Filter string_view from the nullptr diagnosis of bugprone-string-constructor to prevent duplicate warnings with bugprone-stringview-nullptr

[CJ-Johnson]

Summary

Updates the check and tests to not diagnose the null case for string_view (but retains it for string). This prevents the check from giving duplicate warnings that are caught by bugprone-stringview-nullptr (D113148).

[ymandel]

CJ -- please mention "bugprone-stringview-nullptr" explicitly and explain that it will cover those features, etc. so that the description will be clear on its own. Thanks!

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

178–179

Slightly prefer an early return here:
Filter out basic_string_view to avoid conflicts with
bugprone-stringview-nullptr
if (Result.Nodes.getNodeAs<CXXRecordDecl>("basic_string_view_decl") != nullptr)

return;

[CJ-Johnson]

Switch to early return for the filter

[njames93]

I'm in 2 minds about this. This diagnostic is a good fit for this warning and shouldn't be removed. Likewise duplicate diagnostics from different checks is annoying. Maybe a better path forward would be to suppress the diagnostic if the bugprone-stringview-nullptr check is enabled. These kinds of intercheck dependencies are found in modernize-prefer-member-init check IIRC. You can just copy the impl from there.

[CJ-Johnson]

Only filter out basic_string_view when bugprone-stringview-nullptr is enabled

[CJ-Johnson]

In D114823#3229990, @njames93 wrote:

I'm in 2 minds about this. This diagnostic is a good fit for this warning and shouldn't be removed. Likewise duplicate diagnostics from different checks is annoying. Maybe a better path forward would be to suppress the diagnostic if the bugprone-stringview-nullptr check is enabled. These kinds of intercheck dependencies are found in modernize-prefer-member-init check IIRC. You can just copy the impl from there.

Great suggestion! I've updated this diff to do exactly that. Thanks! :)

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

Finder->addMatcher(
                     hasArgument(1, ignoringParenImpCasts(
                                        integerLiteral().bind("int"))))))
           .bind("constructor"),
       this);
 
   // Check the literal string constructor with char pointer.
   // [i.e. string (const char* s);]
   Finder->addMatcher(
-      traverse(TK_AsIs,
-               cxxConstructExpr(
-                   hasDeclaration(cxxConstructorDecl(ofClass(cxxRecordDecl(
-                       hasAnyName(removeNamespaces(StringNames)))))),
-                   hasArgument(0, expr().bind("from-ptr")),
-                   // do not match std::string(ptr, int)
-                   // match std::string(ptr, alloc)
-                   // match std::string(ptr)
-                   anyOf(hasArgument(1, unless(hasType(isInteger()))),
-                         argumentCountIs(1)))
-                   .bind("constructor")),
+      traverse(
+          TK_AsIs,
+              hasDeclaration(cxxConstructorDecl(ofClass(anyOf(
+                  cxxRecordDecl(hasName("basic_string_view"))
+                      .bind("basic_string_view_decl"),
+                  cxxRecordDecl(hasAnyName(removeNamespaces(StringNames))))))),
       this);
 }
 
 void StringConstructorCheck::check(const MatchFinder::MatchResult &Result) {
   const ASTContext &Ctx = *Result.Context;
   const auto *E = Result.Nodes.getNodeAs<CXXConstructExpr>("constructor");
   assert(E && "missing constructor expression");
   SourceLocation Loc = E->getBeginLoc();
if (Lit->getValue().ugt(Str->getLength())) {
       diag(Loc, "length is bigger than string literal size");
     }
   } else if (const auto *Ptr = Result.Nodes.getNodeAs<Expr>("from-ptr")) {
     Expr::EvalResult ConstPtr;
     if (!Ptr->isInstantiationDependent() &&
         Ptr->EvaluateAsRValue(ConstPtr, Ctx) &&
         ((ConstPtr.Val.isInt() && ConstPtr.Val.getInt().isZero()) ||
          (ConstPtr.Val.isLValue() && ConstPtr.Val.isNullPointer()))) {
+      if (Result.Nodes.getNodeAs<CXXRecordDecl>("basic_string_view_decl")) {
+        // Filter out basic_string_view to avoid conflicts with
+        // bugprone-stringview-nullptr
+        return;
+      }
       diag(Loc, "constructing string from nullptr is undefined behaviour");
     }
   }
 }
 
 } // namespace bugprone
 } // namespace tidy
 } // namespace clang

clang-tools-extra/test/clang-tidy/checkers/bugprone-string-constructor.cpp

void TestView() {
   std::string_view q2("test", 200);
   // CHECK-MESSAGES: [[@LINE-1]]:20: warning: length is bigger than string literal size
   std::string_view q3(kText, 200);
   // CHECK-MESSAGES: [[@LINE-1]]:20: warning: length is bigger than string literal size
   std::string_view q4(kText2, 200);
   // CHECK-MESSAGES: [[@LINE-1]]:20: warning: length is bigger than string literal size
   std::string_view q5(kText3, 0x1000000);
   // CHECK-MESSAGES: [[@LINE-1]]:20: warning: suspicious large length parameter
-  std::string_view q6(nullptr);
-  // CHECK-MESSAGES: [[@LINE-1]]:20: warning: constructing string from nullptr is undefined behaviour
-  std::string_view q7 = 0;
-  // CHECK-MESSAGES: [[@LINE-1]]:25: warning: constructing string from nullptr is undefined behaviour
 }
 
 std::string StringFromZero() {
   return 0;
   // CHECK-MESSAGES: [[@LINE-1]]:10: warning: constructing string from nullptr is undefined behaviour
 }
 
-std::string_view StringViewFromZero() {
-  return 0;
-  // CHECK-MESSAGES: [[@LINE-1]]:10: warning: constructing string from nullptr is undefined behaviour
-}
-
 void Valid() {
   std::string empty();
   std::string str(4, 'x');
   std::wstring wstr(4, L'x');
   std::string s1("test", 4);
   std::string s2("test", 3);
   std::string s3("test");