diff --git a/clang-tools-extra/clang-tidy/.clang-tidy b/clang-tools-extra/clang-tidy/.clang-tidy
index 0c2f34b529016..2cd9af494c1ec 100644
--- a/clang-tools-extra/clang-tidy/.clang-tidy
+++ b/clang-tools-extra/clang-tidy/.clang-tidy
@@ -15,7 +15,6 @@ Checks: >
   performance-*,
   -performance-enum-size,
   -performance-no-int-to-ptr,
-  -performance-unnecessary-value-param,
   readability-*,
   -readability-avoid-nested-conditional-operator,
   -readability-braces-around-statements,
diff --git a/clang-tools-extra/clang-tidy/ClangTidy.cpp b/clang-tools-extra/clang-tidy/ClangTidy.cpp
index 7e18f3806a143..e9fd6da15e1dc 100644
--- a/clang-tools-extra/clang-tidy/ClangTidy.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidy.cpp
@@ -450,8 +450,8 @@ ClangTidyASTConsumerFactory::createASTConsumer(
 
   if (Context.canEnableModuleHeadersParsing() &&
       Context.getLangOpts().Modules && OverlayFS != nullptr) {
-    auto ModuleExpander =
-        std::make_unique<ExpandModularHeadersPPCallbacks>(&Compiler, OverlayFS);
+    auto ModuleExpander = std::make_unique<ExpandModularHeadersPPCallbacks>(
+        &Compiler, *OverlayFS);
     ModuleExpanderPP = ModuleExpander->getPreprocessor();
     PP->addPPCallbacks(std::move(ModuleExpander));
   }
diff --git a/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp b/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp
index 487e5e299d132..9a4fc7a30b472 100644
--- a/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp
+++ b/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp
@@ -65,8 +65,7 @@ class ExpandModularHeadersPPCallbacks::FileRecorder {
 };
 
 ExpandModularHeadersPPCallbacks::ExpandModularHeadersPPCallbacks(
-    CompilerInstance *CI,
-    IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> OverlayFS)
+    CompilerInstance *CI, llvm::vfs::OverlayFileSystem &OverlayFS)
     : Recorder(std::make_unique<FileRecorder>()), Compiler(*CI),
       InMemoryFs(new llvm::vfs::InMemoryFileSystem),
       Sources(Compiler.getSourceManager()),
@@ -76,7 +75,7 @@ ExpandModularHeadersPPCallbacks::ExpandModularHeadersPPCallbacks(
       LangOpts(Compiler.getLangOpts()), HSOpts(Compiler.getHeaderSearchOpts()) {
   // Add a FileSystem containing the extra files needed in place of modular
   // headers.
-  OverlayFS->pushOverlay(InMemoryFs);
+  OverlayFS.pushOverlay(InMemoryFs);
 
   Diags.setSourceManager(&Sources);
   // FIXME: Investigate whatever is there better way to initialize DiagEngine
diff --git a/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.h b/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.h
index e599bda92c25c..0cfdfa9a1f6cf 100644
--- a/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.h
+++ b/clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.h
@@ -41,9 +41,8 @@ namespace tooling {
 /// non-modular way.
 class ExpandModularHeadersPPCallbacks : public PPCallbacks {
 public:
-  ExpandModularHeadersPPCallbacks(
-      CompilerInstance *CI,
-      IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> OverlayFS);
+  ExpandModularHeadersPPCallbacks(CompilerInstance *CI,
+                                  llvm::vfs::OverlayFileSystem &OverlayFS);
   ~ExpandModularHeadersPPCallbacks();
 
   /// Returns the preprocessor that provides callbacks for the whole
diff --git a/clang-tools-extra/clang-tidy/android/CloexecCheck.cpp b/clang-tools-extra/clang-tidy/android/CloexecCheck.cpp
index cd83423adae05..c9c077817773c 100644
--- a/clang-tools-extra/clang-tidy/android/CloexecCheck.cpp
+++ b/clang-tools-extra/clang-tidy/android/CloexecCheck.cpp
@@ -39,7 +39,7 @@ const char *CloexecCheck::FuncDeclBindingStr = "funcDecl";
 const char *CloexecCheck::FuncBindingStr = "func";
 
 void CloexecCheck::registerMatchersImpl(
-    MatchFinder *Finder, internal::Matcher<FunctionDecl> Function) {
+    MatchFinder *Finder, const internal::Matcher<FunctionDecl> &Function) {
   // We assume all the checked APIs are C functions.
   Finder->addMatcher(
       callExpr(
diff --git a/clang-tools-extra/clang-tidy/android/CloexecCheck.h b/clang-tools-extra/clang-tidy/android/CloexecCheck.h
index 79f7ab3354d8d..c700348cb2ff4 100644
--- a/clang-tools-extra/clang-tidy/android/CloexecCheck.h
+++ b/clang-tools-extra/clang-tidy/android/CloexecCheck.h
@@ -29,9 +29,9 @@ class CloexecCheck : public ClangTidyCheck {
       : ClangTidyCheck(Name, Context) {}
 
 protected:
-  void
-  registerMatchersImpl(ast_matchers::MatchFinder *Finder,
-                       ast_matchers::internal::Matcher<FunctionDecl> Function);
+  void registerMatchersImpl(
+      ast_matchers::MatchFinder *Finder,
+      const ast_matchers::internal::Matcher<FunctionDecl> &Function);
 
   /// Currently, we have three types of fixes.
   ///
diff --git a/clang-tools-extra/clang-tidy/boost/UseRangesCheck.cpp b/clang-tools-extra/clang-tidy/boost/UseRangesCheck.cpp
index 34ecee5badb15..93ac20b0f1beb 100644
--- a/clang-tools-extra/clang-tidy/boost/UseRangesCheck.cpp
+++ b/clang-tools-extra/clang-tidy/boost/UseRangesCheck.cpp
@@ -18,6 +18,7 @@
 #include <initializer_list>
 #include <optional>
 #include <string>
+#include <utility>
 
 // FixItHint - Let the docs script know that this class does provide fixits
 
@@ -217,11 +218,11 @@ utils::UseRangesCheck::ReplacerMap UseRangesCheck::getReplacerMap() const {
   const auto AddFromStd =
       [&](llvm::IntrusiveRefCntPtr<UseRangesCheck::Replacer> Replacer,
           std::initializer_list<StringRef> Names) {
-        AddFrom(Replacer, Names, "std");
+        AddFrom(std::move(Replacer), Names, "std");
       };
 
   const auto AddFromBoost =
-      [&](llvm::IntrusiveRefCntPtr<UseRangesCheck::Replacer> Replacer,
+      [&](const llvm::IntrusiveRefCntPtr<UseRangesCheck::Replacer> &Replacer,
           std::initializer_list<
               std::pair<StringRef, std::initializer_list<StringRef>>>
               NamespaceAndNames) {
diff --git a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
index 86af5cbd94374..0c88e695aca52 100644
--- a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
@@ -435,7 +435,7 @@ void SignalHandlerCheck::check(const MatchFinder::MatchResult &Result) {
 
 bool SignalHandlerCheck::checkFunction(
     const FunctionDecl *FD, const Expr *CallOrRef,
-    std::function<void(bool)> ChainReporter) {
+    llvm::function_ref<void(bool)> ChainReporter) {
   bool FunctionIsCalled = isa<CallExpr>(CallOrRef);
 
   if (isStandardFunction(FD)) {
@@ -471,7 +471,7 @@ bool SignalHandlerCheck::checkFunction(
 
 bool SignalHandlerCheck::checkFunctionCPP14(
     const FunctionDecl *FD, const Expr *CallOrRef,
-    std::function<void(bool)> ChainReporter) {
+    llvm::function_ref<void(bool)> ChainReporter) {
   if (!FD->isExternC()) {
     diag(CallOrRef->getBeginLoc(),
          "functions without C linkage are not allowed as signal "
diff --git a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.h b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.h
index b5317793cbf45..703e92baac8e4 100644
--- a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.h
+++ b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.h
@@ -48,10 +48,10 @@ class SignalHandlerCheck : public ClangTidyCheck {
   /// The bool parameter is used like \c SkipPathEnd in \c reportHandlerChain .
   /// \return Returns true if a diagnostic was emitted for this function.
   bool checkFunction(const FunctionDecl *FD, const Expr *CallOrRef,
-                     std::function<void(bool)> ChainReporter);
+                     llvm::function_ref<void(bool)> ChainReporter);
   /// Similar as \c checkFunction but only check for C++14 rules.
   bool checkFunctionCPP14(const FunctionDecl *FD, const Expr *CallOrRef,
-                          std::function<void(bool)> ChainReporter);
+                          llvm::function_ref<void(bool)> ChainReporter);
   /// Returns true if a standard library function is considered
   /// asynchronous-safe.
   bool isStandardFunctionAsyncSafe(const FunctionDecl *FD) const;
diff --git a/clang-tools-extra/clang-tidy/bugprone/UnusedRaiiCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/UnusedRaiiCheck.cpp
index dae679baf14e5..6502fc9bfb89e 100644
--- a/clang-tools-extra/clang-tidy/bugprone/UnusedRaiiCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/UnusedRaiiCheck.cpp
@@ -37,8 +37,8 @@ void UnusedRaiiCheck::registerMatchers(MatchFinder *Finder) {
 }
 
 template <typename T>
-static void reportDiagnostic(DiagnosticBuilder D, const T *Node, SourceRange SR,
-                             bool DefaultConstruction) {
+static void reportDiagnostic(const DiagnosticBuilder &D, const T *Node,
+                             SourceRange SR, bool DefaultConstruction) {
   const char *Replacement = " give_me_a_name";
 
   // If this is a default ctor we have to remove the parens or we'll introduce a
diff --git a/clang-tools-extra/clang-tidy/cert/NonTrivialTypesLibcMemoryCallsCheck.cpp b/clang-tools-extra/clang-tidy/cert/NonTrivialTypesLibcMemoryCallsCheck.cpp
index e266cf995e8a7..c69c0cd954318 100644
--- a/clang-tools-extra/clang-tidy/cert/NonTrivialTypesLibcMemoryCallsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cert/NonTrivialTypesLibcMemoryCallsCheck.cpp
@@ -65,7 +65,8 @@ void NonTrivialTypesLibcMemoryCallsCheck::storeOptions(
 void NonTrivialTypesLibcMemoryCallsCheck::registerMatchers(
     MatchFinder *Finder) {
   using namespace ast_matchers::internal;
-  auto IsStructPointer = [](Matcher<CXXRecordDecl> Constraint = anything(),
+  auto IsStructPointer = [](const Matcher<CXXRecordDecl> &Constraint =
+                                anything(),
                             bool Bind = false) {
     return expr(unaryOperator(
         hasOperatorName("&"),
@@ -76,7 +77,7 @@ void NonTrivialTypesLibcMemoryCallsCheck::registerMatchers(
   auto IsRecordSizeOf =
       expr(sizeOfExpr(hasArgumentOfType(equalsBoundNode("Record"))));
   auto ArgChecker = [&](Matcher<CXXRecordDecl> RecordConstraint,
-                        BindableMatcher<Stmt> SecondArg = expr()) {
+                        const BindableMatcher<Stmt> &SecondArg = expr()) {
     return allOf(argumentCountIs(3),
                  hasArgument(0, IsStructPointer(RecordConstraint, true)),
                  hasArgument(1, SecondArg), hasArgument(2, IsRecordSizeOf));
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h b/clang-tools-extra/clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
index 8cdaf315eac52..81f707464ed2f 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
@@ -93,7 +93,7 @@ struct DenseMapInfo<
             "TOMBSTONE"};
   }
 
-  static unsigned getHashValue(ClassDefId Val) {
+  static unsigned getHashValue(const ClassDefId &Val) {
     assert(Val != getEmptyKey() && "Cannot hash the empty key!");
     assert(Val != getTombstoneKey() && "Cannot hash the tombstone key!");
 
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/RestrictSystemLibcHeadersCheck.cpp b/clang-tools-extra/clang-tidy/llvmlibc/RestrictSystemLibcHeadersCheck.cpp
index 129b8a9a30a59..ecd8e19b8b2c6 100644
--- a/clang-tools-extra/clang-tidy/llvmlibc/RestrictSystemLibcHeadersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/llvmlibc/RestrictSystemLibcHeadersCheck.cpp
@@ -22,11 +22,11 @@ namespace {
 class RestrictedIncludesPPCallbacks
     : public portability::RestrictedIncludesPPCallbacks {
 public:
-  explicit RestrictedIncludesPPCallbacks(
-      RestrictSystemLibcHeadersCheck &Check, const SourceManager &SM,
-      const SmallString<128> CompilerIncudeDir)
+  explicit RestrictedIncludesPPCallbacks(RestrictSystemLibcHeadersCheck &Check,
+                                         const SourceManager &SM,
+                                         SmallString<128> CompilerIncudeDir)
       : portability::RestrictedIncludesPPCallbacks(Check, SM),
-        CompilerIncudeDir(CompilerIncudeDir) {}
+        CompilerIncudeDir(std::move(CompilerIncudeDir)) {}
 
   void InclusionDirective(SourceLocation HashLoc, const Token &IncludeTok,
                           StringRef FileName, bool IsAngled,
@@ -61,7 +61,7 @@ void RestrictSystemLibcHeadersCheck::registerPPCallbacks(
       StringRef(PP->getHeaderSearchInfo().getHeaderSearchOpts().ResourceDir);
   llvm::sys::path::append(CompilerIncudeDir, "include");
   PP->addPPCallbacks(std::make_unique<RestrictedIncludesPPCallbacks>(
-      *this, SM, CompilerIncudeDir));
+      *this, SM, std::move(CompilerIncudeDir)));
 }
 
 } // namespace clang::tidy::llvm_libc
diff --git a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
index 17a8a50ff04ac..1c7628010b140 100644
--- a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
@@ -1144,16 +1144,18 @@ void RedundantExpressionCheck::checkArithmeticExpr(
   }
 }
 
-static bool exprEvaluatesToZero(BinaryOperatorKind Opcode, APSInt Value) {
+static bool exprEvaluatesToZero(BinaryOperatorKind Opcode,
+                                const APSInt &Value) {
   return (Opcode == BO_And || Opcode == BO_AndAssign) && Value == 0;
 }
 
 static bool exprEvaluatesToBitwiseNegatedZero(BinaryOperatorKind Opcode,
-                                              APSInt Value) {
+                                              const APSInt &Value) {
   return (Opcode == BO_Or || Opcode == BO_OrAssign) && ~Value == 0;
 }
 
-static bool exprEvaluatesToSymbolic(BinaryOperatorKind Opcode, APSInt Value) {
+static bool exprEvaluatesToSymbolic(BinaryOperatorKind Opcode,
+                                    const APSInt &Value) {
   return ((Opcode == BO_Or || Opcode == BO_OrAssign) && Value == 0) ||
          ((Opcode == BO_And || Opcode == BO_AndAssign) && ~Value == 0);
 }
diff --git a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
index 37482583760f2..83a9ddaf1f5e8 100644
--- a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
@@ -92,7 +92,7 @@ static StatementMatcher incrementVarMatcher() {
 }
 
 static StatementMatcher
-arrayConditionMatcher(internal::Matcher<Expr> LimitExpr) {
+arrayConditionMatcher(const internal::Matcher<Expr> &LimitExpr) {
   return binaryOperator(
       anyOf(allOf(hasOperatorName("<"), hasLHS(integerComparisonMatcher()),
                   hasRHS(LimitExpr)),
diff --git a/clang-tools-extra/clang-tidy/modernize/UseStdNumbersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseStdNumbersCheck.cpp
index a04f78c271d42..de2cbadb7f22b 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseStdNumbersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseStdNumbersCheck.cpp
@@ -81,16 +81,17 @@ AST_MATCHER_P(clang::Expr, anyOfExhaustive, std::vector<Matcher<clang::Stmt>>,
 // literals.
 struct MatchBuilder {
   auto
-  ignoreParenAndArithmeticCasting(const Matcher<clang::Expr> Matcher) const {
+  ignoreParenAndArithmeticCasting(const Matcher<clang::Expr> &Matcher) const {
     return expr(hasType(qualType(isArithmetic())), ignoringParenCasts(Matcher));
   }
 
-  auto ignoreParenAndFloatingCasting(const Matcher<clang::Expr> Matcher) const {
+  auto
+  ignoreParenAndFloatingCasting(const Matcher<clang::Expr> &Matcher) const {
     return expr(hasType(qualType(isFloating())), ignoringParenCasts(Matcher));
   }
 
   auto matchMathCall(const StringRef FunctionName,
-                     const Matcher<clang::Expr> ArgumentMatcher) const {
+                     const Matcher<clang::Expr> &ArgumentMatcher) const {
     auto HasAnyPrecisionName = hasAnyName(
         FunctionName, (FunctionName + "l").str(),
         (FunctionName + "f").str()); // Support long double(l) and float(f).
@@ -100,7 +101,7 @@ struct MatchBuilder {
                  hasArgument(0, ArgumentMatcher))));
   }
 
-  auto matchSqrt(const Matcher<clang::Expr> ArgumentMatcher) const {
+  auto matchSqrt(const Matcher<clang::Expr> &ArgumentMatcher) const {
     return matchMathCall("sqrt", ArgumentMatcher);
   }
 
@@ -148,7 +149,7 @@ struct MatchBuilder {
     return expr(anyOf(Int, Float, Dref));
   }
 
-  auto match1Div(const Matcher<clang::Expr> Match) const {
+  auto match1Div(const Matcher<clang::Expr> &Match) const {
     return binaryOperator(hasOperatorName("/"), hasLHS(matchValue(1)),
                           hasRHS(Match));
   }
diff --git a/clang-tools-extra/clang-tidy/modernize/UseStdPrintCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseStdPrintCheck.cpp
index 99ade046305c1..22dc0683ac348 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseStdPrintCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseStdPrintCheck.cpp
@@ -70,8 +70,8 @@ void UseStdPrintCheck::registerPPCallbacks(const SourceManager &SM,
   this->PP = PP;
 }
 
-static clang::ast_matchers::StatementMatcher
-unusedReturnValue(clang::ast_matchers::StatementMatcher MatchedCallExpr) {
+static clang::ast_matchers::StatementMatcher unusedReturnValue(
+    const clang::ast_matchers::StatementMatcher &MatchedCallExpr) {
   auto UnusedInCompoundStmt =
       compoundStmt(forEach(MatchedCallExpr),
                    // The checker can't currently differentiate between the
diff --git a/clang-tools-extra/clang-tidy/portability/RestrictSystemIncludesCheck.h b/clang-tools-extra/clang-tidy/portability/RestrictSystemIncludesCheck.h
index e37f89336bc92..29e6c4b972c10 100644
--- a/clang-tools-extra/clang-tidy/portability/RestrictSystemIncludesCheck.h
+++ b/clang-tools-extra/clang-tidy/portability/RestrictSystemIncludesCheck.h
@@ -23,7 +23,7 @@ namespace clang::tidy::portability {
 class RestrictSystemIncludesCheck : public ClangTidyCheck {
 public:
   RestrictSystemIncludesCheck(StringRef Name, ClangTidyContext *Context,
-                              std::string DefaultAllowedIncludes = "*")
+                              StringRef DefaultAllowedIncludes = "*")
       : ClangTidyCheck(Name, Context),
         AllowedIncludes(Options.get("Includes", DefaultAllowedIncludes)),
         AllowedIncludesGlobList(AllowedIncludes) {}
@@ -36,7 +36,7 @@ class RestrictSystemIncludesCheck : public ClangTidyCheck {
   }
 
 private:
-  std::string AllowedIncludes;
+  StringRef AllowedIncludes;
   GlobList AllowedIncludesGlobList;
 };
 
diff --git a/clang-tools-extra/clang-tidy/readability/ContainerContainsCheck.cpp b/clang-tools-extra/clang-tidy/readability/ContainerContainsCheck.cpp
index 850ef86c85b17..c979c8b7894ab 100644
--- a/clang-tools-extra/clang-tidy/readability/ContainerContainsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/ContainerContainsCheck.cpp
@@ -47,9 +47,8 @@ void ContainerContainsCheck::registerMatchers(MatchFinder *Finder) {
   const auto StringNpos = anyOf(declRefExpr(to(varDecl(hasName("npos")))),
                                 memberExpr(member(hasName("npos"))));
 
-  auto AddSimpleMatcher = [&](auto Matcher) {
-    Finder->addMatcher(
-        traverse(TK_IgnoreUnlessSpelledInSource, std::move(Matcher)), this);
+  auto AddSimpleMatcher = [&](const auto &Matcher) {
+    Finder->addMatcher(traverse(TK_IgnoreUnlessSpelledInSource, Matcher), this);
   };
 
   // Find membership tests which use `count()`.
diff --git a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
index 64157f530b8c0..71746377d706f 100644
--- a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
+++ b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
@@ -464,10 +464,9 @@ createOptionsProvider(llvm::IntrusiveRefCntPtr<vfs::FileSystem> FS) {
 }
 
 static llvm::IntrusiveRefCntPtr<vfs::FileSystem>
-getVfsFromFile(const std::string &OverlayFile,
-               llvm::IntrusiveRefCntPtr<vfs::FileSystem> BaseFS) {
+getVfsFromFile(const std::string &OverlayFile, vfs::FileSystem &BaseFS) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
-      BaseFS->getBufferForFile(OverlayFile);
+      BaseFS.getBufferForFile(OverlayFile);
   if (!Buffer) {
     llvm::errs() << "Can't load virtual filesystem overlay file '"
                  << OverlayFile << "': " << Buffer.getError().message()
@@ -583,7 +582,7 @@ static llvm::IntrusiveRefCntPtr<vfs::OverlayFileSystem> createBaseFS() {
 
   if (!VfsOverlay.empty()) {
     IntrusiveRefCntPtr<vfs::FileSystem> VfsFromFile =
-        getVfsFromFile(VfsOverlay, BaseFS);
+        getVfsFromFile(VfsOverlay, *BaseFS);
     if (!VfsFromFile)
       return nullptr;
     BaseFS->pushOverlay(std::move(VfsFromFile));
diff --git a/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.cpp b/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.cpp
index 14770c49c2e25..9e462fb18cbc9 100644
--- a/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.cpp
+++ b/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.cpp
@@ -139,7 +139,7 @@ BraceInsertionHints getBraceInsertionsHints(const Stmt *const S,
 
   // StartLoc points at the location of the opening brace to be inserted.
   SourceLocation EndLoc;
-  std::string ClosingInsertion;
+  StringRef ClosingInsertion;
   if (EndLocHint.isValid()) {
     EndLoc = EndLocHint;
     ClosingInsertion = "} ";
diff --git a/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.h b/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.h
index 699d75435db7b..456702acf5845 100644
--- a/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.h
+++ b/clang-tools-extra/clang-tidy/utils/BracesAroundStatement.h
@@ -36,7 +36,7 @@ struct BraceInsertionHints {
   /// Constructor for a hint offering fix-its for brace insertion. Both
   /// positions must be valid.
   BraceInsertionHints(SourceLocation OpeningBracePos,
-                      SourceLocation ClosingBracePos, std::string ClosingBrace)
+                      SourceLocation ClosingBracePos, StringRef ClosingBrace)
       : DiagnosticPos(OpeningBracePos), OpeningBracePos(OpeningBracePos),
         ClosingBracePos(ClosingBracePos), ClosingBrace(ClosingBrace) {
     assert(offersFixIts());
@@ -61,7 +61,7 @@ struct BraceInsertionHints {
 private:
   SourceLocation OpeningBracePos;
   SourceLocation ClosingBracePos;
-  std::string ClosingBrace;
+  StringRef ClosingBrace;
 };
 
 /// Create fix-it hints for braces that wrap the given statement when applied.
diff --git a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
index fd4320eb8144b..65c8a92cf809e 100644
--- a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
+++ b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
@@ -193,7 +193,7 @@ bool isFunctionPointerConvertible(QualType From, QualType To) {
 //
 // The function should only be called in C++ mode.
 bool isQualificationConvertiblePointer(QualType From, QualType To,
-                                       LangOptions LangOpts) {
+                                       const LangOptions &LangOpts) {
 
   // [N4659 7.5 (1)]
   // A cv-decomposition of a type T is a sequence of cv_i and P_i such that T is
diff --git a/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.cpp b/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.cpp
index 87602d1187d59..df2bc2c8658bd 100644
--- a/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.cpp
+++ b/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.cpp
@@ -66,7 +66,7 @@ TransformerClangTidyCheck::TransformerClangTidyCheck(StringRef Name,
 // we would be accessing `getLangOpts` and `Options` before the underlying
 // `ClangTidyCheck` instance was properly initialized.
 TransformerClangTidyCheck::TransformerClangTidyCheck(
-    std::function<std::optional<RewriteRuleWith<std::string>>(
+    llvm::function_ref<std::optional<RewriteRuleWith<std::string>>(
         const LangOptions &, const OptionsView &)>
         MakeRule,
     StringRef Name, ClangTidyContext *Context)
diff --git a/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.h b/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.h
index ad20fbd475759..6fec2e05b963b 100644
--- a/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.h
+++ b/clang-tools-extra/clang-tidy/utils/TransformerClangTidyCheck.h
@@ -48,8 +48,9 @@ class TransformerClangTidyCheck : public ClangTidyCheck {
   ///
   /// See \c setRule for constraints on the rule.
   TransformerClangTidyCheck(
-      std::function<std::optional<transformer::RewriteRuleWith<std::string>>(
-          const LangOptions &, const OptionsView &)>
+      llvm::function_ref<
+          std::optional<transformer::RewriteRuleWith<std::string>>(
+              const LangOptions &, const OptionsView &)>
           MakeRule,
       StringRef Name, ClangTidyContext *Context);
 
diff --git a/clang-tools-extra/clang-tidy/utils/UseRangesCheck.cpp b/clang-tools-extra/clang-tidy/utils/UseRangesCheck.cpp
index cb1495163a2f9..c117613a5d169 100644
--- a/clang-tools-extra/clang-tidy/utils/UseRangesCheck.cpp
+++ b/clang-tools-extra/clang-tidy/utils/UseRangesCheck.cpp
@@ -55,7 +55,7 @@ AST_MATCHER(Expr, hasSideEffects) {
 } // namespace
 
 static auto
-makeExprMatcher(ast_matchers::internal::Matcher<Expr> ArgumentMatcher,
+makeExprMatcher(const ast_matchers::internal::Matcher<Expr> &ArgumentMatcher,
                 ArrayRef<StringRef> MethodNames,
                 ArrayRef<StringRef> FreeNames) {
   return expr(