Use 'ref readonly' in 'IndexOf<T>' APIs

src/CommunityToolkit.HighPerformance/Enumerables/SpanTokenizer{T}.cs

@@ -73,7 +73,10 @@ public bool MoveNext()
         {
             this.start = newEnd;
 
-            int index = this.span.Slice(newEnd).IndexOf(this.separator);
+            // Here we're inside the 'CommunityToolkit.HighPerformance.Enumerables' namespace, so the
+            // 'MemoryExtensions' type from the .NET Community Toolkit would be bound instead. Because
+            // want the one from the BCL (to search by value), we can use its fully qualified name.
+            int index = System.MemoryExtensions.IndexOf(this.span.Slice(newEnd), this.separator);
 
             // Extract the current subsequence
             if (index >= 0)

src/CommunityToolkit.HighPerformance/Extensions/ReadOnlySpanExtensions.cs

@@ -210,7 +210,7 @@ public static ReadOnlySpan2D<T> AsSpan2D<T>(this ReadOnlySpan<T> span, int offse
     /// <param name="value">The reference to the target item to get the index for.</param>
     /// <returns>The index of <paramref name="value"/> within <paramref name="span"/>, or <c>-1</c>.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static unsafe int IndexOf<T>(this ReadOnlySpan<T> span, in T value)
+    public static unsafe int IndexOf<T>(this ReadOnlySpan<T> span, ref readonly T value)
     {
         ref T r0 = ref MemoryMarshal.GetReference(span);
         ref T r1 = ref Unsafe.AsRef(in value);

src/CommunityToolkit.HighPerformance/Extensions/SpanExtensions.cs

@@ -148,10 +148,15 @@ public static Span<TTo> Cast<TFrom, TTo>(this Span<TFrom> span)
     /// <param name="value">The reference to the target item to get the index for.</param>
     /// <returns>The index of <paramref name="value"/> within <paramref name="span"/>, or <c>-1</c>.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static unsafe int IndexOf<T>(this Span<T> span, ref T value)
+    public static unsafe int IndexOf<T>(this Span<T> span, ref readonly T value)
     {
         ref T r0 = ref MemoryMarshal.GetReference(span);
-        IntPtr byteOffset = Unsafe.ByteOffset(ref r0, ref value);
+        IntPtr byteOffset =
+#if NET8_0_OR_GREATER
+            Unsafe.ByteOffset(ref r0, in value);
+#else
+            Unsafe.ByteOffset(ref r0, ref Unsafe.AsRef(in value));
+#endif
 
         nint elementOffset = byteOffset / (nint)(uint)sizeof(T);