C#: Introduce a class for ref structs.

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

@@ -656,7 +656,7 @@ private predicate convBoxingValueType(ValueType fromType, Type toType) {
     or
     toType instanceof SystemValueTypeClass
   ) and
-  not fromType.(Struct).isRef()
+  not fromType.isRefLikeType()
   or
   toType = fromType.getABaseInterface+()
 }

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

@@ -48,6 +48,11 @@ class Type extends Member, TypeContainer, @type {
 
   /** Holds if this type is a value type, or a type parameter that is a value type. */
   predicate isValueType() { none() }
+
+  /**
+   * Holds if this type is a ref like type, or a type parameter that is a ref like type.
+   */
+  predicate isRefLikeType() { none() }
 }
 
 pragma[nomagic]
@@ -704,15 +709,38 @@ class Enum extends ValueType, @enum_type {
  * ```
  */
 class Struct extends ValueType, @struct_type {
-  /** Holds if this `struct` has a `ref` modifier. */
-  predicate isRef() { this.hasModifier("ref") }
+  /**
+   * DEPRECATED: Use `instanceof RefStruct` instead.
+   *
+   * Holds if this `struct` has a `ref` modifier.
+   */
+  deprecated predicate isRef() { this.hasModifier("ref") }
 
   /** Holds if this `struct` has a `readonly` modifier. */
   predicate isReadonly() { this.hasModifier("readonly") }
 
   override string getAPrimaryQlClass() { result = "Struct" }
 }
 
+/**
+ * A `ref struct`, for example
+ *
+ * ```csharp
+ * ref struct S {
+ *  ...
+ * }
+ * ```
+ */
+class RefStruct extends Struct {
+  RefStruct() { this.hasModifier("ref") }
+
+  override string getAPrimaryQlClass() { result = "RefStruct" }
+
+  override predicate isValueType() { none() }
+
+  override predicate isRefLikeType() { any() }
+}
+
 /**
  * A `record struct`, for example
  * ```csharp

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -703,7 +703,7 @@ module LocalFlow {
       or
       t = any(TypeParameter tp | not tp.isValueType())
       or
-      t.(Struct).isRef()
+      t.isRefLikeType()
     ) and
     not exists(getALastEvalNode(result))
   }