[clang] [alpha.webkit.UncountedLocalVarsChecker] Warn the use of a raw pointer/reference when the guardian variable gets mutated. (PR #113859)

[Rashmi Mudduluru via cfe-commits]

================
@@ -48,6 +48,64 @@ bool isRefcountedStringsHack(const VarDecl *V) {
   return false;
 }
 
+struct GuardianVisitor : public RecursiveASTVisitor<GuardianVisitor> {
+  using Base = RecursiveASTVisitor<GuardianVisitor>;
+
+  const VarDecl *Guardian{nullptr};
+
+public:
+  explicit GuardianVisitor(const VarDecl *Guardian) : Guardian(Guardian) {
+    assert(Guardian);
+  }
+
+  bool VisitBinaryOperator(const BinaryOperator *BO) {
+    if (BO->isAssignmentOp()) {
+      if (auto *VarRef = dyn_cast<DeclRefExpr>(BO->getLHS())) {
+        if (VarRef->getDecl() == Guardian)
+          return false;
+      }
+    }
+    return true;
+  }
+
+  bool VisitCXXConstructExpr(const CXXConstructExpr *CE) {
+    if (auto *Ctor = CE->getConstructor()) {
+      if (Ctor->isMoveConstructor() && CE->getNumArgs() == 1) {
+        auto *Arg = CE->getArg(0)->IgnoreParenCasts();
+        if (auto *VarRef = dyn_cast<DeclRefExpr>(Arg)) {
+          if (VarRef->getDecl() == Guardian)
+            return false;
+        }
+      }
+    }
+    return true;
+  }
+
+  bool VisitCXXMemberCallExpr(const CXXMemberCallExpr *MCE) {
+    auto MethodName = safeGetName(MCE->getMethodDecl());
+    if (MethodName == "swap" || MethodName == "leakRef" ||
+        MethodName == "releaseNonNull") {
+      auto *ThisArg = MCE->getImplicitObjectArgument()->IgnoreParenCasts();
+      if (auto *VarRef = dyn_cast<DeclRefExpr>(ThisArg)) {
+        if (VarRef->getDecl() == Guardian)
+          return false;
+      }
+    }
+    return true;
+  }
+
+  bool VisitCXXOperatorCallExpr(const CXXOperatorCallExpr *OCE) {
+    if (OCE->isAssignmentOp() && OCE->getNumArgs() == 2) {
----------------
t-rasmud wrote:

Is the check `OCE->getNumArgs() == 2` necessary here? What are the cases in which `CXXOperatorCallExpr` is an assignment op but has a different number of args?

https://github.com/llvm/llvm-project/pull/113859