Index: clang-tidy/ClangTidy.h
===================================================================
--- clang-tidy/ClangTidy.h
+++ clang-tidy/ClangTidy.h
@@ -6,11 +6,14 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+// This file defines the public ClangTidy API exposed to checks and modules.
+//
+// It should remain as stable as possible, as many out-of-tree checks exist.
+//===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_CLANGTIDY_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_CLANGTIDY_H
 
-#include "ClangTidyDiagnosticConsumer.h"
 #include "ClangTidyOptions.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Basic/Diagnostic.h"
@@ -30,6 +33,8 @@
 }
 
 namespace tidy {
+// Checks should treat ClangTidyContext as opaque.
+class ClangTidyContext;
 
 /// \brief Provides access to the ``ClangTidyCheck`` options via check-local
 /// names.
@@ -131,12 +136,7 @@
   /// Derived classes must implement the constructor with this signature or
   /// delegate it. If a check needs to read options, it can do this in the
   /// constructor using the Options.get() methods below.
-  ClangTidyCheck(StringRef CheckName, ClangTidyContext *Context)
-      : CheckName(CheckName), Context(Context),
-        Options(CheckName, Context->getOptions().CheckOptions) {
-    assert(Context != nullptr);
-    assert(!CheckName.empty());
-  }
+  ClangTidyCheck(StringRef CheckName, ClangTidyContext *Context);
 
   /// \brief Override this to register ``PPCallbacks`` with ``Compiler``.
   ///
@@ -181,10 +181,10 @@
 
 protected:
   OptionsView Options;
-  /// \brief Returns the main file name of the current translation unit.
-  StringRef getCurrentMainFile() const { return Context->getCurrentFile(); }
-  /// \brief Returns the language options from the context.
-  LangOptions getLangOpts() const { return Context->getLangOpts(); }
+  /// Returns the main file name of the current translation unit.
+  StringRef getCurrentMainFile() const;
+  /// Returns the language options from the current translation unit.
+  LangOptions getLangOpts() const;
 };
 
 class ClangTidyCheckFactories;
@@ -247,12 +247,6 @@
                   unsigned &WarningsAsErrorsCount,
                   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS);
 
-/// \brief Serializes replacements into YAML and writes them to the specified
-/// output stream.
-void exportReplacements(StringRef MainFilePath,
-                        const std::vector<ClangTidyError> &Errors,
-                        raw_ostream &OS);
-
 } // end namespace tidy
 } // end namespace clang
 
Index: clang-tidy/ClangTidy.cpp
===================================================================
--- clang-tidy/ClangTidy.cpp
+++ clang-tidy/ClangTidy.cpp
@@ -39,9 +39,7 @@
 #include "clang/StaticAnalyzer/Core/BugReporter/PathDiagnostic.h"
 #include "clang/StaticAnalyzer/Frontend/AnalysisConsumer.h"
 #endif // CLANG_ENABLE_STATIC_ANALYZER
-#include "clang/Tooling/DiagnosticsYaml.h"
 #include "clang/Tooling/Refactoring.h"
-#include "clang/Tooling/ReplacementsYaml.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
@@ -364,7 +362,9 @@
     Context.setCurrentBuildDirectory(WorkingDir.get());
 
   std::vector<std::unique_ptr<ClangTidyCheck>> Checks;
-  CheckFactories->createChecks(&Context, Checks);
+  CheckFactories->createChecks(
+      [this](StringRef Name) { return Context.isCheckEnabled(Name); }, &Context,
+      Checks);
 
   ast_matchers::MatchFinder::MatchFinderOptions FinderOptions;
 
@@ -429,12 +429,21 @@
 ClangTidyOptions::OptionMap ClangTidyASTConsumerFactory::getCheckOptions() {
   ClangTidyOptions::OptionMap Options;
   std::vector<std::unique_ptr<ClangTidyCheck>> Checks;
-  CheckFactories->createChecks(&Context, Checks);
+  CheckFactories->createChecks(
+      [this](StringRef Name) { return Context.isCheckEnabled(Name); }, &Context,
+      Checks);
   for (const auto &Check : Checks)
     Check->storeOptions(Options);
   return Options;
 }
 
+ClangTidyCheck::ClangTidyCheck(StringRef CheckName, ClangTidyContext *Context)
+    : CheckName(CheckName), Context(Context),
+      Options(CheckName, Context->getOptions().CheckOptions) {
+  assert(Context != nullptr);
+  assert(!CheckName.empty());
+}
+
 DiagnosticBuilder ClangTidyCheck::diag(SourceLocation Loc, StringRef Message,
                                        DiagnosticIDs::Level Level) {
   return Context->diag(CheckName, Loc, Message, Level);
@@ -445,6 +454,14 @@
   check(Result);
 }
 
+StringRef ClangTidyCheck::getCurrentMainFile() const {
+  return Context->getCurrentFile();
+}
+
+LangOptions ClangTidyCheck::getLangOpts() const {
+  return Context->getLangOpts();
+}
+
 OptionsView::OptionsView(StringRef CheckName,
                          const ClangTidyOptions::OptionMap &CheckOptions)
     : NamePrefix(CheckName.str() + "."), CheckOptions(CheckOptions) {}
@@ -614,19 +631,5 @@
   WarningsAsErrorsCount += Reporter.getWarningsAsErrorsCount();
 }
 
-void exportReplacements(const llvm::StringRef MainFilePath,
-                        const std::vector<ClangTidyError> &Errors,
-                        raw_ostream &OS) {
-  TranslationUnitDiagnostics TUD;
-  TUD.MainSourceFile = MainFilePath;
-  for (const auto &Error : Errors) {
-    tooling::Diagnostic Diag = Error;
-    TUD.Diagnostics.insert(TUD.Diagnostics.end(), Diag);
-  }
-
-  yaml::Output YAML(OS);
-  YAML << TUD;
-}
-
 } // namespace tidy
 } // namespace clang
Index: clang-tidy/ClangTidyDiagnosticConsumer.h
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.h
+++ clang-tidy/ClangTidyDiagnosticConsumer.h
@@ -6,6 +6,9 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+// This file contains private APIs that checks should not depend on.
+// FIXME: the contents have diverged significantly from the name, split this up.
+//===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_CLANGTIDYDIAGNOSTICCONSUMER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_CLANGTIDYDIAGNOSTICCONSUMER_H
@@ -48,6 +51,11 @@
   bool IsWarningAsError;
 };
 
+/// Serializes replacements into YAML and writes them to the specified stream.
+void exportReplacements(StringRef MainFilePath,
+                        const std::vector<ClangTidyError> &Errors,
+                        raw_ostream &OS);
+
 /// \brief Read-only set of strings represented as a list of positive and
 /// negative globs. Positive globs add all matched strings to the set, negative
 /// globs remove them in the order of appearance in the list.
Index: clang-tidy/ClangTidyDiagnosticConsumer.cpp
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -21,6 +21,8 @@
 #include "clang/AST/ASTDiagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Frontend/DiagnosticRenderer.h"
+#include "clang/Tooling/DiagnosticsYaml.h"
+#include "clang/Tooling/ReplacementsYaml.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include <tuple>
@@ -113,6 +115,20 @@
     : tooling::Diagnostic(CheckName, DiagLevel, BuildDirectory),
       IsWarningAsError(IsWarningAsError) {}
 
+void tidy::exportReplacements(const llvm::StringRef MainFilePath,
+                        const std::vector<ClangTidyError> &Errors,
+                        raw_ostream &OS) {
+  clang::tooling::TranslationUnitDiagnostics TUD;
+  TUD.MainSourceFile = MainFilePath;
+  for (const auto &Error : Errors) {
+    tooling::Diagnostic Diag = Error;
+    TUD.Diagnostics.insert(TUD.Diagnostics.end(), Diag);
+  }
+
+  llvm::yaml::Output YAML(OS);
+  YAML << TUD;
+}
+
 // Returns true if GlobList starts with the negative indicator ('-'), removes it
 // from the GlobList.
 static bool ConsumeNegativeIndicator(StringRef &GlobList) {
Index: clang-tidy/ClangTidyModule.h
===================================================================
--- clang-tidy/ClangTidyModule.h
+++ clang-tidy/ClangTidyModule.h
@@ -63,11 +63,11 @@
                          });
   }
 
-  /// \brief Create instances of all checks matching \p CheckRegexString and
-  /// store them in \p Checks.
+  /// Create instances of all checks that match the filter and store them.
   ///
   /// The caller takes ownership of the return \c ClangTidyChecks.
-  void createChecks(ClangTidyContext *Context,
+  void createChecks(llvm::function_ref<bool(StringRef)> Filter,
+                    ClangTidyContext *Context,
                     std::vector<std::unique_ptr<ClangTidyCheck>> &Checks);
 
   typedef std::map<std::string, CheckFactory> FactoryMap;
Index: clang-tidy/ClangTidyModule.cpp
===================================================================
--- clang-tidy/ClangTidyModule.cpp
+++ clang-tidy/ClangTidyModule.cpp
@@ -22,10 +22,11 @@
 }
 
 void ClangTidyCheckFactories::createChecks(
+    llvm::function_ref<bool(StringRef)> Filter,
     ClangTidyContext *Context,
     std::vector<std::unique_ptr<ClangTidyCheck>> &Checks) {
   for (const auto &Factory : Factories) {
-    if (Context->isCheckEnabled(Factory.first))
+    if (Filter(Factory.first))
       Checks.emplace_back(Factory.second(Factory.first, Context));
   }
 }
Index: clang-tidy/google/TodoCommentCheck.cpp
===================================================================
--- clang-tidy/google/TodoCommentCheck.cpp
+++ clang-tidy/google/TodoCommentCheck.cpp
@@ -8,6 +8,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "TodoCommentCheck.h"
+// FIXME: provide some other way to get at the username.
+#include "../ClangTidyDiagnosticConsumer.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Lex/Preprocessor.h"
 
@@ -52,9 +54,10 @@
 };
 
 TodoCommentCheck::TodoCommentCheck(StringRef Name, ClangTidyContext *Context)
-    : ClangTidyCheck(Name, Context),
-      Handler(llvm::make_unique<TodoCommentHandler>(
-          *this, Context->getOptions().User)) {}
+    : ClangTidyCheck(Name, Context) {
+  Handler =
+      llvm::make_unique<TodoCommentHandler>(*this, Context->getOptions().User);
+}
 
 void TodoCommentCheck::registerPPCallbacks(CompilerInstance &Compiler) {
   Compiler.getPreprocessor().addCommentHandler(Handler.get());
Index: clang-tidy/plugin/ClangTidyPlugin.cpp
===================================================================
--- clang-tidy/plugin/ClangTidyPlugin.cpp
+++ clang-tidy/plugin/ClangTidyPlugin.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "../ClangTidy.h"
+#include "../ClangTidyDiagnosticConsumer.h"
 #include "../ClangTidyModule.h"
 #include "clang/Config/config.h"
 #include "clang/Frontend/CompilerInstance.h"
Index: clang-tidy/tool/ClangTidyMain.cpp
===================================================================
--- clang-tidy/tool/ClangTidyMain.cpp
+++ clang-tidy/tool/ClangTidyMain.cpp
@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "../ClangTidy.h"
+#include "../ClangTidyDiagnosticConsumer.h"
 #include "clang/Config/config.h"
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "llvm/Support/Process.h"