Emit private swift interfaces as long as public interfaces are emitted

With the introduction of features like @_spi_available, we may print public and private interfaces differently even from the same codebase witout @_spi.
For this reason, we should always print private interfaces so that we don’t mix the public interfaces with private Clang modules.

rdar://91061334

Author: nkcsgexi

Sources/SwiftDriver/Driver/Driver.swift

@@ -767,7 +767,7 @@ public struct Driver {
         emitModuleSeparately: emitModuleSeparately,
         outputFileMap: self.outputFileMap,
         moduleName: moduleOutputInfo.name)
-    self.swiftPrivateInterfacePath = try Self.computeSupplementaryOutputPath(
+    let givenPrivateInterfacePath = try Self.computeSupplementaryOutputPath(
         &parsedOptions, type: .privateSwiftInterface, isOutputOptions: [],
         outputPath: .emitPrivateModuleInterfacePath,
         compilerOutputType: compilerOutputType,
@@ -776,6 +776,19 @@ public struct Driver {
         outputFileMap: self.outputFileMap,
         moduleName: moduleOutputInfo.name)
 
+    // Always emitting private swift interfaces if public interfaces are emitted.'
+    // With the introduction of features like @_spi_available, we may print public
+    // and private interfaces differently even from the same codebase. For this reason,
+    // we should always print private interfaces so that we don’t mix the public interfaces
+    // with private Clang modules.
+    if let swiftInterfacePath = self.swiftInterfacePath,
+        givenPrivateInterfacePath == nil {
+      self.swiftPrivateInterfacePath = VirtualPath.lookup(swiftInterfacePath)
+        .replacingExtension(with: .privateSwiftInterface).intern()
+    } else {
+      self.swiftPrivateInterfacePath = givenPrivateInterfacePath
+    }
+
     var optimizationRecordFileType = FileType.yamlOptimizationRecord
     if let argument = parsedOptions.getLastArgument(.saveOptimizationRecordEQ)?.asSingle {
       switch argument {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2292,6 +2292,18 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+  func testPrivateInterfacePathImplicit() throws {
+    var driver1 = try Driver(args: ["swiftc", "foo.swift", "-emit-module", "-module-name",
+                                   "foo", "-emit-module-interface",
+                                   "-enable-library-evolution"])
+
+    let plannedJobs = try driver1.planBuild()
+    XCTAssertEqual(plannedJobs.count, 2)
+    let emitInterfaceJob = plannedJobs[0]
+    XCTAssertTrue(emitInterfaceJob.commandLine.contains(.flag("-emit-module-interface-path")))
+    XCTAssertTrue(emitInterfaceJob.commandLine.contains(.flag("-emit-private-module-interface-path")))
+  }
+
   func testSingleThreadedWholeModuleOptimizationCompiles() throws {
     var envVars = ProcessEnv.vars
     envVars["SWIFT_DRIVER_LD_EXEC"] = ld.nativePathString(escaped: false)