[Frontend] Factor out textual interface generation into its own file

No functionality change.

Author: jrose-apple

include/swift/AST/PrintOptions.h

@@ -424,6 +424,12 @@ struct PrintOptions {
     return result;
   }
 
+  /// Retrieve the set of options suitable for stable textual interfaces.
+  ///
+  /// This is a format that will be parsed again later, so the output must be
+  /// consistent and well-formed.
+  ///
+  /// \see swift::emitModuleInterface
   static PrintOptions printTextualInterfaceFile();
 
   static PrintOptions printModuleInterface();

include/swift/Basic/SupplementaryOutputPaths.h

@@ -121,7 +121,7 @@ struct SupplementaryOutputPaths {
   ///
   /// Currently only makes sense when the compiler has whole-module knowledge.
   ///
-  /// \sa ModuleOutputPath
+  /// \sa swift::emitModuleInterface
   std::string ModuleInterfaceOutputPath;
 
   SupplementaryOutputPaths() = default;

lib/FrontendTool/CMakeLists.txt

@@ -3,6 +3,7 @@ add_swift_library(swiftFrontendTool STATIC
   ImportedModules.cpp
   ReferenceDependencies.cpp
   TBD.cpp
+  TextualInterfaceGeneration.cpp
   DEPENDS
     swift-syntax-generated-headers SwiftOptions
   LINK_LIBRARIES

lib/FrontendTool/FrontendTool.cpp

@@ -24,6 +24,7 @@
 #include "ImportedModules.h"
 #include "ReferenceDependencies.h"
 #include "TBD.h"
+#include "TextualInterfaceGeneration.h"
 
 #include "swift/Subsystems.h"
 #include "swift/AST/ASTScope.h"
@@ -358,21 +359,14 @@ static bool printAsObjCIfNeeded(StringRef outputPath, ModuleDecl *M,
 /// ...unless \p outputPath is empty, in which case it does nothing.
 ///
 /// \returns true if there were any errors
+///
+/// \see swift::emitModuleInterface
 static bool printModuleInterfaceIfNeeded(StringRef outputPath, ModuleDecl *M) {
   if (outputPath.empty())
     return false;
   return atomicallyWritingToTextFile(outputPath, M->getDiags(),
-                                     [&](raw_ostream &out) -> bool {
-    auto printOptions = PrintOptions::printTextualInterfaceFile();
-    SmallVector<Decl *, 16> topLevelDecls;
-    M->getTopLevelDecls(topLevelDecls);
-    for (const Decl *D : topLevelDecls) {
-      if (!D->shouldPrintInContext(printOptions))
-        continue;
-      D->print(out, printOptions);
-      out << "\n";
-    }
-    return false;
+                                     [M](raw_ostream &out) -> bool {
+    return swift::emitModuleInterface(out, M);
   });
 }
 

lib/FrontendTool/TextualInterfaceGeneration.cpp

@@ -0,0 +1,31 @@
+//===--- TextualInterfaceGeneration.cpp - swiftinterface files ------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2018 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "TextualInterfaceGeneration.h"
+
+#include "swift/AST/Decl.h"
+#include "swift/AST/Module.h"
+
+using namespace swift;
+
+bool swift::emitModuleInterface(raw_ostream &out, ModuleDecl *M) {
+  const PrintOptions printOptions = PrintOptions::printTextualInterfaceFile();
+  SmallVector<Decl *, 16> topLevelDecls;
+  M->getTopLevelDecls(topLevelDecls);
+  for (const Decl *D : topLevelDecls) {
+    if (!D->shouldPrintInContext(printOptions))
+      continue;
+    D->print(out, printOptions);
+    out << "\n";
+  }
+  return false;
+}

lib/FrontendTool/TextualInterfaceGeneration.h

@@ -0,0 +1,39 @@
+//===--- TextualInterfaceGeneration.h - swiftinterface files ----*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2018 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_FRONTENDTOOL_TEXTUALINTERFACEGENERATION_H
+#define SWIFT_FRONTENDTOOL_TEXTUALINTERFACEGENERATION_H
+
+#include "swift/Basic/LLVM.h"
+
+namespace swift {
+
+class ModuleDecl;
+
+/// Emit a stable, textual interface for \p M, which can be used by a client
+/// source file to import this module.
+///
+/// Unlike a serialized module, the textual format generated by
+/// emitModuleInterface is intended to be stable across compiler versions while
+/// still describing the full ABI of the module in question.
+///
+/// The initial plan for this format can be found at
+/// https://forums.swift.org/t/plan-for-module-stability/14551/
+///
+/// \return true if an error occurred
+///
+/// \sa swift::serialize
+bool emitModuleInterface(raw_ostream &out, ModuleDecl *M);
+
+} // end namespace swift
+
+#endif