Pass SDK to Compiler when Cross‐Compiling

Package.swift

@@ -169,7 +169,7 @@ let package = Package(
             dependencies: ["swift-build", "swift-package", "swift-test", "swift-run", "Commands", "Workspace", "SPMTestSupport"]),
         .testTarget(
             name: "WorkspaceTests",
-            dependencies: ["Workspace", "SPMTestSupport"]),
+            dependencies: ["Workspace", "SPMTestSupport", "SPMBuildCore"]),
         .testTarget(
             name: "FunctionalTests",
             dependencies: ["swift-build", "swift-package", "swift-test", "PackageModel", "SPMTestSupport"]),

Sources/SPMBuildCore/Triple.swift

@@ -18,7 +18,7 @@ import TSCBasic
 /// @see Destination.target
 /// @see https://github.com/apple/swift-llvm/blob/stable/include/llvm/ADT/Triple.h
 ///
-public struct Triple: Encodable {
+public struct Triple: Encodable, Equatable {
     public let tripleString: String
 
     public let arch: Arch

Sources/Workspace/Destination.swift

@@ -52,6 +52,23 @@ public struct Destination: Encodable {
     /// Additional flags to be passed when compiling with C++.
     public let extraCPPFlags: [String]
 
+    /// Creates a compilation destination with the specified properties.
+    public init(
+      target: Triple? = nil,
+      sdk: AbsolutePath,
+      binDir: AbsolutePath,
+      extraCCFlags: [String] = [],
+      extraSwiftCFlags: [String] = [],
+      extraCPPFlags: [String] = []
+    ) {
+      self.target = target
+      self.sdk = sdk
+      self.binDir = binDir
+      self.extraCCFlags = extraCCFlags
+      self.extraSwiftCFlags = extraSwiftCFlags
+      self.extraCPPFlags = extraCPPFlags
+    }
+
     /// Returns the bin directory for the host.
     ///
     /// - Parameter originalWorkingDirectory: The working directory when the program was launched.

Sources/Workspace/UserToolchain.swift

@@ -238,11 +238,20 @@ public final class UserToolchain: Toolchain {
       #endif
 
         // Use the triple from destination or compute the host triple using swiftc.
-        self.triple = destination.target ?? Triple.getHostTriple(usingSwiftCompiler: swiftCompilers.compile)
-        self.extraSwiftCFlags = (triple.isDarwin()
+        let hostTriple: Triple = Triple.getHostTriple(usingSwiftCompiler: swiftCompilers.compile)
+        let triple = destination.target ?? hostTriple
+        let crossCompiling = triple != hostTriple
+        self.triple = triple
+
+        var extraSwiftCFlags = (triple.isDarwin()
                                     ? ["-sdk", destination.sdk.pathString]
                                     : [])
                                   + destination.extraSwiftCFlags
+        if crossCompiling,
+          !extraSwiftCFlags.contains("-sdk") {
+          extraSwiftCFlags += ["-sdk", destination.sdk.pathString]
+        }
+        self.extraSwiftCFlags = extraSwiftCFlags
 
         self.extraCCFlags = [
             triple.isDarwin() ? "-isysroot" : "--sysroot", destination.sdk.pathString

Tests/WorkspaceTests/WorkspaceTests.swift

@@ -16,6 +16,7 @@ import PackageModel
 import PackageGraph
 import SourceControl
 import TSCUtility
+import SPMBuildCore
 import Workspace
 
 import SPMTestSupport
@@ -4479,6 +4480,25 @@ final class WorkspaceTests: XCTestCase {
             }
         }
     }
+
+    func testCrossCompilationDefaults() throws {
+      let target = try Triple("x86_64-unknown-linux-android")
+      let sdk = AbsolutePath("/some/path/to/an/SDK.sdk")
+      let toolchainPath = AbsolutePath("/some/path/to/a/toolchain.xctoolchain")
+
+      let destination = Destination(
+        target: target,
+        sdk: sdk,
+        binDir: toolchainPath.appending(components: "usr", "bin")
+      )
+      let toolchain = try UserToolchain(destination: destination)
+
+      #if !(os(Android) && arch(x86_64))  // ← Wouldn’t be cross‐compiling.
+        XCTAssertEqual(toolchain.extraSwiftCFlags, [
+          "-sdk", sdk.pathString
+        ])
+      #endif
+    }
 }
 
 extension PackageGraph {