UnusedUsingDeclsCheck.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 "UnusedUsingDeclsCheck.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::misc {
 
namespace {
 
AST_MATCHER_P(DeducedTemplateSpecializationType, refsToTemplatedDecl,
              clang::ast_matchers::internal::Matcher<NamedDecl>, DeclMatcher) {
  if (const auto *TD = Node.getTemplateName().getAsTemplateDecl())
    return DeclMatcher.matches(*TD, Finder, Builder);
  return false;
}
 
AST_MATCHER_P(Type, asTagDecl, clang::ast_matchers::internal::Matcher<TagDecl>,
              DeclMatcher) {
  if (const TagDecl *ND = Node.getAsTagDecl())
    return DeclMatcher.matches(*ND, Finder, Builder);
  return false;
}
 
} // namespace
 
// A function that helps to tell whether a TargetDecl in a UsingDecl will be
// checked. Only variable, function, function template, class template, class,
// enum declaration and enum constant declaration are considered.
static bool shouldCheckDecl(const Decl *TargetDecl) {
  return isa<RecordDecl>(TargetDecl) || isa<ClassTemplateDecl>(TargetDecl) ||
         isa<FunctionDecl>(TargetDecl) || isa<VarDecl>(TargetDecl) ||
         isa<FunctionTemplateDecl>(TargetDecl) || isa<EnumDecl>(TargetDecl) ||
         isa<EnumConstantDecl>(TargetDecl);
}
 
UnusedUsingDeclsCheck::UnusedUsingDeclsCheck(StringRef Name,
                                             ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      HeaderFileExtensions(Context->getHeaderFileExtensions()) {}
 
void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
  // We don't emit warnings on unused-using-decls from headers, so bail out if
  // the main file is a header.
  if (utils::isFileExtension(getCurrentMainFile(), HeaderFileExtensions))
    return;
  Finder->addMatcher(usingDecl(isExpansionInMainFile()).bind("using"), this);
  auto DeclMatcher = hasDeclaration(namedDecl().bind("used"));
  Finder->addMatcher(loc(templateSpecializationType(DeclMatcher)), this);
  Finder->addMatcher(loc(deducedTemplateSpecializationType(
                         refsToTemplatedDecl(namedDecl().bind("used")))),
                     this);
  Finder->addMatcher(callExpr(callee(unresolvedLookupExpr().bind("used"))),
                     this);
  Finder->addMatcher(
      callExpr(hasDeclaration(functionDecl(
          forEachTemplateArgument(templateArgument().bind("used"))))),
      this);
  Finder->addMatcher(loc(templateSpecializationType(forEachTemplateArgument(
                         templateArgument().bind("used")))),
                     this);
  Finder->addMatcher(userDefinedLiteral().bind("used"), this);
  Finder->addMatcher(loc(asTagDecl(tagDecl().bind("used"))), this);
  // Cases where we can identify the UsingShadowDecl directly, rather than
  // just its target.
  // FIXME: cover more cases in this way, as the AST supports it.
  auto ThroughShadowMatcher = throughUsingDecl(namedDecl().bind("usedShadow"));
  Finder->addMatcher(declRefExpr(ThroughShadowMatcher), this);
  Finder->addMatcher(loc(usingType(ThroughShadowMatcher)), this);
}
 
void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
  if (Result.Context->getDiagnostics().hasUncompilableErrorOccurred())
    return;
 
  if (const auto *Using = Result.Nodes.getNodeAs<UsingDecl>("using")) {
    // Ignores using-declarations defined in macros.
    if (Using->getLocation().isMacroID())
      return;
 
    // Ignores using-declarations defined in class definition.
    if (isa<CXXRecordDecl>(Using->getDeclContext()))
      return;
 
    // FIXME: We ignore using-decls defined in function definitions at the
    // moment because of false positives caused by ADL and different function
    // scopes.
    if (isa<FunctionDecl>(Using->getDeclContext()))
      return;
 
    UsingDeclContext Context(Using);
    Context.UsingDeclRange = CharSourceRange::getCharRange(
        Using->getBeginLoc(),
        Lexer::findLocationAfterToken(
            Using->getEndLoc(), tok::semi, *Result.SourceManager, getLangOpts(),
            /*SkipTrailingWhitespaceAndNewLine=*/true));
    for (const auto *UsingShadow : Using->shadows()) {
      const auto *TargetDecl = UsingShadow->getTargetDecl()->getCanonicalDecl();
      if (shouldCheckDecl(TargetDecl)) {
        Context.UsingTargetDecls.insert(TargetDecl);
        UsingTargetDeclsCache.insert(TargetDecl);
      }
    }
    if (!Context.UsingTargetDecls.empty())
      Contexts.push_back(Context);
    return;
  }
 
  // Mark a corresponding using declaration as used.
  auto RemoveNamedDecl = [&](const NamedDecl *Used) {
    removeFromFoundDecls(Used);
    // Also remove variants of Used.
    if (const auto *FD = dyn_cast<FunctionDecl>(Used)) {
      removeFromFoundDecls(FD->getPrimaryTemplate());
      return;
    }
    if (const auto *Specialization =
            dyn_cast<ClassTemplateSpecializationDecl>(Used)) {
      removeFromFoundDecls(Specialization->getSpecializedTemplate());
      return;
    }
    if (const auto *ECD = dyn_cast<EnumConstantDecl>(Used)) {
      if (const auto *ET = ECD->getType()->getAsCanonical<EnumType>())
        removeFromFoundDecls(ET->getDecl());
    }
  };
  // We rely on the fact that the clang AST is walked in order, usages are only
  // marked after a corresponding using decl has been found.
  if (const auto *Used = Result.Nodes.getNodeAs<NamedDecl>("used")) {
    RemoveNamedDecl(Used);
    return;
  }
 
  if (const auto *UsedShadow =
          Result.Nodes.getNodeAs<UsingShadowDecl>("usedShadow")) {
    removeFromFoundDecls(UsedShadow->getTargetDecl());
    return;
  }
 
  if (const auto *Used = Result.Nodes.getNodeAs<TemplateArgument>("used")) {
    if (Used->getKind() == TemplateArgument::Template) {
      if (const auto *TD = Used->getAsTemplate().getAsTemplateDecl())
        removeFromFoundDecls(TD);
      return;
    }
 
    if (Used->getKind() == TemplateArgument::Type) {
      if (auto *RD = Used->getAsType()->getAsCXXRecordDecl())
        removeFromFoundDecls(RD);
      return;
    }
 
    if (Used->getKind() == TemplateArgument::Declaration)
      RemoveNamedDecl(Used->getAsDecl());
    return;
  }
 
  if (const auto *DRE = Result.Nodes.getNodeAs<DeclRefExpr>("used")) {
    RemoveNamedDecl(DRE->getDecl());
    return;
  }
  // Check the uninstantiated template function usage.
  if (const auto *ULE = Result.Nodes.getNodeAs<UnresolvedLookupExpr>("used")) {
    for (const NamedDecl *ND : ULE->decls())
      if (const auto *USD = dyn_cast<UsingShadowDecl>(ND))
        removeFromFoundDecls(USD->getTargetDecl()->getCanonicalDecl());
    return;
  }
  // Check user-defined literals
  if (const auto *UDL = Result.Nodes.getNodeAs<UserDefinedLiteral>("used")) {
    const Decl *CalleeDecl = UDL->getCalleeDecl();
    if (const auto *FD = dyn_cast<FunctionDecl>(CalleeDecl)) {
      if (const FunctionTemplateDecl *FPT = FD->getPrimaryTemplate()) {
        removeFromFoundDecls(FPT);
        return;
      }
    }
    removeFromFoundDecls(CalleeDecl);
  }
}
 
void UnusedUsingDeclsCheck::removeFromFoundDecls(const Decl *D) {
  if (!D)
    return;
  const Decl *CanonicalDecl = D->getCanonicalDecl();
  if (!UsingTargetDeclsCache.contains(CanonicalDecl))
    return;
  // FIXME: Currently, we don't handle the using-decls being used in different
  // scopes (such as different namespaces, different functions). Instead of
  // giving an incorrect message, we mark all of them as used.
  for (auto &Context : Contexts) {
    if (Context.IsUsed)
      continue;
    if (Context.UsingTargetDecls.contains(CanonicalDecl))
      Context.IsUsed = true;
  }
}
 
void UnusedUsingDeclsCheck::onEndOfTranslationUnit() {
  for (const auto &Context : Contexts) {
    if (!Context.IsUsed) {
      diag(Context.FoundUsingDecl->getLocation(), "using decl %0 is unused")
          << Context.FoundUsingDecl;
      // Emit a fix and a fix description of the check;
      diag(Context.FoundUsingDecl->getLocation(),
           /*Description=*/"remove the using", DiagnosticIDs::Note)
          << FixItHint::CreateRemoval(Context.UsingDeclRange);
    }
  }
  Contexts.clear();
  UsingTargetDeclsCache.clear();
}
 
} // namespace clang::tidy::misc