacheFile =
+    sourcesDirectory
+    .appendingPathComponent("SwiftFormat")
+    .appendingPathComponent("Core")
+    .appendingPathComponent("RuleNameCache+Generated.swift")
+  public static let ruleDocumentationFile =
+    sourcesDirectory
+    .appendingPathComponent("..")
+    .appendingPathComponent("Documentation")
+    .appendingPathComponent("RuleDocumentation.md")
+}

Sources/generate-swift-format/PipelineGenerator.swift → Sources/_GenerateSwiftFormat/PipelineGenerator.swift

@@ -13,24 +13,24 @@
 import Foundation
 
 /// Generates the extensions to the lint and format pipelines.
-final class PipelineGenerator: FileGenerator {
+@_spi(Internal) public final class PipelineGenerator: FileGenerator {
 
   /// The rules collected by scanning the formatter source code.
   let ruleCollector: RuleCollector
 
   /// Creates a new pipeline generator.
-  init(ruleCollector: RuleCollector) {
+  public init(ruleCollector: RuleCollector) {
     self.ruleCollector = ruleCollector
   }
 
-  func write(into handle: FileHandle) throws {
-    handle.write(
-      """
+  public func generateContent() -> String {
+    var result = ""
+    result += """
       //===----------------------------------------------------------------------===//
       //
       // This source file is part of the Swift.org open source project
       //
-      // Copyright (c) 2014 - 2019 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
@@ -65,58 +65,41 @@ final class PipelineGenerator: FileGenerator {
         }
 
       """
-    )
 
     for (nodeType, lintRules) in ruleCollector.syntaxNodeLinters.sorted(by: { $0.key < $1.key }) {
-      handle.write(
-        """
+      result += """
 
           override func visit(_ node: \(nodeType)) -> SyntaxVisitorContinueKind {
 
         """
-      )
-
       for ruleName in lintRules.sorted() {
-        handle.write(
-          """
+        result += """
               visitIfEnabled(\(ruleName).visit, for: node)
 
           """
-        )
       }
-
-      handle.write(
-        """
+      result += """
             return .visitChildren
           }
 
         """
-      )
-
-      handle.write(
-        """
+      result += """
           override func visitPost(_ node: \(nodeType)) {
 
         """
-      )
       for ruleName in lintRules.sorted() {
-        handle.write(
-          """
+        result += """
               onVisitPost(rule: \(ruleName).self, for: node)
 
           """
-        )
       }
-      handle.write(
-        """
+      result += """
           }
 
         """
-      )
     }
 
-    handle.write(
-      """
+    result += """
       }
 
       extension FormatPipeline {
@@ -125,24 +108,18 @@ final class PipelineGenerator: FileGenerator {
           var node = node
 
       """
-    )
-
     for ruleName in ruleCollector.allFormatters.map({ $0.typeName }).sorted() {
-      handle.write(
-        """
+      result += """
             node = \(ruleName)(context: context).rewrite(node)
 
         """
-      )
     }
-
-    handle.write(
-      """
+    result += """
           return node
         }
       }
 
       """
-    )
+    return result
   }
 }

Sources/generate-swift-format/RuleCollector.swift → Sources/_GenerateSwiftFormat/RuleCollector.swift

@@ -16,7 +16,7 @@ import SwiftParser
 import SwiftSyntax
 
 /// Collects information about rules in the formatter code base.
-final class RuleCollector {
+@_spi(Internal) public final class RuleCollector {
   /// Information about a detected rule.
   struct DetectedRule: Hashable {
     /// The type name of the rule.
@@ -45,10 +45,12 @@ final class RuleCollector {
   /// A dictionary mapping syntax node types to the lint/format rules that visit them.
   var syntaxNodeLinters = [String: [String]]()
 
+  public init() {}
+
   /// Populates the internal collections with rules in the given directory.
   ///
   /// - Parameter url: The file system URL that should be scanned for rules.
-  func collect(from url: URL) throws {
+  public func collect(from url: URL) throws {
     // For each file in the Rules directory, find types that either conform to SyntaxLintRule or
     // inherit from SyntaxFormatRule.
     let fm = FileManager.default

Sources/generate-swift-format/RuleDocumentationGenerator.swift → Sources/_GenerateSwiftFormat/RuleDocumentationGenerator.swift

@@ -14,19 +14,19 @@ import Foundation
 import SwiftFormat
 
 /// Generates the markdown file with extended documenation on the available rules.
-final class RuleDocumentationGenerator: FileGenerator {
+@_spi(Internal) public final class RuleDocumentationGenerator: FileGenerator {
 
   /// The rules collected by scanning the formatter source code.
   let ruleCollector: RuleCollector
 
   /// Creates a new rule registry generator.
-  init(ruleCollector: RuleCollector) {
+  public init(ruleCollector: RuleCollector) {
     self.ruleCollector = ruleCollector
   }
 
-  func write(into handle: FileHandle) throws {
-    handle.write(
-      """
+  public func generateContent() -> String {
+    var result = ""
+    result += """
       <!-- This file is automatically generated with generate-swift-format. Do not edit! -->
 
       # `swift-format` Lint and Format Rules
@@ -41,29 +41,20 @@ final class RuleDocumentationGenerator: FileGenerator {
 
 
       """
-    )
-
     for detectedRule in ruleCollector.allLinters.sorted(by: { $0.typeName < $1.typeName }) {
-      handle.write(
-        """
-        - [\(detectedRule.typeName)](#\(detectedRule.typeName))
-
-        """
-      )
+      result += "- [\(detectedRule.typeName)](#\(detectedRule.typeName))\n"
     }
-
     for detectedRule in ruleCollector.allLinters.sorted(by: { $0.typeName < $1.typeName }) {
-      handle.write(
-        """
+      result += """
 
         ### \(detectedRule.typeName)
 
         \(detectedRule.description ?? "")
         \(ruleFormatSupportDescription(for: detectedRule))
 
         """
-      )
     }
+    return result
   }
 
   private func ruleFormatSupportDescription(for rule: RuleCollector.DetectedRule) -> String {

Sources/generate-swift-format/RuleNameCacheGenerator.swift → Sources/_GenerateSwiftFormat/RuleNameCacheGenerator.swift

@@ -13,24 +13,24 @@
 import Foundation
 
 /// Generates the rule registry file used to populate the default configuration.
-final class RuleNameCacheGenerator: FileGenerator {
+@_spi(Internal) public final class RuleNameCacheGenerator: FileGenerator {
 
   /// The rules collected by scanning the formatter source code.
   let ruleCollector: RuleCollector
 
   /// Creates a new rule registry generator.
-  init(ruleCollector: RuleCollector) {
+  public init(ruleCollector: RuleCollector) {
     self.ruleCollector = ruleCollector
   }
 
-  func write(into handle: FileHandle) throws {
-    handle.write(
-      """
+  public func generateContent() -> String {
+    var result = ""
+    result += """
       //===----------------------------------------------------------------------===//
       //
       // This source file is part of the Swift.org open source project
       //
-      // Copyright (c) 2014 - 2019 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
@@ -45,11 +45,10 @@ final class RuleNameCacheGenerator: FileGenerator {
       public let ruleNameCache: [ObjectIdentifier: String] = [
 
       """
-    )
-
     for detectedRule in ruleCollector.allLinters.sorted(by: { $0.typeName < $1.typeName }) {
-      handle.write("  ObjectIdentifier(\(detectedRule.typeName).self): \"\(detectedRule.typeName)\",\n")
+      result += "  ObjectIdentifier(\(detectedRule.typeName).self): \"\(detectedRule.typeName)\",\n"
     }
-    handle.write("]\n")
+    result += "]\n"
+    return result
   }
 }