Add the ability to set feature flags

Sources/_OpenAPIGeneratorCore/Config.swift

@@ -18,8 +18,6 @@
 /// A single generator pipeline run produces exactly one file, so for
 /// generating multiple files, create multiple configuration values, each with
 /// a different generator mode.
-///
-/// TODO: Rename to something more specific, like "GeneratorConfig"
 public struct Config {
 
     /// The generator mode to use.
@@ -28,14 +26,22 @@ public struct Config {
     /// Additional imports to add to each generated file.
     public var additionalImports: [String]
 
+    /// Additional pre-release features to enable.
+    public var featureFlags: FeatureFlags
+
     /// Creates a configuration with the specified generator mode and imports.
     /// - Parameters:
     ///   - mode: The mode to use for generation.
-    ///   - additionalImports:  Additional imports to add to each generated
-    ///   file.
-    public init(mode: GeneratorMode, additionalImports: [String] = []) {
+    ///   - additionalImports: Additional imports to add to each generated file.
+    ///   - featureFlags: Additional pre-release features to enable.
+    public init(
+        mode: GeneratorMode,
+        additionalImports: [String] = [],
+        featureFlags: FeatureFlags = []
+    ) {
         self.mode = mode
         self.additionalImports = additionalImports
+        self.featureFlags = featureFlags
     }
 }
 

Sources/_OpenAPIGeneratorCore/FeatureFlags.swift

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// A feature that can be explicitly enabled before being released.
+///
+/// Commonly used to get early feedback on breaking changes, before
+/// they are enabled by default, which can only be done in a major version.
+///
+/// Once a feature is enabled unconditionally in the next major version,
+/// the corresponding feature flag should be removed at the same time.
+///
+/// For example: a breaking feature is being built while version 0.1 is out,
+/// and is hidden behind a feature flag. Once ready, the feature is
+/// enabled unconditionally on main and the feature flag removed, and version
+/// 0.2 is tagged. (This is for pre-1.0 versioning, would be 1.0 and 2.0 after
+/// 1.0 is released.)
+public enum FeatureFlag: String, Hashable, Equatable, Codable, CaseIterable {
+
+    /// Multiple request and response body content types.
+    ///
+    /// Tracking issues:
+    /// - https://github.com/apple/swift-openapi-generator/issues/6
+    /// - https://github.com/apple/swift-openapi-generator/issues/7
+    case multipleContentTypes
+}
+
+/// A set of enabled feature flags.
+public typealias FeatureFlags = Set<FeatureFlag>

Sources/swift-openapi-generator/Extensions.swift

@@ -23,6 +23,7 @@ extension URL: ExpressibleByArgument {
 }
 
 extension GeneratorMode: ExpressibleByArgument {}
+extension FeatureFlag: ExpressibleByArgument {}
 
 extension CaseIterable where Self: RawRepresentable, Self.RawValue == String {
 

Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift

@@ -33,10 +33,12 @@ extension _GenerateOptions {
         let config = try loadedConfig()
         let sortedModes = try resolvedModes(config)
         let resolvedAdditionalImports = resolvedAdditionalImports(config)
+        let resolvedFeatureFlags = resolvedFeatureFlags(config)
         let configs: [Config] = sortedModes.map {
             .init(
                 mode: $0,
-                additionalImports: resolvedAdditionalImports
+                additionalImports: resolvedAdditionalImports,
+                featureFlags: resolvedFeatureFlags
             )
         }
         let diagnostics: any DiagnosticCollector
@@ -57,6 +59,7 @@ extension _GenerateOptions {
             - OpenAPI document path: \(doc.path)
             - Configuration path: \(self.config?.path ?? "<none>")
             - Generator modes: \(sortedModes.map(\.rawValue).joined(separator: ", "))
+            - Feature flags: \(resolvedFeatureFlags.isEmpty ? "<none>" : resolvedFeatureFlags.map(\.rawValue).joined(separator: ", "))
             - Output file names: \(sortedModes.map(\.outputFileName).joined(separator: ", "))
             - Output directory: \(outputDirectory.path)
             - Diagnostics output path: \(diagnosticsOutputPath?.path ?? "<none - logs to stderr>")

Sources/swift-openapi-generator/GenerateOptions.swift

@@ -31,9 +31,12 @@ struct _GenerateOptions: ParsableArguments {
     )
     var mode: [GeneratorMode] = []
 
-    @Option(help: "Additional imports to add to all generated files.")
+    @Option(help: "Additional import to add to all generated files.")
     var additionalImport: [String] = []
 
+    @Option(help: "Pre-release feature to enable. Options: \(FeatureFlag.prettyListing).")
+    var featureFlag: [FeatureFlag] = []
+
     @Option(
         help: "When specified, writes out the diagnostics into a YAML file instead of emitting them to standard error."
     )
@@ -77,6 +80,15 @@ extension _GenerateOptions {
         return []
     }
 
+    /// Returns a list of the feature flags requested by the user.
+    /// - Parameter config: The configuration specified by the user.
+    func resolvedFeatureFlags(_ config: _UserConfig?) -> FeatureFlags {
+        if !featureFlag.isEmpty {
+            return Set(featureFlag)
+        }
+        return Set(config?.featureFlags ?? [])
+    }
+
     /// Returns the configuration requested by the user.
     ///
     /// - Returns: Loaded configuration, if found and parsed successfully.

Sources/swift-openapi-generator/UserConfig.swift

@@ -26,4 +26,7 @@ struct _UserConfig: Codable {
     /// A list of names of additional imports that are added to every
     /// generated Swift file.
     var additionalImports: [String]?
+
+    /// A set of features to explicitly enable.
+    var featureFlags: FeatureFlags?
 }

Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift

@@ -20,14 +20,16 @@ struct TestConfig: Encodable {
     var docFilePath: String
     var mode: GeneratorMode
     var additionalImports: [String]?
+    var featureFlags: FeatureFlags?
     var referenceOutputDirectory: String
 }
 
 extension TestConfig {
     var asConfig: Config {
         .init(
             mode: mode,
-            additionalImports: additionalImports ?? []
+            additionalImports: additionalImports ?? [],
+            featureFlags: featureFlags ?? []
         )
     }
 }
@@ -134,6 +136,7 @@ class FileBasedReferenceTests: XCTestCase {
                     docFilePath: "Docs/\(name.fileName)",
                     mode: mode,
                     additionalImports: [],
+                    featureFlags: [],
                     referenceOutputDirectory: "ReferenceSources/\(name.directoryName)"
                 )
             )