Improve ImmutableArrayExtensions.SequenceEqual

src/libraries/System.Collections.Immutable/src/System/Linq/ImmutableArrayExtensions.cs

@@ -187,28 +187,39 @@ public static bool SequenceEqual<TDerived, TBase>(this ImmutableArray<TBase> imm
         /// <typeparam name="TBase">The type of element contained by the collection.</typeparam>
         public static bool SequenceEqual<TDerived, TBase>(this ImmutableArray<TBase> immutableArray, IEnumerable<TDerived> items, IEqualityComparer<TBase>? comparer = null) where TDerived : TBase
         {
-            Requires.NotNull(items, nameof(items));
+            if (items is ICollection<TBase> itemsCol)
+            {
+                immutableArray.ThrowNullRefIfNotInitialized();
+                return Enumerable.SequenceEqual(immutableArray.array, itemsCol, comparer);
+            }
 
-            comparer ??= EqualityComparer<TBase>.Default;
+            return Enumerate(immutableArray, items, comparer);
 
-            int i = 0;
-            int n = immutableArray.Length;
-            foreach (TDerived item in items)
+            static bool Enumerate(ImmutableArray<TBase> immutableArray, IEnumerable<TDerived> items, IEqualityComparer<TBase>? comparer)
             {
-                if (i == n)
-                {
-                    return false;
-                }
+                Requires.NotNull(items, nameof(items));
 
-                if (!comparer.Equals(immutableArray[i], item))
+                comparer ??= EqualityComparer<TBase>.Default;
+
+                int i = 0;
+                int n = immutableArray.Length;
+                foreach (TDerived item in items)
                 {
-                    return false;
+                    if (i == n)
+                    {
+                        return false;
+                    }
+
+                    if (!comparer.Equals(immutableArray[i], item))
+                    {
+                        return false;
+                    }
+
+                    i++;
                 }
 
-                i++;
+                return i == n;
             }
-
-            return i == n;
         }
 
         /// <summary>