[Build] Fix regression in Xcc handling.

 - Noticed by Chris Bailey (see #507).

 - This also adds at least some test coverage of -Xcc and -Xswiftc.

 - This also fixes it so we don't quite the extra C flags when passing them to
   Clang (which would still work, because Clang ignores -Xcc, but was incorrect
   and produced a warning).

Author: ddunbar

Fixtures/ValidLayouts/ExtraCommandLineFlags/Package.swift

@@ -0,0 +1,6 @@
+import PackageDescription
+
+let package = Package(
+    name: "ExtraCommandLineFlags",
+    targets: [
+        Target(name: "SwiftExec", dependencies: ["CLib"])])

Fixtures/ValidLayouts/ExtraCommandLineFlags/Sources/CLib/foo.c

@@ -0,0 +1,3 @@
+#include "CLib.h"
+
+void foo(void) { }

Fixtures/ValidLayouts/ExtraCommandLineFlags/Sources/CLib/include/CLib.h

@@ -0,0 +1,6 @@
+// This is to check a -Xcc arg.
+#if !defined(EXTRA_C_DEFINE) || EXTRA_C_DEFINE != 2
+#error "unexpected compiler flags"
+#endif
+
+void foo(void);

Fixtures/ValidLayouts/ExtraCommandLineFlags/Sources/SwiftExec/main.swift

@@ -0,0 +1,9 @@
+import CLib
+
+// This is expected to be set with -Xswiftc.
+#if !EXTRA_SWIFTC_DEFINE
+doesNotCompile()
+#endif
+
+foo()
+

Sources/Build/describe().swift

@@ -27,11 +27,10 @@ public func describe(_ prefix: AbsolutePath, _ conf: Configuration, _ graph: Pac
         throw Error.onlyCModule(name: module.name)
     }
 
-    let Xcc = flags.cCompilerFlags.flatMap{ ["-Xcc", $0] }
     let Xld = flags.linkerFlags.flatMap{ ["-Xlinker", $0] }
     let prefix = prefix.appending(component: conf.dirname)
     try makeDirectories(prefix)
-    let swiftcArgs = flags.cCompilerFlags + flags.swiftCompilerFlags + verbosity.ccArgs
+    let swiftcArgs = flags.cCompilerFlags.flatMap{ ["-Xcc", $0] } + flags.swiftCompilerFlags + verbosity.ccArgs
 
     let SWIFT_EXEC = toolchain.SWIFT_EXEC
     let CC = getenv("CC") ?? "clang"
@@ -52,7 +51,7 @@ 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, CC: CC, otherArgs: Xcc + toolchain.platformArgsClang)
+            let compile = try Command.compile(clangModule: module, externalModules: graph.externalModules, configuration: conf, prefix: prefix, CC: CC, otherArgs: flags.cCompilerFlags + toolchain.platformArgsClang)
             commands += compile
             targets.append(compile, for: module)
 

Tests/Functional/Utilities.swift

@@ -182,15 +182,17 @@ extension SwiftPMProduct {
 }
 
 @discardableResult
-func executeSwiftBuild(_ chdir: AbsolutePath, configuration: Configuration = .Debug, printIfError: Bool = false, Xld: [String] = [], env: [String: String] = [:]) throws -> String {
+func executeSwiftBuild(_ chdir: AbsolutePath, configuration: Configuration = .Debug, printIfError: Bool = false, Xcc: [String] = [], Xld: [String] = [], Xswiftc: [String] = [], env: [String: String] = [:]) throws -> String {
     var args = ["--configuration"]
     switch configuration {
     case .Debug:
         args.append("debug")
     case .Release:
         args.append("release")
     }
+    args += Xcc.flatMap{ ["-Xcc", $0] }
     args += Xld.flatMap{ ["-Xlinker", $0] }
+    args += Xswiftc.flatMap{ ["-Xswiftc", $0] }
 
     let swiftBuild = SwiftPMProduct.SwiftBuild
     var env = env
@@ -211,11 +213,11 @@ func mktmpdir(function: StaticString = #function, file: StaticString = #file, li
     }
 }
 
-func XCTAssertBuilds(_ path: AbsolutePath, configurations: Set<Configuration> = [.Debug, .Release], file: StaticString = #file, line: UInt = #line, Xld: [String] = [], env: [String: String] = [:]) {
+func XCTAssertBuilds(_ path: AbsolutePath, configurations: Set<Configuration> = [.Debug, .Release], file: StaticString = #file, line: UInt = #line, Xcc: [String] = [], Xld: [String] = [], Xswiftc: [String] = [], env: [String: String] = [:]) {
     for conf in configurations {
         do {
             print("    Building \(conf)")
-            _ = try executeSwiftBuild(path, configuration: conf, printIfError: true, Xld: Xld, env: env)
+            _ = try executeSwiftBuild(path, configuration: conf, printIfError: true, Xcc: Xcc, Xld: Xld, Xswiftc: Xswiftc, env: env)
         } catch {
             XCTFail("`swift build -c \(conf)' failed:\n\n\(error)\n", file: file, line: line)
         }
@@ -230,9 +232,9 @@ func XCTAssertSwiftTest(_ path: AbsolutePath, file: StaticString = #file, line:
     }
 }
 
-func XCTAssertBuildFails(_ path: AbsolutePath, file: StaticString = #file, line: UInt = #line) {
+func XCTAssertBuildFails(_ path: AbsolutePath, file: StaticString = #file, line: UInt = #line, Xcc: [String] = [], Xld: [String] = [], Xswiftc: [String] = [], env: [String: String] = [:]) {
     do {
-        _ = try executeSwiftBuild(path)
+        _ = try executeSwiftBuild(path, Xcc: Xcc, Xld: Xld, Xswiftc: Xswiftc)
 
         XCTFail("`swift build' succeeded but should have failed", file: file, line: line)
 

Tests/Functional/ValidLayoutTests.swift

@@ -101,6 +101,15 @@ class ValidLayoutsTestCase: XCTestCase {
         }
     }
 
+    func testExtraCommandLineFlags() {
+        fixture(name: "ValidLayouts/ExtraCommandLineFlags") { prefix in
+            // This project is expected to require Xcc and Xswiftc overrides.
+            XCTAssertBuildFails(prefix)
+            XCTAssertBuildFails(prefix, Xcc: ["-DEXTRA_C_DEFINE=2"])
+            XCTAssertBuilds(prefix, Xcc: ["-DEXTRA_C_DEFINE=2"], Xswiftc: ["-DEXTRA_SWIFTC_DEFINE"])
+        }
+    }
+
     static var allTests = [
         ("testSingleModuleLibrary", testSingleModuleLibrary),
         ("testSingleModuleExecutable", testSingleModuleExecutable),