Marking {ReadOnly}Span as readonly structs

src/System.Memory/src/System/ReadOnlySpan.cs

@@ -15,6 +15,8 @@ namespace System
     /// ReadOnlySpan represents a contiguous region of arbitrary memory. Unlike arrays, it can point to either managed
     /// or native memory, or to memory allocated on the stack. It is type- and memory-safe.
     /// </summary>
+    [IsReadOnly]
+    [IsByRefLike]
     public struct ReadOnlySpan<T>
     {
         /// <summary>

src/System.Memory/src/System/Span.cs

@@ -17,6 +17,8 @@ namespace System
     /// </summary>
     [DebuggerTypeProxy(typeof(SpanDebugView<>))]
     [DebuggerDisplay("Length = {Length}")]
+    [IsReadOnly]
+    [IsByRefLike]
     public struct Span<T>
     {
         /// <summary>