AST: Clarify SourceFile's existing missing imports record.

In anticipation of adding a new kind of missing import record to `SourceFile`,
clarify the purpose of the existing "missing imports" record with more specific
naming and documentation.

include/swift/AST/FileUnit.h

@@ -273,8 +273,8 @@ public:
                      ModuleDecl::ImportFilter filter) const {}
 
   /// Lists modules that are not imported from this file and used in API.
-  virtual void
-  getMissingImportedModules(SmallVectorImpl<ImportedModule> &imports) const {}
+  virtual void getImplicitImportsForModuleInterface(
+      SmallVectorImpl<ImportedModule> &imports) const {}
 
   /// \see ModuleDecl::getImportedModulesForLookup
   virtual void getImportedModulesForLookup(

include/swift/AST/Module.h

@@ -1034,8 +1034,8 @@ public:
                           ImportFilter filter = ImportFilterKind::Exported) const;
 
   /// Lists modules that are not imported from a file and used in API.
-  void
-  getMissingImportedModules(SmallVectorImpl<ImportedModule> &imports) const;
+  void getImplicitImportsForModuleInterface(
+      SmallVectorImpl<ImportedModule> &imports) const;
 
   /// Looks up which modules are imported by this module, ignoring any that
   /// won't contain top-level decls.

include/swift/AST/SourceFile.h

@@ -159,6 +159,10 @@ private:
   /// The highest access level of declarations referencing each import.
   llvm::DenseMap<const ModuleDecl *, AccessLevel> ImportsUseAccessLevel;
 
+  /// Imports that should be printed in the module interface even though they
+  /// were not written in the source file.
+  llvm::SmallDenseSet<ImportedModule> ImplicitImportsForModuleInterface;
+
   /// A unique identifier representing this file; used to mark private decls
   /// within the file to keep them from conflicting with other files in the
   /// same module.
@@ -510,12 +514,20 @@ public:
 
   SWIFT_DEBUG_DUMPER(dumpSeparatelyImportedOverlays());
 
-  llvm::SmallDenseSet<ImportedModule> MissingImportedModules;
-
-  void addMissingImportedModule(ImportedModule module) const {
-     const_cast<SourceFile *>(this)->MissingImportedModules.insert(module);
+  /// Record an import that should be printed in the module interface even
+  /// though it was not written in the source file. These imports are needed in
+  /// Swift 5 mode to preserve the integrity of swiftinterface files when code
+  /// publicly use declarations from modules that were \c `@_implementationOnly`
+  /// imported in other source files.
+  void addImplicitImportForModuleInterface(ImportedModule module) {
+    ImplicitImportsForModuleInterface.insert(module);
   }
 
+  /// Gather implicit imports that should printed in swiftinterfaces for
+  /// compatibility with code in some Swift 5 modules.
+  void getImplicitImportsForModuleInterface(
+      SmallVectorImpl<ImportedModule> &imports) const override;
+
   /// Record the source range info for a parsed \c #if clause.
   void recordIfConfigClauseRangeInfo(const IfConfigClauseRangeInfo &range);
 
@@ -527,10 +539,10 @@ public:
   ArrayRef<IfConfigClauseRangeInfo>
   getIfConfigClausesWithin(SourceRange outer) const;
 
-  void getMissingImportedModules(
-         SmallVectorImpl<ImportedModule> &imports) const override;
-
+  /// Record visible declarations for use in code completion and refactoring.
   void cacheVisibleDecls(SmallVectorImpl<ValueDecl *> &&globals) const;
+
+  /// Retrieve visible declarations for use in code completion and refactoring.
   const SmallVectorImpl<ValueDecl *> &getCachedVisibleDecls() const;
 
   virtual void lookupValue(DeclName name, NLKind lookupKind,

lib/AST/Module.cpp

@@ -1720,9 +1720,9 @@ void ModuleDecl::getImportedModules(SmallVectorImpl<ImportedModule> &modules,
   FORWARD(getImportedModules, (modules, filter));
 }
 
-void ModuleDecl::getMissingImportedModules(
+void ModuleDecl::getImplicitImportsForModuleInterface(
     SmallVectorImpl<ImportedModule> &imports) const {
-  FORWARD(getMissingImportedModules, (imports));
+  FORWARD(getImplicitImportsForModuleInterface, (imports));
 }
 
 const llvm::DenseMap<const clang::Module *, ModuleDecl *> &
@@ -1811,9 +1811,9 @@ SourceFile::getImportedModules(SmallVectorImpl<ImportedModule> &modules,
   }
 }
 
-void SourceFile::getMissingImportedModules(
+void SourceFile::getImplicitImportsForModuleInterface(
     SmallVectorImpl<ImportedModule> &modules) const {
-  for (auto module : MissingImportedModules)
+  for (auto module : ImplicitImportsForModuleInterface)
     modules.push_back(module);
 }
 

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -87,7 +87,7 @@ static void printToolVersionAndFlagsComment(raw_ostream &out,
 
     SmallVector<ImportedModule> imports;
     M->getImportedModules(imports, filter);
-    M->getMissingImportedModules(imports);
+    M->getImplicitImportsForModuleInterface(imports);
 
     for (ImportedModule import: imports) {
       StringRef importedName = import.importedModule->getNameStr();
@@ -337,7 +337,7 @@ static void printImports(raw_ostream &out,
   M->getImportedModules(allImports, allImportFilter);
 
   if (Opts.PrintMissingImports)
-    M->getMissingImportedModules(allImports);
+    M->getImplicitImportsForModuleInterface(allImports);
 
   ImportedModule::removeDuplicates(allImports);
   diagnoseScopedImports(ctx.Diags, allImports);

lib/Sema/ResilienceDiagnostics.cpp

@@ -45,7 +45,7 @@ static bool addMissingImport(SourceLoc loc, const Decl *D,
   // API.
   auto missingImport = ImportedModule(ImportPath::Access(),
                                       const_cast<ModuleDecl *>(M));
-  SF->addMissingImportedModule(missingImport);
+  SF->addImplicitImportForModuleInterface(missingImport);
   ctx.Diags.diagnose(loc, diag::missing_import_inserted, M->getName());
   return true;
 }