Improve diagnostic for ignored GNU 'used' attribute

Differential Revision: https://reviews.llvm.org/D97161

Author: MaskRay

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3159,6 +3159,9 @@ def warn_attribute_ignored : Warning<"%0 attribute ignored">,
 def warn_nothrow_attribute_ignored : Warning<"'nothrow' attribute conflicts with"
   " exception specification; attribute ignored">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_ignored_on_non_definition :
+  Warning<"%0 attribute ignored on a non-definition declaration">,
+  InGroup<IgnoredAttributes>;
 def warn_attribute_ignored_on_inline :
   Warning<"%0 attribute ignored on inline function">,
   InGroup<IgnoredAttributes>;

clang/lib/Sema/SemaDecl.cpp

@@ -13295,7 +13295,8 @@ void Sema::FinalizeDeclaration(Decl *ThisDecl) {
 
   if (UsedAttr *Attr = VD->getAttr<UsedAttr>()) {
     if (!Attr->isInherited() && !VD->isThisDeclarationADefinition()) {
-      Diag(Attr->getLocation(), diag::warn_attribute_ignored) << Attr;
+      Diag(Attr->getLocation(), diag::warn_attribute_ignored_on_non_definition)
+          << Attr;
       VD->dropAttr<UsedAttr>();
     }
   }

clang/test/Sema/attr-used.c

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -verify -fsyntax-only -Wno-private-extern %s
 
-extern int l0 __attribute__((used)); // expected-warning {{'used' attribute ignored}}
-__private_extern__ int l1 __attribute__((used)); // expected-warning {{'used' attribute ignored}}
+extern int l0 __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
+__private_extern__ int l1 __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
 
 struct __attribute__((used)) s { // expected-warning {{'used' attribute only applies to variables with non-local storage, functions, and Objective-C methods}}
   int x;

clang/test/SemaCXX/attr-used.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
-extern char test1[] __attribute__((used)); // expected-warning {{'used' attribute ignored}}
-extern const char test2[] __attribute__((used)); // expected-warning {{'used' attribute ignored}}
+extern char test1[] __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
+extern const char test2[] __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
 extern const char test3[] __attribute__((used)) = "";