[Traits] Replace isDefault with defaults trait

# Motivation

The current spelling uses `isDefault` per trait to declare the default traits. This makes it hard to see which traits are actually the default traits.

# Modification

Instead of using an explicit `isDefault` per trait this PR now uses a `.defaults(enabledTraits:)` overload which just uses the `"default"` trait under the hood. This simplifies the logic tremendously to handle default traits.

# Result

Nicer spelling for default traits and easier implementation.

Author: FranzBusch

Fixtures/Traits/Example/Package.swift

@@ -5,15 +5,24 @@
 let package = Package(
     name: "TraitsExample",
     traits: [
-        Trait(name: "Package1", isDefault: true),
-        Trait(name: "Package2", isDefault: true),
-        Trait(name: "Package3", isDefault: true),
-        Trait(name: "Package4", isDefault: true),
+        .default(
+            enabledTraits: [
+                "Package1",
+                "Package2",
+                "Package3",
+                "Package4",
+                "BuildCondition1",
+            ]
+        ),
+        "Package1",
+        "Package2",
+        "Package3",
+        "Package4",
         "Package5",
         "Package7",
         "Package9",
         "Package10",
-        Trait(name: "BuildCondition1", isDefault: true),
+        "BuildCondition1",
         "BuildCondition2",
         "BuildCondition3",
     ],

Fixtures/Traits/Package3/Package.swift

@@ -11,9 +11,10 @@ let package = Package(
         ),
     ],
     traits: [
-        Trait(name: "Package3Trait1", enabledTraits: ["Package3Trait2"]),
-        Trait(name: "Package3Trait2", enabledTraits: ["Package3Trait3"]),
-        Trait(name: "Package3Trait3", isDefault: true),
+        .default(enabledTraits: ["Package3Trait3"]),
+        .trait(name: "Package3Trait1", enabledTraits: ["Package3Trait2"]),
+        .trait(name: "Package3Trait2", enabledTraits: ["Package3Trait3"]),
+        "Package3Trait3",
     ],
     targets: [
         .target(

Fixtures/Traits/Package4/Package.swift

@@ -11,7 +11,8 @@ let package = Package(
         ),
     ],
     traits: [
-        Trait(name: "Package4Trait1", isDefault: true),
+        .default(enabledTraits: ["Package4Trait1"]),
+        "Package4Trait1",
     ],
     targets: [
         .target(

Sources/PackageDescription/PackageDependencyTrait.swift

@@ -16,7 +16,7 @@ extension Package.Dependency {
     @available(_PackageDescription, introduced: 999.0)
     public struct Trait: Hashable, Sendable, ExpressibleByStringLiteral {
         /// Enables all default traits of a package.
-        public static let defaults = Self.init(name: "defaults")
+        public static let defaults = Self.init(name: "default")
 
         /// A condition that limits the application of a dependencies trait.
         public struct Condition: Hashable, Sendable {

Sources/PackageDescription/PackageDescriptionSerialization.swift

@@ -270,7 +270,6 @@ enum Serialization {
     struct Trait: Hashable, Codable {
         let name: String
         let description: String?
-        let isDefault: Bool
         let enabledTraits: Set<String>
     }
 

Sources/PackageDescription/PackageDescriptionSerializationConversion.swift

@@ -382,7 +382,6 @@ extension Serialization.Trait {
     init(_ trait: PackageDescription.Trait) {
         self.name = trait.name
         self.description = trait.description
-        self.isDefault = trait.isDefault
         self.enabledTraits = trait.enabledTraits
     }
 }

Sources/PackageDescription/Trait.swift

@@ -18,6 +18,15 @@
 @_spi(ExperimentalTraits)
 @available(_PackageDescription, introduced: 999.0)
 public struct Trait: Hashable, ExpressibleByStringLiteral {
+    /// Declares the default traits for this package.
+    public static func `default`(enabledTraits: Set<String>) -> Self {
+        .init(
+            name: "default",
+            description: "The default traits of this package.",
+            enabledTraits: enabledTraits
+        )
+    }
+
     /// The trait's canonical name.
     ///
     /// This is used when enabling the trait or when referring to it from other modifiers in the manifest.
@@ -35,9 +44,6 @@ public struct Trait: Hashable, ExpressibleByStringLiteral {
     /// Use this to explain what functionality this trait enables.
     public var description: String?
 
-    /// A boolean indicating wether the trait is enabled by default.
-    public var isDefault: Bool
-
     /// A set of other traits of this package that this trait enables.
     public var enabledTraits: Set<String>
 
@@ -46,17 +52,14 @@ public struct Trait: Hashable, ExpressibleByStringLiteral {
     /// - Parameters:
     ///   - name: The trait's canonical name.
     ///   - description: The trait's description.
-    ///   - isDefault: A boolean indicating wether the trait is enabled by default.
     ///   - enabledTraits: A set of other traits of this package that this trait enables.
     public init(
         name: String,
         description: String? = nil,
-        isDefault: Bool = false,
         enabledTraits: Set<String> = []
     ) {
         self.name = name
         self.description = description
-        self.isDefault = isDefault
         self.enabledTraits = enabledTraits
     }
 
@@ -69,18 +72,15 @@ public struct Trait: Hashable, ExpressibleByStringLiteral {
     /// - Parameters:
     ///   - name: The trait's canonical name.
     ///   - description: The trait's description.
-    ///   - isDefault: A boolean indicating wether the trait is enabled by default.
     ///   - enabledTraits: A set of other traits of this package that this trait enables.
     public static func trait(
         name: String,
         description: String? = nil,
-        isDefault: Bool = false,
         enabledTraits: Set<String> = []
     ) -> Trait {
         .init(
             name: name,
             description: description,
-            isDefault: isDefault,
             enabledTraits: enabledTraits
         )
     }

Sources/PackageGraph/ModulesGraph+Loading.swift

@@ -799,7 +799,7 @@ private func calculateEnabledTraits(
 ) throws -> Set<String> {
     // This the point where we flatten the enabled traits and resolve the recursive traits
     var recursiveEnabledTraits = explictlyEnabledTraits ?? []
-    let areDefaultsEnabled = recursiveEnabledTraits.remove("defaults") != nil
+    let areDefaultsEnabled = recursiveEnabledTraits.remove("default") != nil
 
     // We are going to calculate which traits are actually enabled for a node here. To do this
     // we have to check if default traits should be used and then flatten all the enabled traits.
@@ -813,7 +813,7 @@ private func calculateEnabledTraits(
 
     // We have to enable all default traits if no traits are enabled or the defaults are explicitly enabled
     if explictlyEnabledTraits == nil || areDefaultsEnabled {
-        recursiveEnabledTraits.formUnion(manifest.traits.lazy.filter { $0.isDefault }.map { $0.name })
+        recursiveEnabledTraits.formUnion(manifest.traits.first { $0.name == "default" }?.enabledTraits ?? [])
     }
 
     while true {

Sources/PackageLoading/ManifestJSONParser.swift

@@ -605,7 +605,6 @@ extension TraitDescription {
         self.init(
             name: trait.name,
             description: trait.description,
-            isDefault: trait.isDefault,
             enabledTraits: trait.enabledTraits
         )
     }

Sources/PackageLoading/ManifestLoader+Validation.swift

@@ -107,11 +107,6 @@ public struct ManifestValidator {
                 continue
             }
 
-            guard traitName.lowercased() != "default" && traitName.lowercased() != "defaults" else {
-                diagnostics.append(.defaultTraitName())
-                continue
-            }
-
             for (index, unicodeScalar) in traitName.unicodeScalars.enumerated() {
                 let properties = unicodeScalar.properties
 
@@ -370,10 +365,6 @@ extension Basics.Diagnostic {
         .error("Empty strings are not allowed as trait names")
     }
 
-    static func defaultTraitName() -> Self {
-        .error("Traits are not allowed to be named 'default' or 'defaults' to avoid confusion with default traits")
-    }
-
     static func invalidFirstCharacterInTrait(firstCharater: UnicodeScalar, trait: String) -> Self {
         .error("Invalid first character (\(firstCharater)) in trait \(trait). The first character must be a Unicode XID start character (most letters), a digit, or _.")
     }

Sources/PackageModel/Manifest/TraitDescription.swift

@@ -19,9 +19,6 @@ public struct TraitDescription: Sendable, Hashable, Codable, ExpressibleByString
     /// The trait's description.
     public var description: String?
 
-    /// A boolean indicating wether the trait is enabled by default.
-    public var isDefault: Bool
-
     /// A set of other traits of this package that this trait enables.
     public var enabledTraits: Set<String>
 
@@ -30,17 +27,14 @@ public struct TraitDescription: Sendable, Hashable, Codable, ExpressibleByString
     /// - Parameters:
     ///   - name: The trait's canonical name.
     ///   - description: The trait's description.
-    ///   - isDefault: A boolean indicating wether the trait is enabled by default.
     ///   - enabledTraits: A set of other traits of this package that this trait enables.
     public init(
         name: String,
         description: String? = nil,
-        isDefault: Bool = false,
         enabledTraits: Set<String> = []
     ) {
         self.name = name
         self.description = description
-        self.isDefault = isDefault
         self.enabledTraits = enabledTraits
     }
 

Sources/SPMTestSupport/PackageDependencyDescriptionExtensions.swift

@@ -22,7 +22,7 @@ package extension PackageDependency {
         deprecatedName: String? = nil,
         path: AbsolutePath,
         productFilter: ProductFilter = .everything,
-        traits: Set<Trait> = [.init(name: "defaults")]
+        traits: Set<Trait> = [.init(name: "default")]
     ) -> Self {
         let identity = identity ?? PackageIdentity(path: path)
         return .fileSystem(
@@ -40,7 +40,7 @@ package extension PackageDependency {
         path: AbsolutePath,
         requirement: SourceControl.Requirement,
         productFilter: ProductFilter = .everything,
-        traits: Set<Trait> = [.init(name: "defaults")]
+        traits: Set<Trait> = [.init(name: "default")]
     ) -> Self {
         let identity = identity ?? PackageIdentity(path: path)
         return .localSourceControl(
@@ -59,7 +59,7 @@ package extension PackageDependency {
         url: SourceControlURL,
         requirement: SourceControl.Requirement,
         productFilter: ProductFilter = .everything,
-        traits: Set<Trait> = [.init(name: "defaults")]
+        traits: Set<Trait> = [.init(name: "default")]
     ) -> Self {
         let identity = identity ?? PackageIdentity(url: url)
         return .remoteSourceControl(
@@ -76,7 +76,7 @@ package extension PackageDependency {
         identity: String,
         requirement: Registry.Requirement,
         productFilter: ProductFilter = .everything,
-        traits: Set<Trait> = [.init(name: "defaults")]
+        traits: Set<Trait> = [.init(name: "default")]
     ) -> Self {
         return .registry(
             identity: .plain(identity),

Tests/FunctionalTests/TraitTests.swift

@@ -35,7 +35,7 @@ final class TraitTests: XCTestCase {
 
     func testTraits_whenTraitUnification() async throws {
         try await fixture(name: "Traits") { fixturePath in
-            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "defaults,Package9,Package10"])
+            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "default,Package9,Package10"])
             XCTAssertEqual(stdout, """
             Package1Library1 trait1 enabled
             Package2Library1 trait2 enabled
@@ -55,7 +55,7 @@ final class TraitTests: XCTestCase {
 
     func testTraits_whenTraitUnification_whenSecondTraitNotEnabled() async throws {
         try await fixture(name: "Traits") { fixturePath in
-            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "defaults,Package9"])
+            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "default,Package9"])
             XCTAssertEqual(stdout, """
             Package1Library1 trait1 enabled
             Package2Library1 trait2 enabled
@@ -73,7 +73,7 @@ final class TraitTests: XCTestCase {
 
     func testTraits_whenIndividualTraitsEnabled_andDefaultTraits() async throws {
         try await fixture(name: "Traits") { fixturePath in
-            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "defaults,Package5,Package7,BuildCondition3"])
+            let (stdout, _) = try await executeSwiftRun(fixturePath.appending("Example"), "Example", extraArgs: ["--experimental-traits", "default,Package5,Package7,BuildCondition3"])
             XCTAssertEqual(stdout, """
             Package1Library1 trait1 enabled
             Package2Library1 trait2 enabled
@@ -170,7 +170,7 @@ final class TraitTests: XCTestCase {
             let json = try JSON(bytes: ByteString(encodingAsUTF8: dumpOutput))
             guard case let .dictionary(contents) = json else { XCTFail("unexpected result"); return }
             guard case let .array(traits)? = contents["experimentalTraits"] else { XCTFail("unexpected result"); return }
-            XCTAssertEqual(traits.count, 11)
+            XCTAssertEqual(traits.count, 12)
         }
     }
 

Tests/PackageGraphTests/ModulesGraphTests.swift

@@ -2919,7 +2919,7 @@ final class ModulesGraphTests: XCTestCase {
                         ),
                     ],
                     traits: [
-                        .init(name: "Trait1", isDefault: true),
+                        .init(name: "default", enabledTraits: ["Trait1"]),
                         "Trait1",
                     ]
                 ),
@@ -2955,7 +2955,8 @@ final class ModulesGraphTests: XCTestCase {
                         ),
                     ],
                     traits: [
-                        .init(name: "Trait1", isDefault: true, enabledTraits: ["Trait2"]),
+                        .init(name: "default", enabledTraits: ["Trait1"]),
+                        .init(name: "Trait1", enabledTraits: ["Trait2"]),
                         .init(name: "Trait2", enabledTraits: ["Trait3", "Trait4"]),
                         "Trait3",
                         .init(name: "Trait4", enabledTraits: ["Trait5"]),
@@ -3004,7 +3005,7 @@ final class ModulesGraphTests: XCTestCase {
                         ),
                     ],
                     traits: [
-                        .init(name: "Package1Trait1", isDefault: true),
+                        .init(name: "default", enabledTraits: ["Package1Trait1"]),
                         "Package1Trait1",
                     ]
                 ),
@@ -3072,7 +3073,8 @@ final class ModulesGraphTests: XCTestCase {
                     ),
                 ],
                 traits: [
-                    .init(name: "Package1Trait1", isDefault: true)
+                    .init(name: "default", enabledTraits: ["Package1Trait1"]),
+                    .init(name: "Package1Trait1")
                 ]
             ),
             Manifest.createFileSystemManifest(
@@ -3164,8 +3166,9 @@ final class ModulesGraphTests: XCTestCase {
                     ),
                 ],
                 traits: [
-                    .init(name: "Package1Trait1", isDefault: true),
-                    .init(name: "Package1Trait2", isDefault: true),
+                    .init(name: "default", enabledTraits: ["Package1Trait1", "Package1Trait2"]),
+                    .init(name: "Package1Trait1"),
+                    .init(name: "Package1Trait2"),
                 ]
             ),
             Manifest.createFileSystemManifest(