Disallow applying RequiresLocationAttribute to parameters in source (…

…#68871)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5878,6 +5878,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_OutAttrOnInParam" xml:space="preserve">
     <value>An in parameter cannot have the Out attribute.</value>
   </data>
+  <data name="ERR_OutAttrOnRefReadonlyParam" xml:space="preserve">
+    <value>A ref readonly parameter cannot have the Out attribute.</value>
+  </data>
   <data name="ICompoundAssignmentOperationIsNotCSharpCompoundAssignment" xml:space="preserve">
     <value>{0} is not a valid C# compound assignment operation</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2193,6 +2193,7 @@ internal enum ErrorCode
         ERR_UnsupportedPrimaryConstructorParameterCapturingRefAny = 9136,
 
         ERR_RefReadOnlyWrongOrdering = 9501, // PROTOTYPE: Pack numbers
+        ERR_OutAttrOnRefReadonlyParam = 9520,
 
         #endregion
 

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -2311,6 +2311,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.ERR_ConstantValueOfTypeExpected:
                 case ErrorCode.ERR_UnsupportedPrimaryConstructorParameterCapturingRefAny:
                 case ErrorCode.ERR_RefReadOnlyWrongOrdering:
+                case ErrorCode.ERR_OutAttrOnRefReadonlyParam:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs

@@ -802,7 +802,7 @@ protected override void DecodeWellKnownAttributeImpl(ref DecodeWellKnownAttribut
             else if (ReportExplicitUseOfReservedAttributes(in arguments,
                 ReservedAttributes.DynamicAttribute |
                 ReservedAttributes.IsReadOnlyAttribute |
-                // PROTOTYPE: RequiresLocationAttribute
+                ReservedAttributes.RequiresLocationAttribute |
                 ReservedAttributes.IsUnmanagedAttribute |
                 ReservedAttributes.IsByRefLikeAttribute |
                 ReservedAttributes.TupleElementNamesAttribute |
@@ -1412,6 +1412,13 @@ internal override void PostDecodeWellKnownAttributes(ImmutableArray<CSharpAttrib
                             diagnostics.Add(ErrorCode.ERR_OutAttrOnInParam, this.GetFirstLocation());
                         }
                         break;
+                    case RefKind.RefReadOnlyParameter:
+                        if (data.HasOutAttribute)
+                        {
+                            // error: A ref readonly parameter cannot have the Out attribute.
+                            diagnostics.Add(ErrorCode.ERR_OutAttrOnRefReadonlyParam, this.GetFirstLocation());
+                        }
+                        break;
                 }
             }
 

src/Compilers/CSharp/Portable/Symbols/Symbol.cs

@@ -1432,6 +1432,7 @@ internal enum ReservedAttributes
             RequiredMemberAttribute = 1 << 11,
             ScopedRefAttribute = 1 << 12,
             RefSafetyRulesAttribute = 1 << 13,
+            RequiresLocationAttribute = 1 << 14,
         }
 
         internal bool ReportExplicitUseOfReservedAttributes(in DecodeWellKnownAttributeArguments<AttributeSyntax, CSharpAttributeData, AttributeLocation> arguments, ReservedAttributes reserved)
@@ -1449,6 +1450,10 @@ internal bool ReportExplicitUseOfReservedAttributes(in DecodeWellKnownAttributeA
                 reportExplicitUseOfReservedAttribute(attribute, arguments, AttributeDescription.IsReadOnlyAttribute))
             {
             }
+            else if ((reserved & ReservedAttributes.RequiresLocationAttribute) != 0 &&
+                reportExplicitUseOfReservedAttribute(attribute, arguments, AttributeDescription.RequiresLocationAttribute))
+            {
+            }
             else if ((reserved & ReservedAttributes.IsUnmanagedAttribute) != 0 &&
                 reportExplicitUseOfReservedAttribute(attribute, arguments, AttributeDescription.IsUnmanagedAttribute))
             {