Add support for C++ attribute nodiscard for functions. (#1269)

lib/checkfunctions.cpp

@@ -194,10 +194,13 @@ void CheckFunctions::checkIgnoredReturnValue()
                 continue;
             }
 
-            if ((!tok->function() || !Token::Match(tok->function()->retDef, "void %name%")) && _settings->library.isUseRetVal(tok) && !WRONG_DATA(!tok->next()->astOperand1(), tok))
+            if ((!tok->function() || !Token::Match(tok->function()->retDef, "void %name%")) &&
+                (_settings->library.isUseRetVal(tok) || (tok->function() && tok->function()->isAttributeNodiscard())) &&
+                !WRONG_DATA(!tok->next()->astOperand1(), tok)) {
                 ignoredReturnValueError(tok, tok->next()->astOperand1()->expressionString());
             }
         }
+    }
 }
 
 void CheckFunctions::ignoredReturnValueError(const Token* tok, const std::string& function)

lib/symboldatabase.cpp

@@ -2732,6 +2732,8 @@ void SymbolDatabase::printOut(const char *title) const
                 std::cout << " constructor ";
             if (func->isAttributeDestructor())
                 std::cout << " destructor ";
+            if (func->isAttributeNodiscard())
+                std::cout << " nodiscard ";
             std::cout << std::endl;
             std::cout << "        noexceptArg: " << (func->noexceptArg ? func->noexceptArg->str() : "none") << std::endl;
             std::cout << "        throwArg: " << (func->throwArg ? func->throwArg->str() : "none") << std::endl;

lib/symboldatabase.h

@@ -747,6 +747,9 @@ public:
     bool isAttributeNothrow() const {
         return tokenDef->isAttributeNothrow();
     }
+    bool isAttributeNodiscard() const {
+        return tokenDef->isAttributeNodiscard();
+    }
 
     bool hasBody() const {
         return getFlag(fHasBody);

lib/token.h

@@ -413,6 +413,12 @@ public:
     void isAttributePacked(const bool value) {
         setFlag(fIsAttributePacked, value);
     }
+    bool isAttributeNodiscard() const {
+        return getFlag(fIsAttributeNodiscard);
+    }
+    void isAttributeNodiscard(const bool value) {
+        setFlag(fIsAttributeNodiscard, value);
+    }
     bool isControlFlowKeyword() const {
         return getFlag(fIsControlFlowKeyword);
     }
@@ -949,6 +955,7 @@ private:
         fIsName                 = (1 << 20),
         fIsLiteral              = (1 << 21),
         fIsTemplateArg          = (1 << 22),
+        fIsAttributeNodiscard   = (1 << 23), // __attribute__ ((warn_unused_result)), [[nodiscard]]
     };
 
     unsigned int _flags;

lib/tokenize.cpp

@@ -8945,7 +8945,7 @@ void Tokenizer::simplifyAttribute()
                 }
             }
 
-            else if (Token::Match(tok->tokAt(2), "( pure|__pure__|const|__const__|noreturn|__noreturn__|nothrow|__nothrow__ )")) {
+            else if (Token::Match(tok->tokAt(2), "( pure|__pure__|const|__const__|noreturn|__noreturn__|nothrow|__nothrow__|warn_unused_result )")) {
                 Token *functok = nullptr;
 
                 // type func(...) __attribute__((attribute));
@@ -8971,6 +8971,8 @@ void Tokenizer::simplifyAttribute()
                         functok->isAttributeNoreturn(true);
                     else if (attribute.find("nothrow") != std::string::npos)
                         functok->isAttributeNothrow(true);
+                    else if (attribute.find("warn_unused_result") != std::string::npos)
+                        functok->isAttributeNodiscard(true);
                 }
             }
 
@@ -8992,12 +8994,16 @@ void Tokenizer::simplifyCPPAttribute()
     for (Token *tok = list.front(); tok; tok = tok->next()) {
         if (!tok->link() || !Token::Match(tok, "[ [ %name%"))
             continue;
-        if (tok->strAt(2) == "noreturn") {
+        if (Token::Match(tok->tokAt(2), "noreturn|nodiscard")) {
             const Token * head = tok->link()->next();
             while (Token::Match(head, "%name%|::|*|&"))
                 head = head->next();
-            if (head && head->str() == "(" && isFunctionHead(head, "{|;"))
+            if (head && head->str() == "(" && isFunctionHead(head, "{|;")) {
+                if (tok->strAt(2) == "noreturn")
                     head->previous()->isAttributeNoreturn(true);
+                else
+                    head->previous()->isAttributeNodiscard(true);
+            }
         }
         Token::eraseTokens(tok, tok->link()->next());
         tok->deleteThis();

test/testfunctions.cpp

@@ -926,6 +926,20 @@ private:
         check("void foo() {\n" // don't crash
               "  DEBUG(123)(mystrcmp(a,b))(fd);\n"
               "}", "test.c", &settings2);
+        check("struct teststruct {\n"
+              "    int testfunc1() __attribute__ ((warn_unused_result)) { return 1; }\n"
+              "    [[nodiscard]] int testfunc2() { return 1; }\n"
+              "    void foo() { testfunc1(); testfunc2(); }\n"
+              "};\n"
+              "int main() {\n"
+              "    teststruct TestStruct1;\n"
+              "    TestStruct1.testfunc1();\n"
+              "    TestStruct1.testfunc2();\n"
+              "}", "test.cpp", &settings2);
+        ASSERT_EQUALS("[test.cpp:4]: (warning) Return value of function testfunc1() is not used.\n"
+                      "[test.cpp:4]: (warning) Return value of function testfunc2() is not used.\n"
+                      "[test.cpp:8]: (warning) Return value of function TestStruct1.testfunc1() is not used.\n"
+                      "[test.cpp:9]: (warning) Return value of function TestStruct1.testfunc2() is not used.\n", errout.str());
     }
 
     void memsetZeroBytes() {