[Parser] Improving diagnostics for access-level modifiers

Improves the diagnostics for situations where multiple access-level modifiers are used on the same declaration.

Author: Visckmart

include/swift/AST/DiagnosticsParse.def

@@ -1485,6 +1485,10 @@ ERROR(duplicate_attribute,none,
       "duplicate %select{attribute|modifier}0", (bool))
 NOTE(previous_attribute,none,
      "%select{attribute|modifier}0 already specified here", (bool))
+ERROR(multiple_access_level_modifiers,none,
+      "multiple access-level modifiers specified", ())
+NOTE(previous_access_level_modifier,none,
+     "'%0' previously specified here", (StringRef))
 ERROR(mutually_exclusive_attrs,none,
       "'%0' contradicts previous %select{attribute|modifier}2 '%1'", (StringRef, StringRef, bool))
 

lib/Parse/ParseDecl.cpp

@@ -2926,9 +2926,25 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
     if (!Tok.is(tok::l_paren)) {
       // Normal access control attribute.
       AttrRange = Loc;
-      DuplicateAttribute = Attributes.getAttribute<AccessControlAttr>();
-      if (!DuplicateAttribute)
+      
+      auto PrevAccessControlAttr = Attributes.getAttribute<AccessControlAttr>();
+      
+      if (!PrevAccessControlAttr) {
         Attributes.add(new (Context) AccessControlAttr(AtLoc, Loc, access));
+      } else {
+        // Diagnose if there is already an access control attribute on
+        // this declaration.
+        diagnose(Loc, diag::multiple_access_level_modifiers)
+          .highlight(AttrRange);
+        diagnose(PrevAccessControlAttr->getLocation(),
+                 diag::previous_access_level_modifier,
+                 PrevAccessControlAttr->getAttrName())
+          .highlight(PrevAccessControlAttr->getRange());
+        
+        // Removes the reference to the duplicate attribute as we don't need the
+        // extra diagnostic.
+        DuplicateAttribute = nullptr;
+      }
       break;
     }