new file mode 100644
--- /dev/null
+++ b/build/build-clang/clang-tidy-ci.patch
@@ -0,0 +1,26 @@
+diff --git a/clang-tools-extra/clang-tidy/ClangTidy.cpp b/clang-tools-extra/clang-tidy/ClangTidy.cpp
+index 1d813d65f8d..5ef2e6fe9cd 100644
+--- a/clang-tools-extra/clang-tidy/ClangTidy.cpp
++++ b/clang-tools-extra/clang-tidy/ClangTidy.cpp
+@@ -407,6 +407,7 @@ ClangTidyASTConsumerFactory::CreateASTConsumer(
+
+   for (auto &Check : Checks) {
+     Check->registerMatchers(&*Finder);
++    Check->registerPPCallbacks(Compiler);
+     Check->registerPPCallbacks(*SM, PP, ModuleExpanderPP);
+   }
+
+diff --git a/clang-tools-extra/clang-tidy/ClangTidyCheck.h b/clang-tools-extra/clang-tidy/ClangTidyCheck.h
+index 3064a41caa1..9c7eb4815c4 100644
+--- a/clang-tools-extra/clang-tidy/ClangTidyCheck.h
++++ b/clang-tools-extra/clang-tidy/ClangTidyCheck.h
+@@ -53,6 +53,9 @@ public:
+   /// constructor using the Options.get() methods below.
+   ClangTidyCheck(StringRef CheckName, ClangTidyContext *Context);
+
++  /// This has been deprecated in clang 9 - needed by mozilla-must-override
++   virtual void registerPPCallbacks(CompilerInstance &Compiler) {}
++
+   /// \brief Override this to register ``PPCallbacks`` in the preprocessor.
+   ///
+   /// This should be used for clang-tidy checks that analyze preprocessor-

--- a/build/build-clang/clang-tidy-linux64.json
+++ b/build/build-clang/clang-tidy-linux64.json
@@ -5,10 +5,11 @@
     "assertions": false,
     "build_clang_tidy": true,
     "python_path": "/usr/bin/python2.7",
     "gcc_dir": "{MOZ_FETCHES_DIR}/gcc",
     "cc": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
     "cxx": "{MOZ_FETCHES_DIR}/gcc/bin/g++",
     "as": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
     "patches": [
+        "clang-tidy-ci.patch"
     ]
 }

--- a/build/build-clang/clang-tidy-macosx64.json
+++ b/build/build-clang/clang-tidy-macosx64.json
@@ -10,11 +10,12 @@
     "cc": "{MOZ_FETCHES_DIR}/clang/bin/clang",
     "cxx": "{MOZ_FETCHES_DIR}/clang/bin/clang++",
     "as": "{MOZ_FETCHES_DIR}/clang/bin/clang",
     "ar": "{MOZ_FETCHES_DIR}/cctools/bin/x86_64-apple-darwin-ar",
     "ranlib": "{MOZ_FETCHES_DIR}/cctools/bin/x86_64-apple-darwin-ranlib",
     "libtool": "{MOZ_FETCHES_DIR}/cctools/bin/x86_64-apple-darwin-libtool",
     "ld": "{MOZ_FETCHES_DIR}/clang/bin/clang",
     "patches": [
+      "clang-tidy-ci.patch",
       "compiler-rt-no-codesign.patch"
     ]
 }

--- a/build/build-clang/clang-tidy-win64.json
+++ b/build/build-clang/clang-tidy-win64.json
@@ -4,10 +4,11 @@
     "build_type": "Release",
     "assertions": false,
     "build_clang_tidy": true,
     "python_path": "c:/mozilla-build/python/python.exe",
     "cc": "cl.exe",
     "cxx": "cl.exe",
     "ml": "ml64.exe",
     "patches": [
+        "clang-tidy-ci.patch"
     ]
 }

--- a/build/clang-plugin/BaseCheck.h
+++ b/build/clang-plugin/BaseCheck.h
@@ -7,26 +7,25 @@
 
 class MozContext {};
 typedef MozContext ContextType;
 
 class BaseCheck : public MatchFinder::MatchCallback {
 public:
   BaseCheck(StringRef CheckName, ContextType *Context) {}
   virtual void registerMatchers(MatchFinder *Finder) {}
+  virtual void registerPPCallbacks(CompilerInstance &CI) {}
   virtual void check(const MatchFinder::MatchResult &Result) {}
   DiagnosticBuilder diag(SourceLocation Loc, StringRef Description,
                          DiagnosticIDs::Level Level = DiagnosticIDs::Warning) {
     DiagnosticsEngine &Diag = Context->getDiagnostics();
     unsigned ID = Diag.getDiagnosticIDs()->getCustomDiagID(Level, Description);
     return Diag.Report(Loc, ID);
   }
 
-  virtual void registerCompilerInstance(CompilerInstance &CI) {}
-
 private:
   void run(const MatchFinder::MatchResult &Result) override {
     Context = Result.Context;
     check(Result);
   }
 
 private:
   ASTContext *Context;

--- a/build/clang-plugin/DiagnosticsMatcher.cpp
+++ b/build/clang-plugin/DiagnosticsMatcher.cpp
@@ -2,12 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "DiagnosticsMatcher.h"
 
 DiagnosticsMatcher::DiagnosticsMatcher(CompilerInstance &CI) {
 #define CHECK(cls, name)                                                       \
   cls##_.registerMatchers(&AstMatcher);                                        \
-  cls##_.registerCompilerInstance(CI);
+  cls##_.registerPPCallbacks(CI);
 #include "Checks.inc"
 #undef CHECK
 }

--- a/build/clang-plugin/MustOverrideChecker.cpp
+++ b/build/clang-plugin/MustOverrideChecker.cpp
@@ -4,17 +4,17 @@
 
 #include "MustOverrideChecker.h"
 #include "CustomMatchers.h"
 
 void MustOverrideChecker::registerMatchers(MatchFinder *AstMatcher) {
   AstMatcher->addMatcher(cxxRecordDecl(isDefinition()).bind("class"), this);
 }
 
-void MustOverrideChecker::registerCompilerInstance(CompilerInstance &CI) {
+void MustOverrideChecker::registerPPCallbacks(CompilerInstance &CI) {
   this->CI = &CI;
 }
 
 void MustOverrideChecker::check(const MatchFinder::MatchResult &Result) {
   auto D = Result.Nodes.getNodeAs<CXXRecordDecl>("class");
 
   // Look through all of our immediate bases to find methods that need to be
   // overridden

--- a/build/clang-plugin/MustOverrideChecker.h
+++ b/build/clang-plugin/MustOverrideChecker.h
@@ -7,16 +7,16 @@
 
 #include "plugin.h"
 
 class MustOverrideChecker : public BaseCheck {
 public:
   MustOverrideChecker(StringRef CheckName, ContextType *Context = nullptr)
       : BaseCheck(CheckName, Context), CI(nullptr) {}
   void registerMatchers(MatchFinder *AstMatcher) override;
-  void registerCompilerInstance(CompilerInstance &CI) override;
+  void registerPPCallbacks(CompilerInstance &CI) override;
   void check(const MatchFinder::MatchResult &Result) override;
 
 private:
   const CompilerInstance *CI;
 };
 
 #endif

--- a/build/clang-plugin/plugin.h
+++ b/build/clang-plugin/plugin.h
@@ -39,25 +39,15 @@ typedef ASTConsumer *ASTConsumerPtr;
 #define getEndLoc getLocEnd
 #endif
 
 // In order to support running our checks using clang-tidy, we implement a
 // source compatible base check class called BaseCheck, and we use the
 // preprocessor to decide which base class to pick.
 #ifdef CLANG_TIDY
 #include "../ClangTidy.h"
-typedef clang::tidy::ClangTidyCheck UpstreamBaseCheck;
+typedef clang::tidy::ClangTidyCheck BaseCheck;
 typedef clang::tidy::ClangTidyContext ContextType;
-
-class BaseCheck : public UpstreamBaseCheck {
-public:
-  BaseCheck(StringRef CheckName, ContextType *Context = nullptr)
-      : UpstreamBaseCheck(CheckName, Context) {}
-
-  // Additional functionality needed by Mozilla code
-  virtual void registerCompilerInstance(CompilerInstance &CI) {}
-};
-
 #else
 #include "BaseCheck.h"
 #endif
 
 #endif // plugin_h__