Add generate PCH job

Author: cltnschlosser

Sources/SwiftDriver/Driver/Driver.swift

@@ -1332,9 +1332,7 @@ extension Driver {
       diagnosticEngine.emit(.error_bridging_header_module_interface)
     }
 
-    let objcHeaderPath = try VirtualPath(path: objcHeaderPathArg.asSingle)
-    // FIXME: Precompile bridging header if requested.
-    return objcHeaderPath
+    return try VirtualPath(path: objcHeaderPathArg.asSingle)
   }
 }
 

Sources/SwiftDriver/Jobs/GeneratePCHJob.swift

@@ -0,0 +1,43 @@
+//===--------------- GeneratePCHJob.swift - Swift Autolink Extract ----===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import TSCBasic
+import TSCUtility
+
+extension Driver {
+    mutating func generatePCHJob(input: TypedVirtualPath) throws -> Job {
+        var inputs = [TypedVirtualPath]()
+        var outputs = [TypedVirtualPath]()
+        
+        var commandLine: [Job.ArgTemplate] = swiftCompilerPrefixArgs.map { Job.ArgTemplate.flag($0) }
+        
+        inputs.append(input)
+        commandLine.appendPath(input.file)
+        commandLine.appendFlag(.emitPch)
+        
+        try addCommonFrontendOptions(commandLine: &commandLine)
+        
+        if let outputDirectory = parsedOptions.getLastArgument(.pchOutputDir)?.asSingle {
+            outputs.append(.init(file: try VirtualPath(path: outputDirectory), type: .pch))
+            try commandLine.appendLast(.pchOutputDir, from: &parsedOptions)
+        }
+        
+        return Job(
+            kind: .generatePCH,
+            tool: .absolute(try toolchain.getToolPath(.swiftCompiler)),
+            commandLine: commandLine,
+            displayInputs: [],
+            inputs: inputs,
+            outputs: outputs
+        )
+    }
+}

Sources/SwiftDriver/Jobs/Job.swift

@@ -20,6 +20,7 @@ public struct Job: Codable, Equatable {
     case generateDSYM = "generate-dsym"
     case autolinkExtract = "autolink-extract"
     case emitModule = "emit-module"
+    case generatePCH = "generate-pch"
     case interpret
     case repl
     case verifyDebugInfo = "verify-debug-info"

Sources/SwiftDriver/Jobs/Planning.swift

@@ -45,6 +45,12 @@ extension Driver {
         }
       }
     }
+    
+    if let importedObjCHeader = importedObjCHeader,
+        parsedOptions.hasFlag(positive: .enableBridgingPch, negative: .disableBridgingPch, default: true),
+        importedObjCHeader.extension == FileType.objcHeader.rawValue {
+        jobs.append(try generatePCHJob(input: .init(file: importedObjCHeader, type: .objcHeader)))
+    }
 
     // If we should create emit module job, do so.
     if shouldCreateEmitModuleJob {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -841,7 +841,7 @@ final class SwiftDriverTests: XCTestCase {
 
   func testMergeModulesOnly() throws {
     do {
-      var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module", "-import-objc-header", "TestInputHeader.h", "-emit-dependencies", "-emit-module-doc-path", "/foo/bar/Test.swiftdoc"])
+      var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module", "-disable-bridging-pch", "-import-objc-header", "TestInputHeader.h", "-emit-dependencies", "-emit-module-doc-path", "/foo/bar/Test.swiftdoc"])
       let plannedJobs = try driver.planBuild()
       XCTAssertEqual(plannedJobs.count, 3)
       XCTAssertEqual(plannedJobs[0].outputs.count, 3)