Add bugprone-loop-variable-copied-then-modified clang-tidy check.

Adds a clang-tidy check that alerts when a loop variable is copied and
subsequently modified.

This is a bugprone pattern because the programmer in this case often
assumes they are modifying the original value instead of a copy.

This warning can be suppressed by either converting the loop variable to
a const ref, or by performing the copy explicitly inside the body of the
loop.

Fix llvmGH-155922

Author: flowerhack

clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp

@@ -40,6 +40,7 @@
 #include "IntegerDivisionCheck.h"
 #include "InvalidEnumDefaultInitializationCheck.h"
 #include "LambdaFunctionNameCheck.h"
+#include "LoopVariableCopiedThenModifiedCheck.h"
 #include "MacroParenthesesCheck.h"
 #include "MacroRepeatedSideEffectsCheck.h"
 #include "MisleadingSetterOfReferenceCheck.h"
@@ -154,6 +155,8 @@ class BugproneModule : public ClangTidyModule {
         "bugprone-incorrect-enable-if");
     CheckFactories.registerCheck<IncorrectEnableSharedFromThisCheck>(
         "bugprone-incorrect-enable-shared-from-this");
+    CheckFactories.registerCheck<LoopVariableCopiedThenModifiedCheck>(
+        "bugprone-loop-variable-copied-then-modified");
     CheckFactories.registerCheck<UnintendedCharOstreamOutputCheck>(
         "bugprone-unintended-char-ostream-output");
     CheckFactories.registerCheck<ReturnConstRefFromParameterCheck>(

clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt

@@ -31,6 +31,7 @@ add_clang_library(clangTidyBugproneModule STATIC
   IncorrectEnableIfCheck.cpp
   IncorrectEnableSharedFromThisCheck.cpp
   InvalidEnumDefaultInitializationCheck.cpp
+  LoopVariableCopiedThenModifiedCheck.cpp
   UnintendedCharOstreamOutputCheck.cpp
   ReturnConstRefFromParameterCheck.cpp
   SuspiciousStringviewDataUsageCheck.cpp

clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.cpp

@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "LoopVariableCopiedThenModifiedCheck.h"
+#include "../utils/Matchers.h"
+#include "../utils/TypeTraits.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Analysis/Analyses/ExprMutationAnalyzer.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+namespace {
+AST_MATCHER(VarDecl, isInMacro) { return Node.getBeginLoc().isMacroID(); }
+} // namespace
+
+LoopVariableCopiedThenModifiedCheck::LoopVariableCopiedThenModifiedCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context), IgnoreInexpensiveVariables(Options.get(
+                                         "IgnoreInexpensiveVariables", false)),
+      WarnOnlyOnAutoCopies(Options.get("WarnOnlyOnAutoCopies", false)) {}
+
+void LoopVariableCopiedThenModifiedCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "IgnoreInexpensiveVariables", IgnoreInexpensiveVariables);
+  Options.store(Opts, "WarnOnlyOnAutoCopies", WarnOnlyOnAutoCopies);
+}
+
+void LoopVariableCopiedThenModifiedCheck::registerMatchers(
+    MatchFinder *Finder) {
+  const auto HasReferenceOrPointerType = hasType(qualType(
+      unless(hasCanonicalType(anyOf(referenceType(), pointerType())))));
+  const auto IteratorReturnsValueType = cxxOperatorCallExpr(
+      hasOverloadedOperatorName("*"),
+      callee(
+          cxxMethodDecl(returns(unless(hasCanonicalType(referenceType()))))));
+  const auto NotConstructedByCopy = cxxConstructExpr(
+      hasDeclaration(cxxConstructorDecl(unless(isCopyConstructor()))));
+  const auto ConstructedByConversion =
+      cxxMemberCallExpr(callee(cxxConversionDecl()));
+  const auto LoopVar =
+      varDecl(unless(isInMacro()), HasReferenceOrPointerType,
+              unless(hasInitializer(expr(hasDescendant(expr(
+                  anyOf(materializeTemporaryExpr(), IteratorReturnsValueType,
+                        NotConstructedByCopy, ConstructedByConversion)))))));
+  Finder->addMatcher(cxxForRangeStmt(hasLoopVariable(LoopVar.bind("loopVar")))
+                         .bind("forRange"),
+                     this);
+}
+
+void LoopVariableCopiedThenModifiedCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *LoopVar = Result.Nodes.getNodeAs<VarDecl>("loopVar");
+  std::optional<bool> Expensive = utils::type_traits::isExpensiveToCopy(
+      LoopVar->getType(), *Result.Context);
+  if ((!Expensive || !*Expensive) && IgnoreInexpensiveVariables)
+    return;
+  if (WarnOnlyOnAutoCopies) {
+    if (!isa<AutoType>(LoopVar->getType())) {
+      return;
+    }
+  }
+  const auto *ForRange = Result.Nodes.getNodeAs<CXXForRangeStmt>("forRange");
+
+  if (!ExprMutationAnalyzer(*ForRange->getBody(), *Result.Context)
+           .isMutated(LoopVar)) {
+    return;
+  }
+
+  clang::SourceRange LoopVarSourceRange =
+      LoopVar->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+  clang::SourceLocation EndLoc = clang::Lexer::getLocForEndOfToken(
+      LoopVarSourceRange.getEnd(), 0, Result.Context->getSourceManager(),
+      Result.Context->getLangOpts());
+  diag(LoopVar->getLocation(),
+       "loop variable '%0' is copied and then (possibly) modified; use an "
+       "explicit copy inside the body of the loop or make the variable a "
+       "reference")
+      << LoopVar->getName();
+  diag(LoopVar->getLocation(), "consider making '%0' a reference",
+       DiagnosticIDs::Note)
+      << LoopVar->getName()
+      << FixItHint::CreateInsertion(LoopVarSourceRange.getBegin(), "const ")
+      << FixItHint::CreateInsertion(EndLoc, "&");
+}
+
+} // namespace clang::tidy::bugprone

clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.h

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::bugprone {
+
+/// Finds loop variables that are copied and subsequently modified.
+///
+/// For the user-facing documentation see:
+/// https://clang.llvm.org/extra/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.html
+class LoopVariableCopiedThenModifiedCheck : public ClangTidyCheck {
+public:
+  LoopVariableCopiedThenModifiedCheck(StringRef Name,
+                                      ClangTidyContext *Context);
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+
+private:
+  const bool IgnoreInexpensiveVariables;
+  const bool WarnOnlyOnAutoCopies;
+};
+
+} // namespace clang::tidy::bugprone
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -142,12 +142,23 @@ Improvements to clang-tidy
 New checks
 ^^^^^^^^^^
 
+- New :doc:`bugprone-derived-method-shadowing-base-method
+  <clang-tidy/checks/bugprone/derived-method-shadowing-base-method>` check.
+
+  Finds derived class methods that shadow a (non-virtual) base class method.
+
 - New :doc:`bugprone-invalid-enum-default-initialization
   <clang-tidy/checks/bugprone/invalid-enum-default-initialization>` check.
 
   Detects default initialization (to 0) of variables with ``enum`` type where
   the enum has no enumerator with value of 0.
 
+- New :doc:`bugprone-loop-variable-copied-then-modified
+  <clang-tidy/checks/bugprone/loop-variable-copied-then-modified>` check.
+
+  Detects when a loop variable is copied and then subsequently (possibly) modified
+  and suggests replacing with a reference or an explicit copy.
+
 - New :doc:`cppcoreguidelines-pro-bounds-avoid-unchecked-container-access
   <clang-tidy/checks/cppcoreguidelines/pro-bounds-avoid-unchecked-container-access>`
   check.

clang-tools-extra/docs/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.rst

@@ -0,0 +1,55 @@
+.. title:: clang-tidy - bugprone-loop-variable-copied-then-modified
+
+bugprone-loop-variable-copied-then-modified
+===========================================
+
+Detects when a loop variable is copied and then subsequently (possibly) modified
+and suggests replacing with a reference or an explicit copy.
+
+This pattern is considered bugprone because, frequently, programmers do not
+realize that they are modifying a *copy* rather than an underlying value,
+resulting in subtly erroneous code.
+
+For instance, the following code attempts to null out a value in a map, but only
+succeeds in nulling out a value in a *copy* of the map:
+
+.. code-block:: c++
+
+  for (auto target : target_map) {
+    target.value = nullptr;
+  }
+
+The programmer is likely to have intended this code instead:
+
+.. code-block:: c++
+
+  for (auto& target : target_map) {
+    target.value = nullptr;
+  }
+
+This code can be fixed in one of two ways:
+  - In cases where the programmer did not intend to create a copy, they can
+    convert the loop variable to a reference or a ``const`` reference. A
+    fix-note message will provide a naive suggestion of how to achieve this,
+    which works in most cases.
+  - In cases where the intent is in fact to modify a copy, they may perform the
+    copy explicitly, inside the body of the loop, and perform whatever
+    operations they like on that copy.
+
+This is a conservative check: in cases where it cannot be determined at compile
+time whether or not a particular function modifies the variable, it assumes a
+modification has ocurred and warns accordingly. However, in such cases, the
+warning can still be suppressed by doing one of the actions described above.
+
+Options
+-------
+
+.. option:: IgnoreInexpensiveVariables
+
+   When `true`, this check will only alert on types that are expensive to copy.
+   This will lead to fewer false positives, but will also overlook some
+   instances where there may be an actual bug. Default is `false`.
+
+.. option:: WarnOnlyOnAutoCopies
+
+  When `true`, this check will only alert on `auto` types. Default is `false`.

clang-tools-extra/docs/clang-tidy/checks/list.rst

@@ -108,6 +108,7 @@ Clang-Tidy Checks
    :doc:`bugprone-integer-division <bugprone/integer-division>`,
    :doc:`bugprone-invalid-enum-default-initialization <bugprone/invalid-enum-default-initialization>`,
    :doc:`bugprone-lambda-function-name <bugprone/lambda-function-name>`,
+   :doc:`bugprone-loop-variable-copied-then-modified <bugprone/loop-variable-copied-then-modified>`, "Yes"
    :doc:`bugprone-macro-parentheses <bugprone/macro-parentheses>`, "Yes"
    :doc:`bugprone-macro-repeated-side-effects <bugprone/macro-repeated-side-effects>`,
    :doc:`bugprone-misleading-setter-of-reference <bugprone/misleading-setter-of-reference>`,

clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-generics.cpp

@@ -0,0 +1,74 @@
+// RUN: %check_clang_tidy -std=c++17-or-later %s bugprone-loop-variable-copied-then-modified %t --fix-notes
+
+template <typename T>
+struct Iterator {
+  void operator++() {}
+  const T& operator*() {
+    static T* TT = new T();
+    return *TT;
+  }
+  bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+  T begin() { return T(); }
+  T begin() const { return T(); }
+  T end() { return T(); }
+  T end() const { return T(); }
+};
+
+struct ConstructorConvertible {
+};
+
+struct S {
+  int value;
+
+  S() : value(0) {};
+  S(const S &);
+  S(const ConstructorConvertible&) {}
+  ~S();
+  S &operator=(const S &);
+  void modify() {
+    value++;
+  }
+};
+
+struct Convertible {
+  operator S() const {
+    return S();
+  }
+};
+
+struct PairLike {
+  int id;
+  S data;
+};
+
+template <typename V>
+struct Generic {
+  V value;
+
+  Generic() : value{} {};
+  Generic(const Generic &);
+  ~Generic();
+  Generic &operator=(const Generic &);
+  void modify() {
+    value++;
+  }
+};
+
+void PositiveLoopVariableCopiedAndThenModfiedGeneric() {
+  for (Generic G : View<Iterator<Generic<double>>>()) {
+    // CHECK-MESSAGES: [[@LINE-1]]:16: warning: loop variable 'G' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+    // CHECK-MESSAGES: [[@LINE-2]]:16: note: consider making 'G' a reference
+    // CHECK-FIXES: for (const Generic& G : View<Iterator<Generic<double>>>()) {
+    G.modify();
+  }
+}
+
+void NegativeLoopVariableIsReferenceAndModifiedGeneric() {
+  for (const Generic<double>& G : View<Iterator<Generic<double>>>()) {
+    // It's fine to copy-by-value G into some other G.
+    Generic<double> G2 = G;
+  }
+}

clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-inexpensive.cpp

@@ -0,0 +1,54 @@
+// RUN: %check_clang_tidy %s bugprone-loop-variable-copied-then-modified %t --fix-notes \
+// RUN: -config="{CheckOptions: \
+// RUN: {bugprone-loop-variable-copied-then-modified.IgnoreInexpensiveVariables: true}}" \
+// RUN: -- -I%S
+#include "Inputs/system-header-simulator/sim_initializer_list"
+#include "Inputs/system-header-simulator/sim_vector"
+
+template <typename T>
+struct Iterator {
+  void operator++() {}
+  const T& operator*() {
+    static T* TT = new T();
+    return *TT;
+  }
+  bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+  T begin() { return T(); }
+  T begin() const { return T(); }
+  T end() { return T(); }
+  T end() const { return T(); }
+};
+
+struct S {
+  int value;
+
+  S() : value(0) {};
+  S(const S &);
+  ~S();
+  S &operator=(const S &);
+  void modify() {
+    value++;
+  }
+};
+
+void NegativeOnlyCopyingInts() {
+    std::vector<int> foo;
+    foo.push_back(1);
+    foo.push_back(2);
+    foo.push_back(3);
+    for (int v : foo) {
+        v += 1;
+    }
+}
+
+void PositiveLoopVariableCopiedAndThenModfied() {
+  for (S S1 : View<Iterator<S>>()) {
+    // CHECK-MESSAGES: [[@LINE-1]]:10: warning: loop variable 'S1' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+    // CHECK-MESSAGES: [[@LINE-2]]:10: note: consider making 'S1' a reference
+    // CHECK-FIXES: for (const S& S1 : View<Iterator<S>>()) {
+    S1.modify();
+  }
+}