[AST] emitLetToVarNoteIfSimple should also check if the function or accessor is explicitly 'nonmutating'

Otherwise, we will simply insert the 'mutating' fix-it after 'nonmutating', leading to another error that says both cannot be used at the same time

Author: theblixguy

lib/AST/Decl.cpp

@@ -5683,11 +5683,16 @@ void VarDecl::emitLetToVarNoteIfSimple(DeclContext *UseDC) const {
         if (AD->isGetter() && !AD->getAccessorKeywordLoc().isValid())
           return;
       }
-                   
+
       auto &d = getASTContext().Diags;
-      d.diagnose(FD->getFuncLoc(), diag::change_to_mutating,
-                 isa<AccessorDecl>(FD))
-       .fixItInsert(FD->getFuncLoc(), "mutating ");
+      auto diags = d.diagnose(FD->getFuncLoc(), diag::change_to_mutating,
+                              isa<AccessorDecl>(FD));
+      if (auto nonmutatingAttr =
+              FD->getAttrs().getAttribute<NonMutatingAttr>()) {
+        diags.fixItReplace(nonmutatingAttr->getLocation(), "mutating ");
+      } else {
+        diags.fixItInsert(FD->getFuncLoc(), "mutating ");
+      }
       return;
     }
   }

test/Sema/immutability.swift

@@ -681,3 +681,23 @@ struct SS {
     j = j // expected-error {{cannot assign to value: 'j' is a 'let' constant}}
   }
 }
+
+protocol JustAProtocol {
+  var name: String { get set }
+}
+
+extension JustAProtocol {
+  var foo: String {
+    get { return name }
+    nonmutating set { name = newValue } // expected-error {{cannot assign to property: 'self' is immutable}} 
+    // expected-note@-1 {{mark accessor 'mutating' to make 'self' mutable}}{{5-16=mutating}}
+  }
+
+  nonmutating func bar() { // expected-note {{mark method 'mutating' to make 'self' mutable}}{{3-14=mutating}}
+    name = "Hello" // expected-error {{cannot assign to property: 'self' is immutable}}
+  }
+  
+  func baz() { // expected-note {{mark method 'mutating' to make 'self' mutable}}{{3-3=mutating }}
+    name = "World" // expected-error {{cannot assign to property: 'self' is immutable}}
+  }
+}