[PackageDescription] Use a dedicated type for target dependency condi…

Sources/PackageDescription/Target.swift

@@ -37,9 +37,9 @@ public final class Target {
         case productItem(name: String, package: String?)
         case byNameItem(name: String)
       #else
-        case _targetItem(name: String, condition: BuildSettingCondition?)
-        case _productItem(name: String, package: String?, condition: BuildSettingCondition?)
-        case _byNameItem(name: String, condition: BuildSettingCondition?)
+        case _targetItem(name: String, condition: TargetDependencyCondition?)
+        case _productItem(name: String, package: String?, condition: TargetDependencyCondition?)
+        case _byNameItem(name: String, condition: TargetDependencyCondition?)
       #endif
     }
 
@@ -656,7 +656,7 @@ extension Target.Dependency {
     ///   - name: The name of the target.
     ///   - condition: The condition under which the dependency is exercised.
     @available(_PackageDescription, introduced: 999)
-    public static func target(name: String, condition: BuildSettingCondition? = nil) -> Target.Dependency {
+    public static func target(name: String, condition: TargetDependencyCondition? = nil) -> Target.Dependency {
         return ._targetItem(name: name, condition: condition)
     }
 
@@ -670,7 +670,7 @@ extension Target.Dependency {
     public static func product(
         name: String,
         package: String,
-        condition: BuildSettingCondition? = nil
+        condition: TargetDependencyCondition? = nil
     ) -> Target.Dependency {
         return ._productItem(name: name, package: package, condition: condition)
     }
@@ -682,7 +682,7 @@ extension Target.Dependency {
     ///   - name: The name of the dependency, either a target or a product.
     ///   - condition: The condition under which the dependency is exercised.
     @available(_PackageDescription, introduced: 999)
-    public static func byName(name: String, condition: BuildSettingCondition? = nil) -> Target.Dependency {
+    public static func byName(name: String, condition: TargetDependencyCondition? = nil) -> Target.Dependency {
         return ._byNameItem(name: name, condition: condition)
     }
   #endif
@@ -704,3 +704,25 @@ extension Target.Dependency: ExpressibleByStringLiteral {
       #endif
     }
 }
+
+/// A condition that limits the application of a target's dependency.
+public struct TargetDependencyCondition: Encodable {
+
+    private let platforms: [Platform]?
+
+    private init(platforms: [Platform]?) {
+        self.platforms = platforms
+    }
+
+    /// Create a target dependency condition.
+    ///
+    /// - Parameters:
+    ///   - platforms: The applicable platforms for this target dependency condition.
+    public static func when(
+        platforms: [Platform]? = nil
+    ) -> TargetDependencyCondition {
+        // FIXME: This should be an error, not a precondition.
+        precondition(!(platforms == nil))
+        return TargetDependencyCondition(platforms: platforms)
+    }
+}

Tests/PackageLoadingTests/PDNextLoadingTests.swift

@@ -303,8 +303,8 @@ class PackageDescriptionNextLoadingTests: PackageDescriptionLoadingTests {
                     .target(name: "Foo", dependencies: [
                         .target(name: "Biz"),
                         .target(name: "Bar", condition: .when(platforms: [.linux])),
-                        .product(name: "Baz", package: "Baz", condition: .when(configuration: .release)),
-                        .byName(name: "Bar", condition: .when(platforms: [.watchOS, .iOS], configuration: .debug)),
+                        .product(name: "Baz", package: "Baz", condition: .when(platforms: [.macOS])),
+                        .byName(name: "Bar", condition: .when(platforms: [.watchOS, .iOS])),
                     ]),
                     .target(name: "Bar"),
                     .target(name: "Biz"),
@@ -317,8 +317,8 @@ class PackageDescriptionNextLoadingTests: PackageDescriptionLoadingTests {
 
             XCTAssertEqual(dependencies[0], .target(name: "Biz"))
             XCTAssertEqual(dependencies[1], .target(name: "Bar", condition: .init(platformNames: ["linux"], config: nil)))
-            XCTAssertEqual(dependencies[2], .product(name: "Baz", package: "Baz", condition: .init(platformNames: [], config: "release")))
-            XCTAssertEqual(dependencies[3], .byName(name: "Bar", condition: .init(platformNames: ["watchos", "ios"], config: "debug")))
+            XCTAssertEqual(dependencies[2], .product(name: "Baz", package: "Baz", condition: .init(platformNames: ["macos"])))
+            XCTAssertEqual(dependencies[3], .byName(name: "Bar", condition: .init(platformNames: ["watchos", "ios"])))
         }
     }
 }