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

//===--- ImplicitCastInLoopCheck.cpp - clang-tidy--------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "ImplicitCastInLoopCheck.h"

#include "clang/AST/ASTContext.h"
#include "clang/AST/Decl.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/ASTMatchers/ASTMatchers.h"
#include "clang/Lex/Lexer.h"

namespace clang {

using namespace ast_matchers;

namespace tidy {
namespace performance {

namespace {
// Checks if the stmt is a ImplicitCastExpr with a CastKind that is not a NoOp.
// The subtelty is that in some cases (user defined conversions), we can
// get to ImplicitCastExpr inside each other, with the outer one a NoOp. In this
// case we skip the first cast expr.
bool IsNonTrivialImplicitCast(const Stmt* ST) {
  if (const auto* ICE = dyn_cast<ImplicitCastExpr>(ST)) {
    return (ICE->getCastKind() != CK_NoOp) ||
            IsNonTrivialImplicitCast(ICE->getSubExpr());
  }
  return false;
}
} // namespace

void ImplicitCastInLoopCheck::registerMatchers(
    ast_matchers::MatchFinder* Finder) {
  // We look for const ref loop variables that (optionally inside an
  // ExprWithCleanup) materialize a temporary, and contain a implicit cast. The
  // check on the implicit cast is done in check() because we can't access
  // implicit cast subnode via matchers: has() skips casts and materialize!
  // We also bind on the call to operator* to get the proper type in the
  // diagnostic message.
  // Note that when the implicit cast is done through a user defined cast
  // operator, the node is a CXXMemberCallExpr, not a CXXOperatorCallExpr, so
  // it should not get caught by the cxxOperatorCallExpr() matcher.
  Finder->addMatcher(
      cxxForRangeStmt(hasLoopVariable(
          varDecl(hasType(qualType(references(qualType(isConstQualified())))),
                  hasInitializer(expr(hasDescendant(cxxOperatorCallExpr().bind(
                                          "operator-call")))
                                     .bind("init")))
              .bind("faulty-var"))),
      this);
}

void ImplicitCastInLoopCheck::check(
    const ast_matchers::MatchFinder::MatchResult &Result) {
  const auto* VD = Result.Nodes.getNodeAs<VarDecl>("faulty-var");
  const auto* Init = Result.Nodes.getNodeAs<Expr>("init");
  const auto* OperatorCall =
      Result.Nodes.getNodeAs<CXXOperatorCallExpr>("operator-call");

  if (const auto* Cleanup = dyn_cast<ExprWithCleanups>(Init)) {
    Init = Cleanup->getSubExpr();
  }
  const auto* Materialized = dyn_cast<MaterializeTemporaryExpr>(Init);
  if (!Materialized) {
    return;
  }

  // We ignore NoOp casts. Those are generated if the * operator on the
  // iterator returns a value instead of a reference, and the loop variable
  // is a reference. This situation is fine (it probably produces the same
  // code at the end).
  if (IsNonTrivialImplicitCast(Materialized->getTemporary())) {
    ReportAndFix(Result.Context, VD, OperatorCall);
  }
}

void ImplicitCastInLoopCheck::ReportAndFix(
    const ASTContext *Context, const VarDecl *VD,
    const CXXOperatorCallExpr *OperatorCall) {
  // We only match on const ref, so we should print a const ref version of the
  // type.
  QualType ConstType = OperatorCall->getType().withConst();
  QualType ConstRefType = Context->getLValueReferenceType(ConstType);
  const char Message[] =
      "the type of the loop variable '%0' is different from the one returned "
      "by the iterator and generates an implicit cast; you can either "
      "change the type to the correct one ('%1' but 'const auto&' is always a "
      "valid option) or remove the reference to make it explicit that you are "
      "creating a new value";
  PrintingPolicy Policy(Context->getLangOpts());
  Policy.SuppressTagKeyword = true;

  diag(VD->getLocStart(), Message) << VD->getName()
                                   << ConstRefType.getAsString(Policy);
}

} // namespace performance
} // namespace tidy
} // namespace clang