clang performance avoid copy

gistfile1.txt

// File: UnnecessaryByValueCheck.cpp
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Expr.h"
#include "clang/AST/Type.h"
#include "clang/Tidy/ClangTidy.h"

using namespace clang;
using namespace clang::ast_matchers;
using namespace clang::tidy;

namespace {

class UnnecessaryByValueCheck : public ClangTidyCheck {
public:
  UnnecessaryByValueCheck(StringRef Name, ClangTidyContext *Context)
      : ClangTidyCheck(Name, Context) {}
  
  void registerMatchers(MatchFinder *Finder) override {
    Finder->addMatcher(
        varDecl(
            hasType(autoType()),
            hasLocalStorage(),
            hasInitializer(expr().bind("init")),
            unless(isConstQualified()),
            unless(isConstexpr()),
            unless(parmVarDecl())
        ).bind("valVar"),
        this);
  }

  void check(const MatchFinder::MatchResult &Result) override {
    const auto *Var = Result.Nodes.getNodeAs<VarDecl>("valVar");
    const auto *Init = Result.Nodes.getNodeAs<Expr>("init");

    if (!Var || !Init)
      return;

    QualType InitType = Init->getType();
    ASTContext &Ctx = *Result.Context;

    if (InitType->isReferenceType() && !Var->isModifiable()) {
      QualType Pointee = InitType->getPointeeType();
      if (!Pointee.isTrivialType(Ctx)) {
        diag(Var->getLocation(),
             "variable '%0' is initialized from a referenceable expression "
             "but declared by value; consider using 'const auto&'")
            << Var->getName();
      }
    }
  }
};

} // namespace

// Register the check.
namespace clang {
namespace tidy {
namespace yourplugin {
  class UnnecessaryByValueModule : public ClangTidyModule {
  public:
    void addCheckFactories(ClangTidyCheckFactories &Factories) override {
      Factories.registerCheck<UnnecessaryByValueCheck>(
          "yourplugin-unnecessary-by-value");
    }
  };
  static ClangTidyModuleRegistry::Add<UnnecessaryByValueModule>
      X("yourplugin-module", "Adds UnnecessaryByValueCheck.");
} // namespace yourplugin
} // namespace tidy
} // namespace clang