C#: Add extractor and QL library support for ref readonly parameters.

csharp/extractor/Semmle.Extraction.CSharp/Entities/CachedSymbol.cs

@@ -53,6 +53,7 @@ namespace Semmle.Extraction.CSharp.Entities
                     trapFile.type_annotation(this, Kinds.TypeAnnotation.Ref);
                     break;
                 case RefKind.RefReadOnly:
+                case RefKind.RefReadOnlyParameter:
                     trapFile.type_annotation(this, Kinds.TypeAnnotation.ReadonlyRef);
                     break;
             }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs

@@ -24,7 +24,13 @@ namespace Semmle.Extraction.CSharp.Entities
 
         public enum Kind
         {
-            None, Ref, Out, Params, This, In
+            None = 0,
+            Ref = 1,
+            Out = 2,
+            Params = 3,
+            This = 4,
+            In = 5,
+            RefReadOnly = 6
         }
 
         protected virtual int Ordinal => Symbol.Ordinal;
@@ -41,6 +47,8 @@ namespace Semmle.Extraction.CSharp.Entities
                         return Kind.Ref;
                     case RefKind.In:
                         return Kind.In;
+                    case RefKind.RefReadOnlyParameter:
+                        return Kind.RefReadOnly;
                     default:
                         if (Symbol.IsParams)
                             return Kind.Params;

csharp/ql/lib/semmle/code/csharp/Variable.qll

@@ -167,6 +167,18 @@ class Parameter extends DotNet::Parameter, LocalScopeVariable, Attributable, Top
    */
   predicate isParams() { params(this, _, _, _, 3, _, _) }
 
+  /**
+   * Holds if this parameter if a ref readonly parameter.
+   * For example, `p` is a ref readonly parameter in
+   *
+   * ```csharp
+   * void M(ref readonly int p) {
+   *   ...
+   * }
+   * ```
+   */
+  predicate isReadonlyRef() { params(this, _, _, _, 6, _, _) }
+
   /**
    * Holds this parameter is the first parameter of an extension method.
    * For example, `list` is the first parameter of the extension method