Respect the single-input output-file-map entries when emit-module is the sole job.

When the driver is invoked with the primary output of a swift module and we are in the `emit-module-separtely` mode, there will only be a single `emit-module` job, for which we need to respect module-wide supplementary output paths specified in the output file map.

Resolves rdar://84262412

Author: artemcm

Sources/SwiftDriver/Driver/Driver.swift

@@ -2745,6 +2745,16 @@ extension Driver {
       return singleOutputPath
     }
 
+    // The driver lacks a compilerMode for *only* emitting a Swift module, but if the
+    // primary output type is a .swiftmodule and we are using the emit-module-separately
+    // flow, then also consider single output paths specified in the output file-map.
+    if compilerOutputType == .swiftModule &&
+        Driver.shouldEmitModuleSeparately(parsedOptions: &parsedOptions),
+       let singleOutputPath = outputFileMap?.existingOutputForSingleInput(
+           outputType: type) {
+      return singleOutputPath
+    }
+
     // If there is an output argument, derive the name from there.
     if let outputPathArg = parsedOptions.getLastArgument(.o) {
       let path = try VirtualPath(path: outputPathArg.asSingle)

Sources/SwiftDriver/Jobs/EmitModuleJob.swift

@@ -9,6 +9,8 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//
+
+import SwiftOptions
 extension Driver {
   /// Add options that are common to command lines that emit modules, e.g.,
   /// options for the paths of various module files.
@@ -117,13 +119,17 @@ extension Driver {
     }
   }
 
-  /// Returns true if the -emit-module-separately is active.
-  mutating func shouldEmitModuleSeparately() -> Bool {
+  static func shouldEmitModuleSeparately(parsedOptions: inout ParsedOptions) -> Bool {
     return parsedOptions.hasFlag(positive: .emitModuleSeparately,
                                  negative: .noEmitModuleSeparately,
                                  default: true)
-           && !parsedOptions.hasFlag(positive: .wholeModuleOptimization,
-                                     negative: .noWholeModuleOptimization,
-                                     default: false)
+             && !parsedOptions.hasFlag(positive: .wholeModuleOptimization,
+                                       negative: .noWholeModuleOptimization,
+                                       default: false)
+  }
+
+  /// Returns true if the -emit-module-separately is active.
+  mutating func shouldEmitModuleSeparately() -> Bool {
+    return Self.shouldEmitModuleSeparately(parsedOptions: &self.parsedOptions)
   }
 }

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -807,6 +807,29 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertTrue(plannedJobs[0].commandLine.contains(.flag("-serialize-diagnostics-path")))
   }
 
+  func testEmitModuleSepratelyEmittingDiagnosticsWithOutputFileMap() throws {
+    try withTemporaryDirectory { path in
+      let outputFileMap = path.appending(component: "outputFileMap.json")
+      try localFileSystem.writeFileContents(outputFileMap) {
+        $0 <<< """
+        {
+          "": {
+            "diagnostics": "/build/Foo-test.dia"
+          }
+        }
+        """
+      }
+      var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Foo", "-emit-module",
+                                      "-serialize-diagnostics", "-experimental-emit-module-separately",
+                                      "-output-file-map", outputFileMap.description])
+      let plannedJobs = try driver.planBuild().removingAutolinkExtractJobs()
+      
+      XCTAssertEqual(plannedJobs.count, 1)
+      XCTAssertTrue(plannedJobs[0].kind == .emitModule)
+      XCTAssertTrue(plannedJobs[0].commandLine.contains(subsequence: [.flag("-serialize-diagnostics-path"), .path(.absolute(.init("/build/Foo-test.dia")))]))
+    }
+  }
+
   func testReferenceDependencies() throws {
     var driver = try Driver(args: ["swiftc", "foo.swift", "-incremental"])
     let plannedJobs = try driver.planBuild()