Add fast path to Array.ConstrainedCopy for single-dimensional arrays (#122530)

Adds a fast path optimization to `Array.ConstrainedCopy` for single-dimensional zero-based arrays, bringing it to performance parity with `Array.Copy`. 

Fixes #122518

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: jkotas <[email protected]>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/Array.cs

@@ -352,18 +352,18 @@ public static void Copy(Array sourceArray, long sourceIndex, Array destinationAr
             Copy(sourceArray, isourceIndex, destinationArray, idestinationIndex, ilength);
         }
 
+#if !MONO // implementation details of MethodTable
+
         // Provides a strong exception guarantee - either it succeeds, or
         // it throws an exception with no side effects.  The arrays must be
         // compatible array types based on the array element type - this
         // method does not support casting, boxing, or primitive widening.
         // It will up-cast, assuming the array types are correct.
         public static void ConstrainedCopy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length)
         {
-            CopyImpl(sourceArray, sourceIndex, destinationArray, destinationIndex, length, reliable: true);
+            Copy(sourceArray, sourceIndex, destinationArray, destinationIndex, length, reliable: true);
         }
 
-#if !MONO // implementation details of MethodTable
-
         // Copies length elements from sourceArray, starting at index 0, to
         // destinationArray, starting at index 0.
         public static unsafe void Copy(Array sourceArray, Array destinationArray, int length)
@@ -396,7 +396,12 @@ public static unsafe void Copy(Array sourceArray, Array destinationArray, int le
 
         // Copies length elements from sourceArray, starting at sourceIndex, to
         // destinationArray, starting at destinationIndex.
-        public static unsafe void Copy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length)
+        public static void Copy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length)
+        {
+            Copy(sourceArray, sourceIndex, destinationArray, destinationIndex, length, reliable: false);
+        }
+
+        private static unsafe void Copy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length, bool reliable)
         {
             if (sourceArray != null && destinationArray != null)
             {
@@ -423,7 +428,7 @@ public static unsafe void Copy(Array sourceArray, int sourceIndex, Array destina
             }
 
             // Less common
-            CopyImpl(sourceArray, sourceIndex, destinationArray, destinationIndex, length, reliable: false);
+            CopyImpl(sourceArray, sourceIndex, destinationArray, destinationIndex, length, reliable);
         }
 
         // Reliability-wise, this method will either possibly corrupt your

src/mono/System.Private.CoreLib/src/System/Array.Mono.cs

@@ -106,6 +106,11 @@ public static void Copy(Array sourceArray, int sourceIndex, Array destinationArr
             CopyImpl(sourceArray, sourceIndex, destinationArray, destinationIndex, length, false);
         }
 
+        public static void ConstrainedCopy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length)
+        {
+            CopyImpl(sourceArray, sourceIndex, destinationArray, destinationIndex, length, true);
+        }
+
         private static void CopyImpl(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length, bool reliable)
         {
             ArgumentNullException.ThrowIfNull(sourceArray);