command plugins: Build command plugin dependencies for the host, not the target

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Package.swift

@@ -19,8 +19,21 @@ let package = Package(
                 description: "Build a target for testing"
             ))
         ),
+        .plugin(
+            name: "plugin-dependencies-stub",
+            capability: .command(intent: .custom(
+                verb: "build-plugin-dependency",
+                description: "Build a plugin dependency for testing"
+            )),
+            dependencies: [
+                .target(name: "plugintool")
+            ]
+        ),
         .executableTarget(
             name: "placeholder"
         ),
+        .executableTarget(
+            name: "plugintool"
+        ),
     ]
 )

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Plugins/plugin-dependencies-stub/main.swift

@@ -0,0 +1,13 @@
+import PackagePlugin
+
+@main
+struct test: CommandPlugin {
+    // This plugin exists to test that the executable it requires is built correctly when cross-compiling
+    func performCommand(context: PluginContext, arguments: [String]) async throws {
+        print("Hello from dependencies-stub")
+        let _ = try packageManager.build(
+            .product("placeholder"),
+            parameters: .init(configuration: .debug, logging: .concise)
+        )
+    }
+}

Fixtures/Miscellaneous/Plugins/CommandPluginTestStub/Sources/plugintool/main.swift

@@ -0,0 +1 @@
+print("Hello from plugintool")

Sources/Commands/PackageTools/PluginCommand.swift

@@ -319,7 +319,11 @@ struct PluginCommand: SwiftCommand {
         // Build or bring up-to-date any executable host-side tools on which this plugin depends. Add them and any binary dependencies to the tool-names-to-path map.
         let buildSystem = try swiftTool.createBuildSystem(
             explicitBuildSystem: .native,
-            cacheBuildManifest: false
+            cacheBuildManifest: false,
+            // Force all dependencies to be built for the host, to work around the fact that BuildOperation.plan
+            // knows to compile build tool plugin dependencies for the host but does not do the same for command
+            // plugins.
+            productsBuildParameters: buildParameters
         )
         let accessibleTools = try plugin.processAccessibleTools(
             packageGraph: packageGraph,

Tests/BuildTests/PluginsBuildPlanTests.swift

@@ -12,7 +12,9 @@
 
 import Basics
 import SPMTestSupport
+@testable import SPMBuildCore
 import XCTest
+import PackageModel
 
 final class PluginsBuildPlanTests: XCTestCase {
     func testBuildToolsDatabasePath() throws {
@@ -22,4 +24,41 @@ final class PluginsBuildPlanTests: XCTestCase {
             XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/plugins/tools/build.db"))))
         }
     }
+
+    func testCommandPluginDependenciesWhenCrossCompiling() throws {
+        // Command Plugin dependencies must be built for the host.
+        // This test is only supported on macOS because that is the only
+        // platform on which we can currently be sure of having a viable
+        // cross-compilation environment (arm64->x86_64 or vice versa).
+        // On Linux it is typically only possible to build for the host
+        // environment unless cross-compilation SDKs are being used.
+        #if !os(macOS)
+        try XCTSkipIf(true, "test is only supported on macOS")
+        #endif
+
+        let hostToolchain = try UserToolchain(swiftSDK: .hostSwiftSDK())
+        let hostTriple = try! hostToolchain.targetTriple.withoutVersion().tripleString
+
+        let x86Triple = "x86_64-apple-macosx"
+        let armTriple = "arm64-apple-macosx"
+        let targetTriple = hostToolchain.targetTriple.arch == .aarch64 ? x86Triple : armTriple
+
+        // By default, plugin dependencies are built for the host platform
+        try fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            let (stdout, stderr) = try executeSwiftPackage(fixturePath, extraArgs: ["-v", "build-plugin-dependency"])
+            XCTAssertMatch(stdout, .contains("Hello from dependencies-stub"))
+            XCTAssertMatch(stderr, .contains("Build of product 'plugintool' complete!"))
+            XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/\(hostTriple)/debug/plugintool"))))
+            XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/\(hostTriple)/debug/placeholder"))))
+        }
+
+        // When cross compiling the final product, plugin dependencies should still be built for the host
+        try fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
+            let (stdout, stderr) = try executeSwiftPackage(fixturePath, extraArgs: ["--triple", targetTriple, "-v", "build-plugin-dependency"])
+            XCTAssertMatch(stdout, .contains("Hello from dependencies-stub"))
+            XCTAssertMatch(stderr, .contains("Build of product 'plugintool' complete!"))
+            XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/\(hostTriple)/debug/plugintool"))))
+            XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/\(targetTriple)/debug/placeholder"))))
+        }
+    }
 }