[[gnu::nonstring]] should work on pointers too (#150974)

Clang's current implementation only works on array types, but GCC (which
is where we got this attribute) supports it on pointers as well as
arrays.

Fixes #150951

Author: AaronBallman

clang/include/clang/Basic/AttrDocs.td

@@ -9417,9 +9417,9 @@ def NonStringDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{
 The ``nonstring`` attribute can be applied to the declaration of a variable or
-a field whose type is a character array to specify that the character array is
-not intended to behave like a null-terminated string. This will silence
-diagnostics with code like:
+a field whose type is a character pointer or character array to specify that
+the buffer is not intended to behave like a null-terminated string. This will
+silence diagnostics with code like:
 
 .. code-block:: c
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4805,10 +4805,10 @@ void Sema::AddModeAttr(Decl *D, const AttributeCommonInfo &CI,
 
 static void handleNonStringAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // This only applies to fields and variable declarations which have an array
-  // type.
+  // type or pointer type, with character elements.
   QualType QT = cast<ValueDecl>(D)->getType();
-  if (!QT->isArrayType() ||
-      !QT->getBaseElementTypeUnsafe()->isAnyCharacterType()) {
+  if ((!QT->isArrayType() && !QT->isPointerType()) ||
+      !QT->getPointeeOrArrayElementType()->isAnyCharacterType()) {
     S.Diag(D->getBeginLoc(), diag::warn_attribute_non_character_array)
         << AL << AL.isRegularKeywordAttribute() << QT << AL.getRange();
     return;

clang/test/Sema/attr-nonstring.c

@@ -229,3 +229,11 @@ struct Outer o2[] = {
     }
   }
 };
+
+// The attribute also works with a pointer type, not just an array type.
+__attribute__((nonstring)) char *ptr1;
+__attribute__((nonstring)) const unsigned char *ptr2;
+struct GH150951 {
+  __attribute__((nonstring)) char *ptr1;
+  __attribute__((nonstring)) const unsigned char *ptr2;
+};