checkunusedvar: handle array element change using pointer arithmetic (#3000)

Author: miltolstoy

lib/astutils.cpp

@@ -1681,6 +1681,23 @@ bool isVariableChanged(const Token *tok, int indirect, const Settings *settings,
             return true;
         return false;
     }
+
+    if (indirect > 0)
+    {
+        // check for `*(ptr + 1) = new_value` case
+        parent = tok2->astParent();
+        while (parent && parent->isArithmeticalOp() && parent->isBinaryOp()) {
+            parent = parent->astParent();
+        }
+        if (Token::simpleMatch(parent, "*"))
+        {
+            if (parent->astParent() && parent->astParent()->isAssignmentOp() &&
+                (parent->astParent()->astOperand1() == parent)) {
+                return true;
+            }
+        }
+    }
+
     return false;
 }
 

lib/checkunusedvar.cpp

@@ -1620,9 +1620,6 @@ bool CheckUnusedVar::isFunctionWithoutSideEffects(const Function& func, const To
             }
             // check if global variable is changed
             if (bodyVariable->isGlobal() || (pointersToGlobals.find(bodyVariable) != pointersToGlobals.end())) {
-                if (bodyVariable->isPointer() || bodyVariable->isArray()) {
-                    return false; // TODO: Update astutils.cpp:1544 isVariableChanged() and remove this. Unhandled case: `*(global_arr + 1) = new_val`
-                }
                 const int depth = 20;
                 if (isVariableChanged(bodyToken, depth, mSettings, mTokenizer->isCPP())) {
                     return false;

test/testunusedvar.cpp

@@ -748,6 +748,23 @@ class TestUnusedVar : public TestFixture {
             "}");
         ASSERT_EQUALS("", errout.str());
 
+        functionVariableUsage(
+            "int x[] = {0, 1, 3};\n"
+            "int func() {\n"
+            "   *(x) = 2;\n"
+            "   return 1;\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("", errout.str());
+
+        // pointer arithmetic on global array
         functionVariableUsage(
             "int x[] = {0, 1, 3};\n"
             "int func() {\n"
@@ -764,6 +781,87 @@ class TestUnusedVar : public TestFixture {
             "}");
         ASSERT_EQUALS("", errout.str());
 
+        functionVariableUsage(
+            "int x[][] = {{0, 1}, {2, 3}};\n"
+            "int func() {\n"
+            "   *((x + 1) + 1) = 4;\n"
+            "   return 1;\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("", errout.str());
+
+        functionVariableUsage(
+            "int x[] = {0, 1, 3};\n"
+            "int func() {\n"
+            "   int local = *(x + 1);\n"
+            "   (void) local;\n"
+            "   return 1;\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", errout.str());
+
+        functionVariableUsage(
+            "int x[] = {0, 1, 3};\n"
+            "int func() {\n"
+            "   int* local = x + 2;\n"
+            "   (void) local;\n"
+            "   return 1;\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", errout.str());
+
+        functionVariableUsage(
+            "int x[] = {0, 1, 3};\n"
+            "int func() {\n"
+            "   int* local = x + 2;\n"
+            "   return *local;\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("", errout.str());
+
+        functionVariableUsage(
+            "int x[] = {0, 1, 3};\n"
+            "int func() {\n"
+            "   return *(x + 1);\n"
+            "}\n"
+            "class C {\n"
+            "public:\n"
+            "   C() : x(func()) {}\n"
+            "   int x;\n"
+            "};\n"
+            "void f() {\n"
+            "   C c;\n"
+            "}");
+        ASSERT_EQUALS("", errout.str());
+
         // changing local variable
         functionVariableUsage(
             "int func() {\n"
@@ -1064,8 +1162,7 @@ class TestUnusedVar : public TestFixture {
             "void f() {\n"
             "   C c;\n"
             "}");
-        // TODO: see TODO for global vars under CheckUnusedVar::isFunctionWithoutSideEffects()
-        TODO_ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", "", errout.str());
+        ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", errout.str());
 
         // global struct variable modification
         functionVariableUsage(
@@ -1135,8 +1232,7 @@ class TestUnusedVar : public TestFixture {
             "void f() {\n"
             "   C c;\n"
             "}");
-        // TODO: see TODO for global vars under CheckUnusedVar::isFunctionWithoutSideEffects()
-        TODO_ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", "", errout.str());
+        ASSERT_EQUALS("[test.cpp:13]: (style) Unused variable: c\n", errout.str());
     }
 
     // #5355 - False positive: Variable is not assigned a value.