Reorder swiftc args to allow user override

Fixtures/Miscellaneous/OverrideSwiftcArgs/Package.swift

@@ -0,0 +1,5 @@
+import PackageDescription
+
+let package = Package(
+    name: "OverrideSwiftcArgs"
+)

Fixtures/Miscellaneous/OverrideSwiftcArgs/Sources/main.swift

@@ -0,0 +1,7 @@
+/// This file exists to test the ability to override deployment targets via args passed to swiftc
+/// For this test to work, this file must have an API call which was introduced in a version
+/// higher than the default macOS deployment target that is checked in.
+@available(macOS 10.20, *)
+func foo() {}
+
+foo()

Sources/Build/describe().swift

@@ -38,7 +38,7 @@ public func describe(_ prefix: AbsolutePath, _ conf: Configuration, _ graph: Pac
     for module in graph.modules {
         switch module {
         case let module as SwiftModule:
-            let compile = try Command.compile(swiftModule: module, configuration: conf, prefix: prefix, otherArgs: swiftcArgs + toolchain.swiftPlatformArgs, compilerExec: toolchain.swiftCompiler)
+            let compile = try Command.compile(swiftModule: module, configuration: conf, prefix: prefix, otherArgs: toolchain.swiftPlatformArgs + swiftcArgs, compilerExec: toolchain.swiftCompiler)
             commands.append(compile)
             targets.append([compile], for: module)
 
@@ -48,7 +48,7 @@ public func describe(_ prefix: AbsolutePath, _ conf: Configuration, _ graph: Pac
             if module.isTest { continue }
           #endif
             // FIXME: Find a way to eliminate `externalModules` from here.
-            let compile = try Command.compile(clangModule: module, externalModules: graph.externalModules, configuration: conf, prefix: prefix, otherArgs: flags.cCompilerFlags + toolchain.clangPlatformArgs, compilerExec: toolchain.clangCompiler)
+            let compile = try Command.compile(clangModule: module, externalModules: graph.externalModules, configuration: conf, prefix: prefix, otherArgs: toolchain.clangPlatformArgs + flags.cCompilerFlags, compilerExec: toolchain.clangCompiler)
             commands += compile
             targets.append(compile, for: module)
 
@@ -62,7 +62,7 @@ public func describe(_ prefix: AbsolutePath, _ conf: Configuration, _ graph: Pac
 
     for product in graph.products {
         var rpathArgs = [String]()
-        
+
         // On Linux, always embed an RPATH adjacent to the linked binary. Note
         // that the '$ORIGIN' here is literal, it is a reference which is
         // understood by the dynamic linker.
@@ -73,7 +73,7 @@ public func describe(_ prefix: AbsolutePath, _ conf: Configuration, _ graph: Pac
         if product.containsOnlyClangModules {
             command = try Command.linkClangModule(product, configuration: conf, prefix: prefix, otherArgs: Xld, linkerExec: toolchain.clangCompiler)
         } else {
-            command = try Command.linkSwiftModule(product, configuration: conf, prefix: prefix, otherArgs: Xld + swiftcArgs + toolchain.swiftPlatformArgs + rpathArgs, linkerExec: toolchain.swiftCompiler)
+            command = try Command.linkSwiftModule(product, configuration: conf, prefix: prefix, otherArgs: Xld + toolchain.swiftPlatformArgs + swiftcArgs + rpathArgs, linkerExec: toolchain.swiftCompiler)
         }
 
         commands.append(command)

Tests/FunctionalTests/MiscellaneousTests.swift

@@ -21,7 +21,7 @@ import func POSIX.popen
 class MiscellaneousTestCase: XCTestCase {
     func testPrintsSelectedDependencyVersion() {
 
-        // verifies the stdout contains information about 
+        // verifies the stdout contains information about
         // the selected version of the package
 
         fixture(name: "DependencyResolution/External/Simple", tags: ["1.3.5"]) { prefix in
@@ -91,21 +91,21 @@ class MiscellaneousTestCase: XCTestCase {
             XCTAssertNoSuchPath(buildDir.appending(component: "some"))
         }
     }
-    
+
     func testManifestExcludes4() {
-        
+
         // exclude directory is inside Tests folder (Won't build without exclude)
-        
+
         fixture(name: "Miscellaneous/ExcludeDiagnostic4") { prefix in
             XCTAssertBuilds(prefix)
             XCTAssertFileExists(prefix.appending(components: ".build", "debug", "FooPackage.swiftmodule"))
         }
     }
-    
+
     func testManifestExcludes5() {
-        
+
         // exclude directory is Tests folder (Won't build without exclude)
-        
+
         fixture(name: "Miscellaneous/ExcludeDiagnostic5") { prefix in
             XCTAssertBuilds(prefix)
             XCTAssertFileExists(prefix.appending(components: ".build", "debug", "FooPackage.swiftmodule"))
@@ -318,13 +318,13 @@ class MiscellaneousTestCase: XCTestCase {
             XCTAssertFileExists(prefix.appending(components: ".build", "debug", "libProductName.\(Product.dynamicLibraryExtension)"))
         }
     }
-    
+
     func testProductWithNoModules() {
         fixture(name: "Miscellaneous/ProductWithNoModules") { prefix in
             XCTAssertBuildFails(prefix)
         }
     }
-    
+
     func testProductWithMissingModules() {
         fixture(name: "Miscellaneous/ProductWithMissingModules") { prefix in
             XCTAssertBuildFails(prefix)
@@ -387,6 +387,14 @@ class MiscellaneousTestCase: XCTestCase {
         }
     }
 
+    func testOverridingSwiftcArguments() throws {
+#if os(macOS)
+        fixture(name: "Miscellaneous/OverrideSwiftcArgs") { prefix in
+            try executeSwiftBuild(prefix, configuration: .Debug, printIfError: true, Xcc: [], Xld: [], Xswiftc: ["-target", "x86_64-apple-macosx10.20"], env: [:])
+        }
+#endif
+    }
+
     static var allTests = [
         ("testExecutableAsBuildOrderDependency", testExecutableAsBuildOrderDependency),
         ("testPrintsSelectedDependencyVersion", testPrintsSelectedDependencyVersion),
@@ -418,5 +426,6 @@ class MiscellaneousTestCase: XCTestCase {
         ("testSecondBuildIsNullInModulemapGen", testSecondBuildIsNullInModulemapGen),
         ("testSwiftTestParallel", testSwiftTestParallel),
         ("testInitPackageNonc99Directory", testInitPackageNonc99Directory),
+        ("testOverridingSwiftcArguments", testOverridingSwiftcArguments),
     ]
 }