Initial version of misc-unused-using-decl check

[alexfh]

Awesome! Thank you for tackling this! A few comments.

clang-tidy/misc/CMakeLists.txt

39

Please fix file sorting around the new file.

clang-tidy/misc/MiscTidyModule.cpp

123

I think, all "misc-unused-" checks except for the "misc-unused-raii" (which finds actual bugs) mostly relate to readability. I suggest placing the new check to readability/ and we'll then move the other two "misc-unused-" checks there as well.

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

24

Why is it necessarily a recordType? How about typedefs and enums?

clang-tidy/misc/UnusedUsingDeclsCheck.h

33

I'm not sure how the llvm/ADT/DenseMap.h header is pulled here, but DenseMap is not even used in ClangTidy.h, so this transitive inclusion is rather brittle. Please include llvm/ADT/DenseMap.h directly.

docs/clang-tidy/checks/misc-unused-using-decls.rst

7

Add an example, please.

[djasper]

clang-tidy/misc/MiscTidyModule.cpp

123

I am not really certain that "readability" is a reasonable category for unused things. Maybe unused should be its own category? At any rate, I think we should move all of them at the same time and not now go ahead and create a weird intermediate state.

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

24

I'd like to handle those as a follow-up. We only track using declarations of Records at the moment in the callback, too.

[alexfh]

LG with one nit.

Thank you!

clang-tidy/misc/MiscTidyModule.cpp

123

Fair enough. Let's go this way then.

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

67

FoundDecls should be cleared at the end. There are some cases when a check can be used multiple times (e.g. in unit tests). Though we're not adding unit tests here, I think, we shouldn't hide this kind of a surprise in the code ;)

[alexfh]

This seems to have been committed as r266735.

clang-tidy/misc/CMakeLists.txt

add_clang_library(clangTidyMiscModule
   StaticAssertCheck.cpp
   StringIntegerAssignmentCheck.cpp
   StringLiteralWithEmbeddedNulCheck.cpp
   SuspiciousMissingCommaCheck.cpp
   SuspiciousSemicolonCheck.cpp
   SwappedArgumentsCheck.cpp
   ThrowByValueCatchByReferenceCheck.cpp
   UndelegatedConstructor.cpp
+  UniqueptrResetReleaseCheck.cpp
   UnusedAliasDeclsCheck.cpp
   UnusedParametersCheck.cpp
   UnusedRAIICheck.cpp
-  UniqueptrResetReleaseCheck.cpp
+  UnusedUsingDeclsCheck.cpp
   VirtualNearMissCheck.cpp
 
   LINK_LIBS
   clangAST
   clangASTMatchers
   clangBasic
   clangLex
   clangTidy
   clangTidyUtils
   )

clang-tidy/misc/MiscTidyModule.cpp

 #include "SuspiciousSemicolonCheck.h"
 #include "SwappedArgumentsCheck.h"
 #include "ThrowByValueCatchByReferenceCheck.h"
 #include "UndelegatedConstructor.h"
 #include "UniqueptrResetReleaseCheck.h"
 #include "UnusedAliasDeclsCheck.h"
 #include "UnusedParametersCheck.h"
 #include "UnusedRAIICheck.h"
+#include "UnusedUsingDeclsCheck.h"
 #include "VirtualNearMissCheck.h"
 
 namespace clang {
 namespace tidy {
 namespace misc {
 
 class MiscModule : public ClangTidyModule {
 public:
CheckFactories.registerCheck<UndelegatedConstructorCheck>(
         "misc-undelegated-constructor");
     CheckFactories.registerCheck<UniqueptrResetReleaseCheck>(
         "misc-uniqueptr-reset-release");
     CheckFactories.registerCheck<UnusedAliasDeclsCheck>(
         "misc-unused-alias-decls");
     CheckFactories.registerCheck<UnusedParametersCheck>(
         "misc-unused-parameters");
     CheckFactories.registerCheck<UnusedRAIICheck>("misc-unused-raii");
+    CheckFactories.registerCheck<UnusedUsingDeclsCheck>(
+        "misc-unused-using-decls");
     CheckFactories.registerCheck<VirtualNearMissCheck>(
         "misc-virtual-near-miss");
   }
 };
 
 } // namespace misc
 
 // Register the MiscTidyModule using this statically initialized variable.

clang-tidy/misc/UnusedAliasDeclsCheck.h

 //===--- UnusedAliasDeclsCheck.h - clang-tidy--------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_UNUSED_ALIAS_DECLS_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_UNUSED_ALIAS_DECLS_H
 
 #include "../ClangTidy.h"
+#include "llvm/ADT/DenseMap.h"
 
 namespace clang {
 namespace tidy {
 
 /// Finds unused namespace alias declarations.
 class UnusedAliasDeclsCheck : public ClangTidyCheck {
 public:
   UnusedAliasDeclsCheck(StringRef Name, ClangTidyContext *Context)

clang-tidy/misc/UnusedUsingDeclsCheck.h

+//===--- UnusedUsingDeclsCheck.h - clang-tidy--------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_UNUSED_USING_DECLS_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_UNUSED_USING_DECLS_H
+
+#include "../ClangTidy.h"
+#include "llvm/ADT/DenseMap.h"
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+/// Finds unused using declarations.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/misc-unused-using-decls.html
+class UnusedUsingDeclsCheck : public ClangTidyCheck {
+public:
+  UnusedUsingDeclsCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  void onEndOfTranslationUnit() override;
+
+private:
+  llvm::DenseMap<const NamedDecl*, const UsingDecl*> FoundDecls;
+  llvm::DenseMap<const NamedDecl*, CharSourceRange> FoundRanges;
+};
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_UNUSED_USING_DECLS_H

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

+//===--- UnusedUsingDeclsCheck.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 "UnusedUsingDeclsCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(usingDecl(isExpansionInMainFile()).bind("using"), this);
+  Finder->addMatcher(recordType(hasDeclaration(namedDecl().bind("used"))),
+                     this);
+}
+
+void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
+  if (const auto *Using = Result.Nodes.getNodeAs<UsingDecl>("using")) {
+    // FIXME: Implement the correct behavior for using declarations with more
+    // than one shadow.
+    if (Using->shadow_size() != 1)
+      return;
+    const auto* TargetDecl = Using->shadow_begin()->getTargetDecl();
+
+    // FIXME: Handle other target types.
+    if (!isa<RecordDecl>(TargetDecl))
+      return;
+
+    FoundDecls[TargetDecl] = Using;
+    FoundRanges[TargetDecl] = CharSourceRange::getCharRange(
+        Using->getLocStart(),
+        Lexer::findLocationAfterToken(
+            Using->getLocEnd(), tok::semi, *Result.SourceManager,
+            Result.Context->getLangOpts(),
+            /*SkipTrailingWhitespaceAndNewLine=*/true));
+    return;
+  }
+
+  // Mark using declarations as used by setting FoundDecls' value to zero. As
+  // the AST is walked in order, usages are only marked after a the
+  // corresponding using declaration has been found.
+  // FIXME: This currently doesn't look at whether the type reference is
+  // actually found with the help of the using declaration.
+  if (const auto *Used = Result.Nodes.getNodeAs<NamedDecl>("used")) {
+    if (FoundDecls.find(Used) != FoundDecls.end())
+      FoundDecls[Used] = nullptr;
+  }
+}
+
+void UnusedUsingDeclsCheck::onEndOfTranslationUnit() {
+  for (const auto &FoundDecl : FoundDecls) {
+    if (FoundDecl.second == nullptr)
+      continue;
+    diag(FoundDecl.second->getLocation(), "using decl %0 is unused")
+        << FoundDecl.second
+        << FixItHint::CreateRemoval(FoundRanges[FoundDecl.first]);
+  }
+}
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang

docs/clang-tidy/checks/list.rst

.. toctree::
    misc-suspicious-semicolon
    misc-swapped-arguments
    misc-throw-by-value-catch-by-reference
    misc-undelegated-constructor
    misc-uniqueptr-reset-release
    misc-unused-alias-decls
    misc-unused-parameters
    misc-unused-raii
+   misc-unused-using-decls
    misc-virtual-near-miss
    modernize-deprecated-headers
    modernize-loop-convert
    modernize-make-unique
    modernize-pass-by-value
    modernize-raw-string-literal
    modernize-redundant-void-arg
    modernize-replace-auto-ptr

docs/clang-tidy/checks/misc-unused-using-decls.rst

+.. title:: clang-tidy - misc-unused-using-decls
+
+misc-unused-using-decls
+=======================
+
+Finds unused ``using`` declarations.
+
+Example:
+
+.. code:: c++
+
+  namespace n { class C; }
+  using n::C;  // Never actually used.
+

test/clang-tidy/misc-unused-using-decls.cpp

+// RUN: %check_clang_tidy %s misc-unused-using-decls %t
+
+// ----- Definitions -----
+template <typename T> class vector {};
+namespace n {
+class A;
+class B;
+class C;
+class D { public: static int i; };
+}
+
+// ----- Using declarations -----
+// eol-comments aren't removed (yet)
+using n::A; // A
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: using decl 'A' is unused
+// CHECK-FIXES: {{^}}// A
+using n::B;
+using n::C;
+using n::D;
+
+// ----- Usages -----
+void f(B b);
+void g() {
+  vector<C> data;
+  D::i = 1;
+}
+