Fix mutability of ReadOnlySpan2D<T>.GetPinnableReference()

Author: Sergio0694

src/CommunityToolkit.HighPerformance/Memory/ReadOnlySpan2D{T}.cs

@@ -780,14 +780,14 @@ public bool TryCopyTo(Span2D<T> destination)
     /// <returns>A reference to the 0th element, or a <see langword="null"/> reference.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     [EditorBrowsable(EditorBrowsableState.Never)]
-    public unsafe ref T GetPinnableReference()
+    public unsafe ref readonly T GetPinnableReference()
     {
-        ref T r0 = ref Unsafe.AsRef<T>(null);
+        ref readonly T r0 = ref Unsafe.AsRef<T>(null);
 
         if (Length != 0)
         {
 #if NET7_0_OR_GREATER
-            r0 = ref Unsafe.AsRef(in this.reference);
+            r0 = ref this.reference;
 #elif NETSTANDARD2_1_OR_GREATER
             r0 = ref MemoryMarshal.GetReference(this.span);
 #else

tests/CommunityToolkit.HighPerformance.UnitTests/Memory/Test_ReadOnlySpan2D{T}.cs

@@ -351,7 +351,7 @@ public unsafe void Test_ReadOnlySpan2DT_GetPinnableReference()
     {
         Assert.IsTrue(Unsafe.AreSame(
             ref Unsafe.AsRef<int>(null),
-            ref ReadOnlySpan2D<int>.Empty.GetPinnableReference()));
+            ref Unsafe.AsRef(in ReadOnlySpan2D<int>.Empty.GetPinnableReference())));
 
         int[,] array =
         {
@@ -361,7 +361,7 @@ ref Unsafe.AsRef<int>(null),
 
         ReadOnlySpan2D<int> span2d = new(array);
 
-        ref int r0 = ref span2d.GetPinnableReference();
+        ref int r0 = ref Unsafe.AsRef(in span2d.GetPinnableReference());
 
         Assert.IsTrue(Unsafe.AreSame(ref r0, ref array[0, 0]));
     }