[clang-tidy] add option performance-move-const-arg.CheckMoveToConstRef

[devjgm]

Summary

This option allows callers to disable the warning from
https://clang.llvm.org/extra/clang-tidy/checks/performance-move-const-arg.html
that would warn on the following

void f(const string &s);
string s;
f(std::move(s));  // ALLOWED if performance-move-const-arg.CheckMoveToConstRef=false

The reason people might want to disable this check, is because it allows
callers to use std::move() or not based on local reasoning about the
argument, and without having to care about how the function f accepts
the argument. Indeed, f might accept the argument by const-ref today,
but change to by-value tomorrow, and if the caller had moved the
argument that they were finished with, the code would work as
efficiently as possible regardless of how f accepted the parameter.

[devjgm]

Here is also updated the documentation to include the new option.

[ymandel]

clang-tools-extra/clang-tidy/performance/MoveConstArgCheck.cpp

197–198

nit, up to you. I think this way is a little clearer b/c it reads "proceed on this branch if the check is configured to "check move to const ref"".

[devjgm]

Accepted ymandel's suggestion.

[devjgm]

• accepted ymandel's suggestion

clang-tools-extra/clang-tidy/performance/MoveConstArgCheck.h

 #include "llvm/ADT/DenseSet.h"
 
 namespace clang {
 namespace tidy {
 namespace performance {
 
 /// Find casts of calculation results to bigger type. Typically from int to
 ///
-/// There is one option:
+/// The options are
 ///
 ///   - `CheckTriviallyCopyableMove`: Whether to check for trivially-copyable
 //      types as their objects are not moved but copied. Enabled by default.
+//    - `CheckMoveToConstRef`: Whether to check if a `std::move()` is passed
+//      as a const reference argument.
 class MoveConstArgCheck : public ClangTidyCheck {
 public:
   MoveConstArgCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context),
-        CheckTriviallyCopyableMove(
-            Options.get("CheckTriviallyCopyableMove", true)) {}
+      : ClangTidyCheck(Name, Context), CheckTriviallyCopyableMove(Options.get(
+                                           "CheckTriviallyCopyableMove", true)),
+        CheckMoveToConstRef(Options.get("CheckMoveToConstRef", true)) {}
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     return LangOpts.CPlusPlus;
   }
   void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
 
 private:
   const bool CheckTriviallyCopyableMove;
+  const bool CheckMoveToConstRef;
   llvm::DenseSet<const CallExpr *> AlreadyCheckedMoves;
 };
 
 } // namespace performance
 } // namespace tidy
 } // namespace clang
 
 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_MOVECONSTANTARGUMENTCHECK_H

clang-tools-extra/clang-tidy/performance/MoveConstArgCheck.cpp

static void replaceCallWithArg(const CallExpr *Call, DiagnosticBuilder &Diag,
   if (BeforeArgumentsRange.isValid() && AfterArgumentsRange.isValid()) {
     Diag << FixItHint::CreateRemoval(BeforeArgumentsRange)
          << FixItHint::CreateRemoval(AfterArgumentsRange);
   }
 }
 
 void MoveConstArgCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
   Options.store(Opts, "CheckTriviallyCopyableMove", CheckTriviallyCopyableMove);
+  Options.store(Opts, "CheckMoveToConstRef", CheckMoveToConstRef);
 }
 
 void MoveConstArgCheck::registerMatchers(MatchFinder *Finder) {
   auto MoveCallMatcher =
       callExpr(callee(functionDecl(hasName("::std::move"))), argumentCountIs(1),
                unless(isInTemplateInstantiation()))
           .bind("call-move");
 
if (IsRVRefParam) {
       }
 
       diag(InvocationParm->getLocation(),
            "consider changing the %ordinal0 parameter of %1 from %2 to '%3'",
            DiagnosticIDs::Note)
           << (InvocationParm->getFunctionScopeIndex() + 1) << FunctionName
           << *InvocationParmType << ExpectParmTypeName;
     }
-  } else if (ReceivingExpr) {
+  } else if (ReceivingExpr && CheckMoveToConstRef) {
     if ((*InvocationParmType)->isRValueReferenceType())
       return;
 
     auto Diag = diag(FileMoveRange.getBegin(),
                      "passing result of std::move() as a const reference "
                      "argument; no move will actually happen");
 
     replaceCallWithArg(CallMove, Diag, SM, getLangOpts());
   }
 }
 
 } // namespace performance
 } // namespace tidy
 } // namespace clang

clang-tools-extra/docs/clang-tidy/checks/performance-move-const-arg.rst

 
 Options
 -------
 
 .. option:: CheckTriviallyCopyableMove
 
    If `true`, enables detection of trivially copyable types that do not
    have a move constructor. Default is `true`.
+
+.. option:: CheckMoveToConstRef
+
+   If `true`, enables detection of `std::move()` passed as a const
+   reference argument. Default is `true`.

clang-tools-extra/test/clang-tidy/checkers/performance-move-const-arg-const-ref.cpp

+// RUN: %check_clang_tidy %s performance-move-const-arg %t \
+// RUN: -config='{CheckOptions: \
+// RUN:  [{key: performance-move-const-arg.CheckMoveToConstRef, value: false}]}'
+
+namespace std {
+template <typename>
+struct remove_reference;
+
+template <typename _Tp>
+struct remove_reference {
+  typedef _Tp type;
+};
+
+template <typename _Tp>
+struct remove_reference<_Tp &> {
+  typedef _Tp type;
+};
+
+template <typename _Tp>
+struct remove_reference<_Tp &&> {
+  typedef _Tp type;
+};
+
+template <typename _Tp>
+constexpr typename std::remove_reference<_Tp>::type &&move(_Tp &&__t) {
+  return static_cast<typename std::remove_reference<_Tp>::type &&>(__t);
+}
+
+template <typename _Tp>
+constexpr _Tp &&
+forward(typename remove_reference<_Tp>::type &__t) noexcept {
+  return static_cast<_Tp &&>(__t);
+}
+
+} // namespace std
+
+struct TriviallyCopyable {
+  int i;
+};
+
+void f(TriviallyCopyable) {}
+
+void g() {
+  TriviallyCopyable obj;
+  // Some basic test to ensure that other warnings from
+  // performance-move-const-arg are still working and enabled.
+  f(std::move(obj));
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: std::move of the variable 'obj' of the trivially-copyable type 'TriviallyCopyable' has no effect; remove std::move() [performance-move-const-arg]
+  // CHECK-FIXES: f(obj);
+}
+
+class NoMoveSemantics {
+public:
+  NoMoveSemantics();
+  NoMoveSemantics(const NoMoveSemantics &);
+  NoMoveSemantics &operator=(const NoMoveSemantics &);
+};
+
+class MoveSemantics {
+public:
+  MoveSemantics();
+  MoveSemantics(MoveSemantics &&);
+
+  MoveSemantics &operator=(MoveSemantics &&);
+};
+
+void callByConstRef1(const NoMoveSemantics &);
+void callByConstRef2(const MoveSemantics &);
+
+void moveToConstReferencePositives() {
+  NoMoveSemantics a;
+
+  // This call is now allowed since CheckMoveToConstRef is false.
+  callByConstRef1(std::move(a));
+
+  MoveSemantics b;
+
+  // This call is now allowed since CheckMoveToConstRef is false.
+  callByConstRef2(std::move(b));
+}