[Traits] Disallow disabling default traits of a package without traits

Fixtures/Traits/DisablingEmptyDefaultsExample/Package.swift

@@ -0,0 +1,18 @@
+// swift-tools-version: 6.1
+
+import PackageDescription
+
+let package = Package(
+    name: "DisablingEmptyDefaultsExample",
+    dependencies: [
+        .package(
+            path: "../Package11",
+            traits: []
+        ),
+    ],
+    targets: [
+        .executableTarget(
+            name: "DisablingEmptyDefaultsExample"
+        ),
+    ]
+)

Fixtures/Traits/DisablingEmptyDefaultsExample/Sources/DisablingEmptyDefaultsExample/Example.swift

@@ -0,0 +1,6 @@
+@main
+struct Example {
+    static func main() {
+
+    }
+}

Fixtures/Traits/Package11/Package.swift

@@ -0,0 +1,18 @@
+// swift-tools-version: 6.1
+
+import PackageDescription
+
+let package = Package(
+    name: "Package11",
+    products: [
+        .library(
+            name: "Package11Library1",
+            targets: ["Package11Library1"]
+        ),
+    ],
+    targets: [
+        .target(
+            name: "Package11Library1"
+        ),
+    ]
+)

Fixtures/Traits/Package11/Sources/Package11Library1/Library.swift

@@ -0,0 +1,3 @@
+public func hello() {
+    print("Package11Library1")
+}

Sources/PackageGraph/ModulesGraph+Loading.swift

@@ -112,6 +112,7 @@ extension ModulesGraph {
                 manifest: package.manifest,
                 productFilter: .everything,
                 enabledTraits: calculateEnabledTraits(
+                    parentPackage: nil,
                     identity: identity,
                     manifest: package.manifest,
                     explictlyEnabledTraits: enabledTraits
@@ -154,6 +155,7 @@ extension ModulesGraph {
                                 manifest: manifest,
                                 productFilter: dependency.productFilter,
                                 enabledTraits: calculateEnabledTraits(
+                                    parentPackage: node.item.identity,
                                     identity: dependency.identity,
                                     manifest: manifest,
                                     explictlyEnabledTraits: explictlyEnabledTraits.flatMap { Set($0) }
@@ -941,6 +943,7 @@ private func emitDuplicateProductDiagnostic(
 }
 
 private func calculateEnabledTraits(
+    parentPackage: PackageIdentity?,
     identity: PackageIdentity,
     manifest: Manifest,
     explictlyEnabledTraits: Set<String>?
@@ -958,6 +961,15 @@ private func calculateEnabledTraits(
             throw ModuleError.invalidTrait(package: identity, trait: trait)
         }
     }
+
+    if let parentPackage, !(explictlyEnabledTraits == nil || areDefaultsEnabled) && manifest.traits.isEmpty {
+        // We throw an error when default traits are disabled for a package without any traits
+        // This allows packages to initially move new API behind traits once.
+        throw ModuleError.disablingDefaultTraitsOnEmptyTraits(
+            parentPackage: parentPackage,
+            packageName: manifest.displayName
+        )
+    }
 
     // We have to enable all default traits if no traits are enabled or the defaults are explicitly enabled
     if explictlyEnabledTraits == nil || areDefaultsEnabled {

Sources/PackageLoading/PackageBuilder.swift

@@ -97,6 +97,11 @@ public enum ModuleError: Swift.Error {
         package: PackageIdentity,
         trait: String
     )
+
+    case disablingDefaultTraitsOnEmptyTraits(
+        parentPackage: PackageIdentity,
+        packageName: String
+    )
 }
 
 extension ModuleError: CustomStringConvertible {
@@ -179,6 +184,10 @@ extension ModuleError: CustomStringConvertible {
             return """
             Trait '"\(trait)"' is not declared by package '\(package)'.
             """
+        case .disablingDefaultTraitsOnEmptyTraits(let parentPackage, let packageName):
+            return """
+            Disabled default traits by package '\(parentPackage)' on package '\(packageName)' that declares no traits. This is prohibited to allow packages to adopt traits initially without causing an API break.
+            """
         }
     }
 }

Sources/_InternalTestSupport/ManifestExtensions.swift

@@ -32,7 +32,7 @@ extension Manifest {
         dependencies: [PackageDependency] = [],
         products: [ProductDescription] = [],
         targets: [TargetDescription] = [],
-        traits: Set<TraitDescription> = []
+        traits: Set<TraitDescription> = [.init(name: "defaults")]
     ) -> Manifest {
         Self.createManifest(
             displayName: displayName,
@@ -70,7 +70,7 @@ extension Manifest {
         dependencies: [PackageDependency] = [],
         products: [ProductDescription] = [],
         targets: [TargetDescription] = [],
-        traits: Set<TraitDescription> = []
+        traits: Set<TraitDescription> = [.init(name: "defaults")]
     ) -> Manifest {
         Self.createManifest(
             displayName: displayName,
@@ -220,7 +220,7 @@ extension Manifest {
         dependencies: [PackageDependency] = [],
         products: [ProductDescription] = [],
         targets: [TargetDescription] = [],
-        traits: Set<TraitDescription> = []
+        traits: Set<TraitDescription> = [.init(name: "defaults")]
     ) -> Manifest {
         return Manifest(
             displayName: displayName,

Sources/_InternalTestSupport/MockPackage.swift

@@ -35,7 +35,7 @@ public struct MockPackage {
         targets: [MockTarget],
         products: [MockProduct] = [],
         dependencies: [MockDependency] = [],
-        traits: Set<TraitDescription> = [],
+        traits: Set<TraitDescription> = [.init(name: "defaults")],
         versions: [String?] = [],
         revisionProvider: ((String) -> String)? = nil,
         toolsVersion: ToolsVersion? = nil
@@ -60,7 +60,7 @@ public struct MockPackage {
         targets: [MockTarget],
         products: [MockProduct],
         dependencies: [MockDependency] = [],
-        traits: Set<TraitDescription> = [],
+        traits: Set<TraitDescription> = [.init(name: "defaults")],
         versions: [String?] = [],
         revisionProvider: ((String) -> String)? = nil,
         toolsVersion: ToolsVersion? = nil
@@ -86,7 +86,7 @@ public struct MockPackage {
         targets: [MockTarget],
         products: [MockProduct],
         dependencies: [MockDependency] = [],
-        traits: Set<TraitDescription> = [],
+        traits: Set<TraitDescription> = [.init(name: "defaults")],
         versions: [String?] = [],
         revisionProvider: ((String) -> String)? = nil,
         toolsVersion: ToolsVersion? = nil

Tests/FunctionalTests/TraitTests.swift

@@ -264,5 +264,24 @@ final class TraitTests: XCTestCase {
             XCTAssertTrue(symbolGraph.contains("TypeGatedByPackage10Trait2"))
         }
     }
+
+    func testPackageDisablinDefaultsTrait_whenNoTraits() async throws {
+        try await fixture(name: "Traits") { fixturePath in
+            do {
+                let (_, _) = try await executeSwiftRun(fixturePath.appending("DisablingEmptyDefaultsExample"), "DisablingEmptyDefaultsExample")
+            } catch let error as SwiftPMError {
+                switch error {
+                case .packagePathNotFound:
+                    throw error
+                case .executionFailure(_, _, let stderr):
+                    let expectedErr = """
+                    error: Disabled default traits by package 'disablingemptydefaultsexample' on package 'Package11' that declares no traits. This is prohibited to allow packages to adopt traits initially without causing an API break.
+
+                    """
+                    XCTAssertTrue(stderr.contains(expectedErr))
+                }
+            }
+        }
+    }
 }
 #endif