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

@@ -435,6 +435,7 @@ extension Option {
   public static let Fsystem: Option = Option("-Fsystem", .separate, attributes: [.frontend, .argumentIsPath], helpText: "Add directory to system framework search path")
   public static let functionSections: Option = Option("-function-sections", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Emit functions to separate sections.")
   public static let F: Option = Option("-F", .joinedOrSeparate, attributes: [.frontend, .argumentIsPath], helpText: "Add directory to framework search path")
+  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 gdwarfTypes: Option = Option("-gdwarf-types", .flag, attributes: [.frontend], helpText: "Emit full DWARF type info.", group: .g)
   public static let generateEmptyBaseline: Option = Option("-generate-empty-baseline", .flag, attributes: [.noDriver], helpText: "Generate an empty baseline")
   public static let generateEmptyBaseline_: Option = Option("--generate-empty-baseline", .flag, alias: Option.generateEmptyBaseline, attributes: [.noDriver], helpText: "Generate an empty baseline")
@@ -1188,6 +1189,7 @@ extension Option {
       Option.Fsystem,
       Option.functionSections,
       Option.F,
+      Option.gccToolchain,
       Option.gdwarfTypes,
       Option.generateEmptyBaseline,
       Option.generateEmptyBaseline_,

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 {