UseStructuredBindingCheck.cpp

Go to the documentation of this file.

//===----------------------------------------------------------------------===//
//
// 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 "UseStructuredBindingCheck.h"
#include "../utils/DeclRefExprUtils.h"
#include "clang/Lex/Lexer.h"
 
using namespace clang::ast_matchers;
 
namespace clang::tidy::modernize {
 
static constexpr StringRef PairDeclName = "PairVarD";
static constexpr StringRef PairVarTypeName = "PairVarType";
static constexpr StringRef FirstVarDeclName = "FirstVarDecl";
static constexpr StringRef SecondVarDeclName = "SecondVarDecl";
static constexpr StringRef BeginDeclStmtName = "BeginDeclStmt";
static constexpr StringRef EndDeclStmtName = "EndDeclStmt";
static constexpr StringRef FirstTypeName = "FirstType";
static constexpr StringRef SecondTypeName = "SecondType";
static constexpr StringRef ScopeBlockName = "ScopeBlock";
static constexpr StringRef StdTieAssignStmtName = "StdTieAssign";
static constexpr StringRef StdTieExprName = "StdTieExpr";
static constexpr StringRef ForRangeStmtName = "ForRangeStmt";
static constexpr StringRef InitExprName = "init_expr";
 
/// Matches a sequence of VarDecls matching the inner matchers, starting from
/// the \p Iter to \p EndIter and set bindings for the first DeclStmt and the
/// last DeclStmt if matched.
///
/// \p Backwards indicates whether to match the VarDecls in reverse order.
template <typename Iterator>
static bool matchNVarDeclStartingWith(
    Iterator Iter, const Iterator &EndIter,
    ArrayRef<ast_matchers::internal::Matcher<VarDecl>> InnerMatchers,
    ast_matchers::internal::ASTMatchFinder *Finder,
    ast_matchers::internal::BoundNodesTreeBuilder *Builder,
    bool Backwards = false) {
  const DeclStmt *BeginDS = nullptr;
  const DeclStmt *EndDS = nullptr;
  const size_t N = InnerMatchers.size();
  size_t Count = 0;
 
  const auto Matches = [&](const Decl *VD) {
    // We don't want redundant decls in DeclStmt.
    if (Count == N)
      return false;
 
    if (const auto *Var = dyn_cast<VarDecl>(VD);
        Var && InnerMatchers[Backwards ? N - Count - 1 : Count].matches(
                   *Var, Finder, Builder)) {
      ++Count;
      return true;
    }
 
    return false;
  };
 
  for (; Iter != EndIter; ++Iter) {
    EndDS = dyn_cast<DeclStmt>(*Iter);
    if (!EndDS)
      break;
 
    if (!BeginDS)
      BeginDS = EndDS;
 
    for (const auto *VD :
         llvm::reverse_conditionally(EndDS->decls(), Backwards)) {
      if (!Matches(VD))
        return false;
    }
 
    // All the matchers is satisfied in those DeclStmts.
    if (Count == N) {
      if (Backwards)
        std::swap(BeginDS, EndDS);
      Builder->setBinding(BeginDeclStmtName, DynTypedNode::create(*BeginDS));
      Builder->setBinding(EndDeclStmtName, DynTypedNode::create(*EndDS));
      return true;
    }
  }
 
  return false;
}
 
namespace {
/// What qualifiers and specifiers are used to create structured binding
/// declaration, it only supports the following four cases now.
enum TransferType : uint8_t {
  TT_ByVal,
  TT_ByConstVal,
  TT_ByRef,
  TT_ByConstRef
};
 
/// Matches a Stmt whose parent is a CompoundStmt, and which is directly
/// following two VarDecls matching the inner matcher.
AST_MATCHER_P(Stmt, hasPreTwoVarDecl,
              SmallVector<ast_matchers::internal::Matcher<VarDecl>>,
              InnerMatchers) {
  const DynTypedNodeList Parents = Finder->getASTContext().getParents(Node);
  if (Parents.size() != 1)
    return false;
 
  const auto *C = Parents[0].get<CompoundStmt>();
  if (!C)
    return false;
 
  const auto It = llvm::find(llvm::reverse(C->body()), &Node);
  assert(It != C->body_rend() && "C is parent of Node");
  return matchNVarDeclStartingWith(It + 1, C->body_rend(), InnerMatchers,
                                   Finder, Builder, true);
}
 
/// Matches a Stmt whose parent is a CompoundStmt, and which is directly
/// followed by two VarDecls matching the inner matcher.
AST_MATCHER_P(Stmt, hasNextTwoVarDecl,
              SmallVector<ast_matchers::internal::Matcher<VarDecl>>,
              InnerMatchers) {
  const DynTypedNodeList Parents = Finder->getASTContext().getParents(Node);
  if (Parents.size() != 1)
    return false;
 
  const auto *C = Parents[0].get<CompoundStmt>();
  if (!C)
    return false;
 
  const auto *It = llvm::find(C->body(), &Node);
  assert(It != C->body_end() && "C is parent of Node");
  return matchNVarDeclStartingWith(It + 1, C->body_end(), InnerMatchers, Finder,
                                   Builder);
}
 
/// Matches a CompoundStmt which has two VarDecls matching the inner matcher in
/// the beginning.
AST_MATCHER_P(CompoundStmt, hasFirstTwoVarDecl,
              SmallVector<ast_matchers::internal::Matcher<VarDecl>>,
              InnerMatchers) {
  return matchNVarDeclStartingWith(Node.body_begin(), Node.body_end(),
                                   InnerMatchers, Finder, Builder);
}
 
/// It's not very common to have specifiers for variables used to decompose a
/// pair, so we ignore these cases.
AST_MATCHER(VarDecl, hasAnySpecifiersShouldBeIgnored) {
  return Node.isStaticLocal() || Node.isConstexpr() || Node.hasAttrs() ||
         Node.isInlineSpecified() || Node.getStorageClass() != SC_None ||
         Node.getTSCSpec() != TSCS_unspecified;
}
 
// Ignore nodes inside macros.
AST_POLYMORPHIC_MATCHER(isInMacro,
                        AST_POLYMORPHIC_SUPPORTED_TYPES(Stmt, Decl)) {
  return Node.getBeginLoc().isMacroID() || Node.getEndLoc().isMacroID();
}
 
AST_MATCHER_P(Expr, ignoringCopyCtorAndImplicitCast,
              ast_matchers::internal::Matcher<Expr>, InnerMatcher) {
  if (const auto *CtorE = dyn_cast<CXXConstructExpr>(&Node)) {
    if (const CXXConstructorDecl *CtorD = CtorE->getConstructor();
        CtorD->isCopyConstructor() && CtorE->getNumArgs() == 1) {
      return InnerMatcher.matches(*CtorE->getArg(0)->IgnoreImpCasts(), Finder,
                                  Builder);
    }
  }
 
  return InnerMatcher.matches(*Node.IgnoreImpCasts(), Finder, Builder);
}
 
AST_MATCHER(CXXRecordDecl, isPairType) {
  return llvm::all_of(Node.fields(), [](const FieldDecl *FD) {
    return FD->getAccess() == AS_public &&
           (FD->getName() == "first" || FD->getName() == "second");
  });
}
 
AST_MATCHER(VarDecl, isDirectInitialization) {
  return Node.getInitStyle() != VarDecl::InitializationStyle::CInit;
}
 
} // namespace
 
static auto getVarInitWithMemberMatcher(
    StringRef PairName, StringRef MemberName, StringRef TypeName,
    StringRef BindingName,
    const ast_matchers::internal::Matcher<VarDecl> &ExtraMatcher) {
  return varDecl(ExtraMatcher,
                 hasInitializer(ignoringCopyCtorAndImplicitCast(memberExpr(
                     hasObjectExpression(ignoringImpCasts(declRefExpr(
                         to(equalsBoundNode(std::string(PairName)))))),
                     member(fieldDecl(hasName(MemberName),
                                      hasType(qualType().bind(TypeName))))))))
      .bind(BindingName);
}
 
static auto typeOrLValueReferenceTo(
    const ast_matchers::internal::Matcher<QualType> &TypeMatcher) {
  return qualType(
      anyOf(TypeMatcher, lValueReferenceType(pointee(TypeMatcher))));
}
 
void UseStructuredBindingCheck::registerMatchers(MatchFinder *Finder) {
  const auto PairType =
      qualType(unless(isVolatileQualified()),
               hasUnqualifiedDesugaredType(
                   recordType(hasDeclaration(cxxRecordDecl(isPairType())))));
 
  auto UnlessShouldBeIgnored =
      unless(anyOf(hasAnySpecifiersShouldBeIgnored(), isInMacro()));
 
  const auto VarInitWithFirstMember =
      getVarInitWithMemberMatcher(PairDeclName, "first", FirstTypeName,
                                  FirstVarDeclName, UnlessShouldBeIgnored);
  const auto VarInitWithSecondMember =
      getVarInitWithMemberMatcher(PairDeclName, "second", SecondTypeName,
                                  SecondVarDeclName, UnlessShouldBeIgnored);
 
  const auto RefToBindName = [&UnlessShouldBeIgnored](const StringRef &Name) {
    return declRefExpr(to(varDecl(UnlessShouldBeIgnored).bind(Name)));
  };
 
  const auto HasAnyLambdaCaptureThisVar =
      [](const ast_matchers::internal::Matcher<VarDecl> &VDMatcher) {
        return compoundStmt(hasDescendant(
            lambdaExpr(hasAnyCapture(capturesVar(varDecl(VDMatcher))))));
      };
 
  // Captured structured bindings are a C++20 extension
  const auto UnlessFirstVarOrSecondVarIsCapturedByLambda =
      getLangOpts().CPlusPlus20
          ? compoundStmt()
          : compoundStmt(unless(HasAnyLambdaCaptureThisVar(
                anyOf(equalsBoundNode(std::string(FirstVarDeclName)),
                      equalsBoundNode(std::string(SecondVarDeclName))))));
 
  // X x;
  // Y y;
  // std::tie(x, y) = ...;
  Finder->addMatcher(
      exprWithCleanups(
          unless(isInMacro()),
          has(cxxOperatorCallExpr(
                  hasOverloadedOperatorName("="),
                  hasLHS(ignoringImplicit(
                      callExpr(callee(functionDecl(isInStdNamespace(),
                                                   hasName("tie"))),
                               hasArgument(0, RefToBindName(FirstVarDeclName)),
                               hasArgument(1, RefToBindName(SecondVarDeclName)))
                          .bind(StdTieExprName))),
                  hasRHS(expr(hasType(PairType))))
                  .bind(StdTieAssignStmtName)),
          hasPreTwoVarDecl(
              SmallVector<ast_matchers::internal::Matcher<VarDecl>>{
                  varDecl(equalsBoundNode(std::string(FirstVarDeclName))),
                  varDecl(equalsBoundNode(std::string(SecondVarDeclName)))}),
          hasParent(compoundStmt(UnlessFirstVarOrSecondVarIsCapturedByLambda)
                        .bind(ScopeBlockName))),
      this);
 
  // pair<X, Y> p = ...;
  // X x = p.first;
  // Y y = p.second;
  Finder->addMatcher(
      declStmt(
          unless(isInMacro()),
          hasSingleDecl(varDecl(UnlessShouldBeIgnored,
                                unless(isDirectInitialization()),
                                hasType(typeOrLValueReferenceTo(PairType).bind(
                                    PairVarTypeName)),
                                hasInitializer(ignoringCopyCtorAndImplicitCast(
                                    expr().bind(InitExprName))))
                            .bind(PairDeclName)),
          hasNextTwoVarDecl(
              SmallVector<ast_matchers::internal::Matcher<VarDecl>>{
                  VarInitWithFirstMember, VarInitWithSecondMember}),
          hasParent(compoundStmt(UnlessFirstVarOrSecondVarIsCapturedByLambda)
                        .bind(ScopeBlockName))),
      this);
 
  // for (pair<X, Y> p : map) {
  //    X x = p.first;
  //    Y y = p.second;
  // }
  Finder->addMatcher(
      cxxForRangeStmt(
          unless(isInMacro()),
          hasLoopVariable(
              varDecl(hasType(typeOrLValueReferenceTo(PairType).bind(
                          PairVarTypeName)),
                      hasInitializer(ignoringCopyCtorAndImplicitCast(
                          expr().bind(InitExprName))))
                  .bind(PairDeclName)),
          hasBody(compoundStmt(
                      hasFirstTwoVarDecl(
                          SmallVector<ast_matchers::internal::Matcher<VarDecl>>{
                              VarInitWithFirstMember, VarInitWithSecondMember}),
                      UnlessFirstVarOrSecondVarIsCapturedByLambda)
                      .bind(ScopeBlockName)))
          .bind(ForRangeStmtName),
      this);
}
 
static std::optional<TransferType> getTransferType(const ASTContext &Ctx,
                                                   QualType ResultType,
                                                   QualType OriginType) {
  ResultType = ResultType.getCanonicalType();
  OriginType = OriginType.getCanonicalType();
 
  if (ResultType == Ctx.getLValueReferenceType(OriginType.withConst()))
    return TT_ByConstRef;
 
  if (ResultType == Ctx.getLValueReferenceType(OriginType))
    return TT_ByRef;
 
  if (ResultType == OriginType.withConst())
    return TT_ByConstVal;
 
  if (ResultType == OriginType)
    return TT_ByVal;
 
  return std::nullopt;
}
 
void UseStructuredBindingCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *FirstVar = Result.Nodes.getNodeAs<VarDecl>(FirstVarDeclName);
  const auto *SecondVar = Result.Nodes.getNodeAs<VarDecl>(SecondVarDeclName);
 
  const auto *BeginDS = Result.Nodes.getNodeAs<DeclStmt>(BeginDeclStmtName);
  const auto *EndDS = Result.Nodes.getNodeAs<DeclStmt>(EndDeclStmtName);
  const auto *ScopeBlock = Result.Nodes.getNodeAs<CompoundStmt>(ScopeBlockName);
 
  const auto *CFRS = Result.Nodes.getNodeAs<CXXForRangeStmt>(ForRangeStmtName);
  const auto DiagAndFix = [&BeginDS, &EndDS, &FirstVar, &SecondVar, &CFRS,
                           this](SourceLocation DiagLoc,
                                 SourceRange ReplaceRange,
                                 TransferType TT = TT_ByVal) {
    const auto Prefix = [&TT]() -> StringRef {
      switch (TT) {
      case TT_ByVal:
        return "auto";
      case TT_ByConstVal:
        return "const auto";
      case TT_ByRef:
        return "auto&";
      case TT_ByConstRef:
        return "const auto&";
      }
      llvm_unreachable("Unhandled TransferType enum");
    }();
 
    const std::string ReplacementText =
        (Twine(Prefix) + " [" + FirstVar->getNameAsString() + ", " +
         SecondVar->getNameAsString() + "]" + (CFRS ? " :" : ""))
            .str();
    diag(DiagLoc, "use a structured binding to decompose a pair")
        << FixItHint::CreateReplacement(ReplaceRange, ReplacementText)
        << FixItHint::CreateRemoval(
               SourceRange{BeginDS->getBeginLoc(), EndDS->getEndLoc()});
  };
 
  if (const auto *COCE =
          Result.Nodes.getNodeAs<CXXOperatorCallExpr>(StdTieAssignStmtName)) {
    DiagAndFix(COCE->getBeginLoc(),
               Result.Nodes.getNodeAs<Expr>(StdTieExprName)->getSourceRange());
    return;
  }
 
  // Check whether PairVar, FirstVar and SecondVar have the same transfer type,
  // so they can be combined to structured binding.
  const auto *PairVar = Result.Nodes.getNodeAs<VarDecl>(PairDeclName);
 
  const std::optional<TransferType> PairCaptureType =
      getTransferType(*Result.Context, PairVar->getType(),
                      Result.Nodes.getNodeAs<Expr>(InitExprName)->getType());
  const std::optional<TransferType> FirstVarCaptureType =
      getTransferType(*Result.Context, FirstVar->getType(),
                      *Result.Nodes.getNodeAs<QualType>(FirstTypeName));
  const std::optional<TransferType> SecondVarCaptureType =
      getTransferType(*Result.Context, SecondVar->getType(),
                      *Result.Nodes.getNodeAs<QualType>(SecondTypeName));
  if (!PairCaptureType || !FirstVarCaptureType || !SecondVarCaptureType ||
      *PairCaptureType != *FirstVarCaptureType ||
      *FirstVarCaptureType != *SecondVarCaptureType)
    return;
 
  // Check PairVar is not used except for assignment members to firstVar and
  // SecondVar.
  if (const auto AllRef = utils::decl_ref_expr::allDeclRefExprs(
          *PairVar, *ScopeBlock, *Result.Context);
      AllRef.size() != 2)
    return;
 
  DiagAndFix(PairVar->getBeginLoc(),
             CFRS ? PairVar->getSourceRange()
                  : SourceRange(PairVar->getBeginLoc(),
                                Lexer::getLocForEndOfToken(
                                    PairVar->getLocation(), 0,
                                    Result.Context->getSourceManager(),
                                    Result.Context->getLangOpts())),
             *PairCaptureType);
}
 
} // namespace clang::tidy::modernize