Add -gcc-toolchain flag to swift-driver

GCC toolchains are useful when cross-compiling to Linux with a given gcc
toolchain. The gcc toolchain contains tools for linking, libraries for
linking against, and the headers that map to those libraries.

This patch adds a gcc-toolchain flag to the driver so that Swift can
pass that down to both the clang importer and clang linker
appropriately.

Author: etcwilde

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -13,6 +13,7 @@
 import class TSCBasic.LocalFileOutputByteStream
 import class TSCBasic.TerminalController
 import struct TSCBasic.RelativePath
+import struct TSCBasic.AbsolutePath
 import var TSCBasic.stderrStream
 
 /// Whether we should produce color diagnostics by default.
@@ -141,6 +142,11 @@ extension Driver {
     try commandLine.appendAll(.F, .Fsystem, from: &parsedOptions)
     try commandLine.appendAll(.vfsoverlay, from: &parsedOptions)
 
+    if let gccToolchain = parsedOptions.getLastArgument(.gccToolchain) {
+        commandLine.appendFlag(.Xcc)
+        commandLine.appendFlag("--gcc-toolchain=\(gccToolchain.asSingle)")
+    }
+
     try commandLine.appendLast(.AssertConfig, from: &parsedOptions)
     try commandLine.appendLast(.autolinkForceLoad, from: &parsedOptions)
 

Sources/SwiftDriver/Jobs/LinkJob.swift

@@ -55,6 +55,11 @@ extension Driver {
       outputFile = try outputFileForImage
     }
 
+    if let gccToolchain = parsedOptions.getLastArgument(.gccToolchain) {
+        commandLine.appendFlag(.XclangLinker)
+        commandLine.appendFlag("--gcc-toolchain=\(gccToolchain.asSingle)")
+    }
+
     // Defer to the toolchain for platform-specific linking
     let linkTool = try toolchain.addPlatformSpecificLinkerArgs(
       to: &commandLine,

Sources/SwiftOptions/Options.swift

@@ -758,6 +758,7 @@ extension Option {
   public static let Xfrontend: Option = Option("-Xfrontend", .separate, attributes: [.helpHidden], metaVar: "<arg>", helpText: "Pass <arg> to the Swift frontend")
   public static let Xlinker: Option = Option("-Xlinker", .separate, attributes: [.doesNotAffectIncrementalBuild], helpText: "Specifies an option which should be passed to the linker")
   public static let Xllvm: Option = Option("-Xllvm", .separate, attributes: [.helpHidden, .frontend], metaVar: "<arg>", helpText: "Pass <arg> to LLVM.")
+  public static let gccToolchain: Option = Option("-gcc-toolchain", .separate, attributes: [.helpHidden, .argumentIsPath], metaVar: "<path>", helpText: "Specify a directory where the clang importer and clang linker can find headers and libraries" )
   public static let DASHDASH: Option = Option("--", .remaining, attributes: [.frontend, .doesNotAffectIncrementalBuild])
 }
 
@@ -1511,6 +1512,7 @@ extension Option {
       Option.Xfrontend,
       Option.Xlinker,
       Option.Xllvm,
+      Option.gccToolchain,
       Option.DASHDASH,
     ]
   }

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6650,6 +6650,18 @@ final class SwiftDriverTests: XCTestCase {
 #endif
   }
 
+  func testGccToolchainFlags() throws {
+      VirtualPath.resetTemporaryFileStore()
+      var driver = try Driver(args: [
+        "swiftc", "-gcc-toolchain", "foo/as/blarpy", "test.swift"
+      ])
+      let jobs = try driver.planBuild().removingAutolinkExtractJobs()
+      XCTAssertEqual(jobs.count, 2)
+      let (compileJob, linkJob) = (jobs[0], jobs[1])
+      compileJob.commandLine.contains(.flag("--gcc-toolchain=foo/as/blarpy"))
+      linkJob.commandLine.contains(.flag("--gcc-toolchain=foo/as/blarpy"))
+  }
+
   func testPluginPaths() throws {
     var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift"])
     guard driver.isFrontendArgSupported(.pluginPath) else {