Temporarily removing use of ReadOnlySpan indexer. (#25881)

Author: ahsonkhan

src/Common/src/System/Memory/FixedBufferExtensions.cs

@@ -35,17 +35,21 @@ internal unsafe static bool FixedBufferEqualsString(this ReadOnlySpan<char> span
             if (value == null || value.Length > span.Length)
                 return false;
 
-            int i = 0;
-            for (; i < value.Length; i++)
+            fixed (char* spanPtr = &span.DangerousGetPinnableReference())
             {
-                // Strings with embedded nulls can never match as the fixed buffer always null terminates.
-                if (value[i] == '\0' || value[i] != span[i])
-                    return false;
-            }
+                var readWriteSpan = new Span<char>(spanPtr, span.Length);
+                int i = 0;
+                for (; i < value.Length; i++)
+                {
+                    // Strings with embedded nulls can never match as the fixed buffer always null terminates.
+                    if (value[i] == '\0' || value[i] != readWriteSpan[i])
+                        return false;
+                }
 
-            // If we've maxed out the buffer or reached the
-            // null terminator, we're equal.
-            return i == span.Length || span[i] == '\0';
+                // If we've maxed out the buffer or reached the
+                // null terminator, we're equal.
+                return i == span.Length || readWriteSpan[i] == '\0';
+            }
         }
     }
 }

src/System.IO.FileSystem/src/System/IO/PathHelpers.cs

@@ -172,17 +172,21 @@ public static unsafe bool IsDotOrDotDot(ReadOnlySpan<char> fileName)
                 || (fileName.Length == 2 && fileName[1] != '.'));
         }
 
-        public static ReadOnlySpan<char> GetDirectoryNameNoChecks(ReadOnlySpan<char> path)
+        public static unsafe ReadOnlySpan<char> GetDirectoryNameNoChecks(ReadOnlySpan<char> path)
         {
             if (path.Length == 0)
                 return ReadOnlySpan<char>.Empty;
 
             int root = PathInternal.GetRootLength(path);
             int i = path.Length;
-            if (i > root)
+            fixed (char* pathPtr = &path.DangerousGetPinnableReference())
             {
-                while (i > root && !PathInternal.IsDirectorySeparator(path[--i])) ;
-                return path.Slice(0, i);
+                var pathSpan = new Span<char>(pathPtr, path.Length);
+                if (i > root)
+                {
+                    while (i > root && !PathInternal.IsDirectorySeparator(pathSpan[--i])) ;
+                    return pathSpan.Slice(0, i);
+                }
             }
 
             return ReadOnlySpan<char>.Empty;

src/System.Private.Xml/src/System/Xml/Core/XmlTextReaderImpl.cs

@@ -3212,16 +3212,20 @@ private void SetupEncoding(Encoding encoding)
             }
         }
 
-        private void EatPreamble()
+        private unsafe void EatPreamble()
         {
             ReadOnlySpan<byte> preamble = _ps.encoding.Preamble;
             int preambleLen = preamble.Length;
             int i;
-            for (i = 0; i < preambleLen && i < _ps.bytesUsed; i++)
+            fixed (byte* preamblePtr = &preamble.DangerousGetPinnableReference())
             {
-                if (_ps.bytes[i] != preamble[i])
+                var preambleSpan = new Span<byte>(preamblePtr, preambleLen);
+                for (i = 0; i < preambleLen && i < _ps.bytesUsed; i++)
                 {
-                    break;
+                    if (_ps.bytes[i] != preambleSpan[i])
+                    {
+                        break;
+                    }
                 }
             }
             if (i == preambleLen)