Add test cases for target build testability with command plugin

Author: kateinoigakukun

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Package.swift

@@ -29,11 +29,27 @@ let package = Package(
                 .target(name: "plugintool")
             ]
         ),
+        .plugin(
+            name: "check-testability",
+            capability: .command(intent: .custom(
+                verb: "check-testability",
+                description: "Check testability of a target"
+            ))
+        ),
         .executableTarget(
             name: "placeholder"
         ),
         .executableTarget(
             name: "plugintool"
         ),
+        .target(
+            name: "InternalModule"
+        ),
+        .testTarget(
+            name: "InternalModuleTests",
+            dependencies: [
+                .target(name: "InternalModule")
+            ]
+        ),
     ]
 )

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Plugins/check-testability/main.swift

@@ -0,0 +1,45 @@
+import Foundation
+import PackagePlugin
+
+@main
+struct CheckTestability: CommandPlugin {
+    // This is a helper for testing target builds to ensure that they are testable.
+    func performCommand(context: PluginContext, arguments: [String]) async throws {
+        // Parse the arguments: <targetName> <config> <shouldTestable>
+        guard arguments.count == 3 else {
+            fatalError("Usage: <targetName> <config> <shouldTestable>")
+        }
+        let rawSubsetName = arguments[0]
+        var subset: PackageManager.BuildSubset
+        switch rawSubsetName {
+        // Special subset names
+        case "all-with-tests":
+            subset = .all(includingTests: true)
+        // By default, treat the subset as a target name
+        default:
+            subset = .target(rawSubsetName)
+        }
+        guard let config = PackageManager.BuildConfiguration(rawValue: arguments[1]) else {
+            fatalError("Invalid configuration: \(arguments[1])")
+        }
+        let shouldTestable = arguments[2] == "true"
+
+        var parameters = PackageManager.BuildParameters()
+        parameters.configuration = config
+        parameters.logging = .verbose
+
+        // Perform the build
+        let result = try packageManager.build(subset, parameters: parameters)
+
+        // Check if the build was successful
+        guard result.succeeded else {
+            fatalError("Build failed: \(result.logText)")
+        }
+
+        // Check if the build log contains "-enable-testing" flag
+        let isTestable = result.logText.contains("-enable-testing")
+        if isTestable != shouldTestable {
+            fatalError("Testability mismatch: expected \(shouldTestable), but got \(isTestable):\n\(result.logText)")
+        }
+    }
+}

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Sources/InternalModule/InternalModule.swift

@@ -0,0 +1,3 @@
+internal func internalFunction() {
+    print("Internal function")
+}

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Tests/InternalModuleTests/InternalModuleTests.swift

@@ -0,0 +1 @@
+@testable import InternalModule

Tests/CommandsTests/PackageCommandTests.swift

@@ -2448,6 +2448,35 @@ final class PackageCommandTests: CommandsTestCase {
         }
     }
 
+    func testCommandPluginBuildTestability() async throws {
+        // Plugin arguments: check-testability <targetName> <config> <shouldTestable>
+
+        // Overall configuration: debug, plugin build request: debug -> without testability
+        try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            try await XCTAssertAsyncNoThrow(try await SwiftPM.Package.execute(["-c", "debug", "check-testability", "InternalModule", "debug", "true"], packagePath: fixturePath))
+        }
+
+        // Overall configuration: debug, plugin build request: release -> without testability
+        try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            try await XCTAssertAsyncNoThrow(try await SwiftPM.Package.execute(["-c", "debug", "check-testability", "InternalModule", "release", "false"], packagePath: fixturePath))
+        }
+
+        // Overall configuration: release, plugin build request: debug -> with testability
+        try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            try await XCTAssertAsyncNoThrow(try await SwiftPM.Package.execute(["-c", "release", "check-testability", "InternalModule", "debug", "true"], packagePath: fixturePath))
+        }
+
+        // Overall configuration: release, plugin build request: release -> with testability
+        try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            try await XCTAssertAsyncNoThrow(try await SwiftPM.Package.execute(["-c", "release", "check-testability", "InternalModule", "release", "false"], packagePath: fixturePath))
+        }
+
+        // Overall configuration: release, plugin build request: release including tests -> with testability
+        try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            try await XCTAssertAsyncNoThrow(try await SwiftPM.Package.execute(["-c", "release", "check-testability", "all-with-tests", "release", "true"], packagePath: fixturePath))
+        }
+    }
+
     // Test logging of builds initiated by a command plugin
     func testCommandPluginBuildLogs() async throws {
         // Only run the test if the environment in which we're running actually supports Swift concurrency (which the plugin APIs require).