[clang-tidy] Move implicit-cast-in-loop check to upstream.

[hokein]

Summary

This is implemented originally by Alex Pilkiewicz ([e-mail]).

[hokein]

Add documents.

[alexfh]

Looks good with one nit. I'll fix and commit the patch.

Thank you!

clang-tidy/performance/ImplicitCastInLoopCheck.cpp

79

No braces needed around single-line if bodies.

[alexfh]

test/clang-tidy/performance_implicit_cast_in_loop.cpp

1

Also, the name should use dashes, not underscores. I'll fix this as well.

clang-tidy/performance/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS support)
 
 add_clang_library(clangTidyPerformanceModule
+  ImplicitCastInLoopCheck.cpp
   PerformanceTidyModule.cpp
   UnnecessaryCopyInitialization.cpp
 
   LINK_LIBS
   clangAST
   clangASTMatchers
   clangBasic
   clangLex
   clangTidy
   clangTidyUtils
   )

clang-tidy/performance/ImplicitCastInLoopCheck.h

+//===--- ImplicitCastInLoopCheck.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_PERFORMANCE_IMPLICIT_CAST_IN_LOOP_CHECK_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_PERFORMANCE_IMPLICIT_CAST_IN_LOOP_CHECK_H_
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace performance {
+
+// Checks that in a for range loop, if the provided type is a reference, then
+// the underlying type is the one returned by the iterator (i.e. that there
+// isn't any implicit conversion).
+class ImplicitCastInLoopCheck : public ClangTidyCheck {
+ public:
+  using ClangTidyCheck::ClangTidyCheck;
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+ private:
+  void ReportAndFix(const ASTContext *Context, const VarDecl *VD,
+                    const CXXOperatorCallExpr *OperatorCall);
+};
+
+} // namespace performance
+} // namespace tidy
+} // namespace clang
+
+#endif  // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_PERFORMANCE_IMPLICIT_CAST_IN_LOOP_CHECK_H_

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

clang-tidy/performance/PerformanceTidyModule.cpp

 //===--- PeformanceTidyModule.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 "../ClangTidy.h"
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 
+#include "ImplicitCastInLoopCheck.h"
 #include "UnnecessaryCopyInitialization.h"
 
 namespace clang {
 namespace tidy {
 namespace performance {
 
 class PerformanceModule : public ClangTidyModule {
 public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
+    CheckFactories.registerCheck<ImplicitCastInLoopCheck>(
+        "performance-implicit-cast-in-loop");
     CheckFactories.registerCheck<UnnecessaryCopyInitialization>(
         "performance-unnecessary-copy-initialization");
   }
 };
 
 // Register the PerformanceModule using this statically initialized variable.
 static ClangTidyModuleRegistry::Add<PerformanceModule>
     X("performance-module", "Adds performance checks.");

docs/clang-tidy/checks/list.rst

.. toctree::
    modernize-pass-by-value
    modernize-redundant-void-arg
    modernize-replace-auto-ptr
    modernize-shrink-to-fit
    modernize-use-auto
    modernize-use-default
    modernize-use-nullptr
    modernize-use-override
+   performance-implicit-cast-in-loop
    readability-braces-around-statements
    readability-container-size-empty
    readability-else-after-return
    readability-function-size
    readability-identifier-naming
    readability-implicit-bool-cast
    readability-inconsistent-declaration-parameter-name
    readability-named-parameter
    readability-redundant-smartptr-get
    readability-redundant-string-cstr
    readability-simplify-boolean-expr
    readability-uniqueptr-delete-release

docs/clang-tidy/checks/performance-implicit-cast-in-loop.rst

+performance-implicit-cast-in-loop
+=================================
+
+This warning appears in range-based loop with loop variable of const ref type
+where the type of the variable does not match the one returned by the iterator.
+This means that an implicit cast has been added, which can for example result in
+expensive deep copies.
+
+Example:
+
+.. code:: c++
+
+   map<int, vector<string>> my_map;
+   for (const pair<int, vector<string>>& p : my_map) {}
+   // The iterator type is in fact pair<const int, vector<string>>, which means
+   // that the compiler added a cast, resulting in a copy of the vectors.
+
+The easiest solution is usually to use "const auto&" instead of writing the type
+manually.

test/clang-tidy/performance_implicit_cast_in_loop.cpp

+// RUN: %check_clang_tidy %s performance-implicit-cast-in-loop %t
+
+// ---------- Classes used in the tests ----------
+
+// Iterator returning by value.
+template <typename T>
+struct Iterator {
+  void operator++();
+  T operator*();
+  bool operator!=(const Iterator& other);
+};
+
+// Iterator returning by reference.
+template <typename T>
+struct RefIterator {
+  void operator++();
+  T& operator*();
+  bool operator!=(const RefIterator& other);
+};
+
+// The template argument is an iterator type, and a view is an object you can
+// run a for loop on.
+template <typename T>
+struct View {
+  T begin();
+  T end();
+};
+
+// With this class, the implicit cast is a call to the (implicit) constructor of
+// the class.
+template <typename T>
+class ImplicitWrapper {
+ public:
+  // Implicit!
+  ImplicitWrapper(const T& t);
+};
+
+// With this class, the implicit cast is a call to the conversion operators of
+// SimpleClass and ComplexClass.
+template <typename T>
+class OperatorWrapper {
+ public:
+  explicit OperatorWrapper(const T& t);
+};
+
+struct SimpleClass {
+  int foo;
+  operator OperatorWrapper<SimpleClass>();
+};
+
+// The materialize expression is not the same when the class has a destructor,
+// so we make sure we cover that case too.
+class ComplexClass {
+ public:
+  ComplexClass();
+  ~ComplexClass();
+  operator OperatorWrapper<ComplexClass>();
+};
+
+typedef View<Iterator<SimpleClass>> SimpleView;
+typedef View<RefIterator<SimpleClass>> SimpleRefView;
+typedef View<Iterator<ComplexClass>> ComplexView;
+typedef View<RefIterator<ComplexClass>> ComplexRefView;
+
+// ---------- The test themselves ----------
+// For each test we do, in the same order, const ref, non const ref, const
+// value, non const value.
+
+void SimpleClassIterator() {
+  for (const SimpleClass& foo : SimpleView()) {}
+  // This line does not compile because a temporary cannot be assigned to a non
+  // const reference.
+  // for (SimpleClass& foo : SimpleView()) {}
+  for (const SimpleClass foo : SimpleView()) {}
+  for (SimpleClass foo : SimpleView()) {}
+}
+
+void SimpleClassRefIterator() {
+  for (const SimpleClass& foo : SimpleRefView()) {}
+  for (SimpleClass& foo : SimpleRefView()) {}
+  for (const SimpleClass foo : SimpleRefView()) {}
+  for (SimpleClass foo : SimpleRefView()) {}
+}
+
+void ComplexClassIterator() {
+  for (const ComplexClass& foo : ComplexView()) {}
+  // for (ComplexClass& foo : ComplexView()) {}
+  for (const ComplexClass foo : ComplexView()) {}
+  for (ComplexClass foo : ComplexView()) {}
+}
+
+void ComplexClassRefIterator() {
+  for (const ComplexClass& foo : ComplexRefView()) {}
+  for (ComplexClass& foo : ComplexRefView()) {}
+  for (const ComplexClass foo : ComplexRefView()) {}
+  for (ComplexClass foo : ComplexRefView()) {}
+}
+
+void ImplicitSimpleClassIterator() {
+  for (const ImplicitWrapper<SimpleClass>& foo : SimpleView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the loop variable 'foo' is different from the one returned by the iterator and generates an implicit cast; you can either change the type to the correct one ('const SimpleClass &' but 'const auto&' is always a valid option) or remove the reference to make it explicit that you are creating a new value [performance-implicit-cast-in-loop]
+  // for (ImplicitWrapper<SimpleClass>& foo : SimpleView()) {}
+  for (const ImplicitWrapper<SimpleClass> foo : SimpleView()) {}
+  for (ImplicitWrapper<SimpleClass>foo : SimpleView()) {}
+}
+
+void ImplicitSimpleClassRefIterator() {
+  for (const ImplicitWrapper<SimpleClass>& foo : SimpleRefView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const SimpleClass &'.*}}
+  // for (ImplicitWrapper<SimpleClass>& foo : SimpleRefView()) {}
+  for (const ImplicitWrapper<SimpleClass> foo : SimpleRefView()) {}
+  for (ImplicitWrapper<SimpleClass>foo : SimpleRefView()) {}
+}
+
+void ImplicitComplexClassIterator() {
+  for (const ImplicitWrapper<ComplexClass>& foo : ComplexView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const ComplexClass &'.*}}
+  // for (ImplicitWrapper<ComplexClass>& foo : ComplexView()) {}
+  for (const ImplicitWrapper<ComplexClass> foo : ComplexView()) {}
+  for (ImplicitWrapper<ComplexClass>foo : ComplexView()) {}
+}
+
+void ImplicitComplexClassRefIterator() {
+  for (const ImplicitWrapper<ComplexClass>& foo : ComplexRefView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const ComplexClass &'.*}}
+  // for (ImplicitWrapper<ComplexClass>& foo : ComplexRefView()) {}
+  for (const ImplicitWrapper<ComplexClass> foo : ComplexRefView()) {}
+  for (ImplicitWrapper<ComplexClass>foo : ComplexRefView()) {}
+}
+
+void OperatorSimpleClassIterator() {
+  for (const OperatorWrapper<SimpleClass>& foo : SimpleView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const SimpleClass &'.*}}
+  // for (OperatorWrapper<SimpleClass>& foo : SimpleView()) {}
+  for (const OperatorWrapper<SimpleClass> foo : SimpleView()) {}
+  for (OperatorWrapper<SimpleClass>foo : SimpleView()) {}
+}
+
+void OperatorSimpleClassRefIterator() {
+  for (const OperatorWrapper<SimpleClass>& foo : SimpleRefView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const SimpleClass &'.*}}
+  // for (OperatorWrapper<SimpleClass>& foo : SimpleRefView()) {}
+  for (const OperatorWrapper<SimpleClass> foo : SimpleRefView()) {}
+  for (OperatorWrapper<SimpleClass>foo : SimpleRefView()) {}
+}
+
+void OperatorComplexClassIterator() {
+  for (const OperatorWrapper<ComplexClass>& foo : ComplexView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const ComplexClass &'.*}}
+  // for (OperatorWrapper<ComplexClass>& foo : ComplexView()) {}
+  for (const OperatorWrapper<ComplexClass> foo : ComplexView()) {}
+  for (OperatorWrapper<ComplexClass>foo : ComplexView()) {}
+}
+
+void OperatorComplexClassRefIterator() {
+  for (const OperatorWrapper<ComplexClass>& foo : ComplexRefView()) {}
+  // CHECK-MESSAGES: [[@LINE-1]]:{{[0-9]*}}: warning: the type of the{{.*'const ComplexClass &'.*}}
+  // for (OperatorWrapper<ComplexClass>& foo : ComplexRefView()) {}
+  for (const OperatorWrapper<ComplexClass> foo : ComplexRefView()) {}
+  for (OperatorWrapper<ComplexClass>foo : ComplexRefView()) {}
+}