ImplicitConversionInLoopCheck.cpp

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//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
 
#include "ImplicitConversionInLoopCheck.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"
 
using namespace clang::ast_matchers;
 
namespace clang::tidy::performance {
 
// Checks if the stmt is a ImplicitCastExpr with a CastKind that is not a NoOp.
// The subtlety 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.
static bool isNonTrivialImplicitCast(const Stmt *ST) {
  if (const auto *ICE = dyn_cast<ImplicitCastExpr>(ST)) {
    return (ICE->getCastKind() != CK_NoOp) ||
           isNonTrivialImplicitCast(ICE->getSubExpr());
  }
  return false;
}
 
void ImplicitConversionInLoopCheck::registerMatchers(MatchFinder *Finder) {
  // We look for const ref loop variables that (optionally inside an
  // ExprWithCleanup) materialize a temporary, and contain a implicit
  // conversion. The check on the implicit conversion is done in check() because
  // we can't access implicit conversion 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. We use both cxxOperatorCallExpr for user
  // defined operator and unaryOperator when the iterator is a pointer, like
  // for arrays or std::array.
  //
  // Note that when the implicit conversion is done through a user defined
  // conversion operator, the node is a CXXMemberCallExpr, not a
  // CXXOperatorCallExpr, so it should not get caught by the
  // cxxOperatorCallExpr() matcher.
  Finder->addMatcher(
      traverse(
          TK_AsIs,
          cxxForRangeStmt(hasLoopVariable(
              varDecl(
                  hasType(qualType(references(qualType(isConstQualified())))),
                  hasInitializer(
                      expr(anyOf(
                               hasDescendant(
                                   cxxOperatorCallExpr().bind("operator-call")),
                               hasDescendant(unaryOperator(hasOperatorName("*"))
                                                 .bind("operator-call"))))
                          .bind("init")))
                  .bind("faulty-var")))),
      this);
}
 
void ImplicitConversionInLoopCheck::check(
    const 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<Expr>("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->getSubExpr()))
    reportAndFix(Result.Context, VD, OperatorCall);
}
 
void ImplicitConversionInLoopCheck::reportAndFix(const ASTContext *Context,
                                                 const VarDecl *VD,
                                                 const Expr *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 conversion; you can either "
      "change the type to the matching 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";
  diag(VD->getBeginLoc(), Message) << VD << ConstRefType;
}
 
} // namespace clang::tidy::performance