[clang-tools-extra] Add bugprone-loop-variable-copied-then-modified clang-tidy check. (PR #157213)

[Baranov Victor via cfe-commits]

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@@ -0,0 +1,89 @@
+//===----------------------------------------------------------------------===//
+//
+// 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"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+LoopVariableCopiedThenModifiedCheck::LoopVariableCopiedThenModifiedCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      IgnoreInexpensiveVariables(
+          Options.get("IgnoreInexpensiveVariables", false)) {}
+
+void LoopVariableCopiedThenModifiedCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "IgnoreInexpensiveVariables", IgnoreInexpensiveVariables);
+}
+
+void LoopVariableCopiedThenModifiedCheck::registerMatchers(
+    MatchFinder *Finder) {
+  const auto HasReferenceOrPointerTypeOrIsAllowed = 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(HasReferenceOrPointerTypeOrIsAllowed,
+              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");
+  if (LoopVar->getBeginLoc().isMacroID())
+    return;
+  std::optional<bool> Expensive = utils::type_traits::isExpensiveToCopy(
+      LoopVar->getType(), *Result.Context);
+  if ((!Expensive || !*Expensive) && IgnoreInexpensiveVariables)
+    return;
+  const auto *ForRange = Result.Nodes.getNodeAs<CXXForRangeStmt>("forRange");
+
+  std::string HintString = "";
+
+  if (ExprMutationAnalyzer(*ForRange->getBody(), *Result.Context)
+          .isMutated(LoopVar)) {
+    if (isa<AutoType>(LoopVar->getType())) {
+      HintString = "const auto&";
+    } else {
+      const std::string CanonicalTypeStr =
+          LoopVar->getType().getAsString(Result.Context->getLangOpts());
+      HintString = "const " + CanonicalTypeStr + "&";
+    }
+    clang::SourceRange LoopVarSourceRange =
+        LoopVar->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+    diag(LoopVar->getLocation(), "loop variable '%0' is copied and then "
+                                 "modified, which is likely a bug; you "
+                                 "probably want to modify the underlying "
+                                 "object and not this copy. If you "
+                                 "*did* intend to modify this copy, "
+                                 "please use an explicit copy inside the "
+                                 "body of the loop")
----------------
vbvictor wrote:

The user would read documentation if necessary - the diagnostic should be concise.

https://github.com/llvm/llvm-project/pull/157213