Optimize ImmutableSortedSet<T>.SetEquals to avoid unnecessary allocations

Author: Ahmed

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableSortedSet_1.cs

@@ -375,24 +375,73 @@ public bool SetEquals(IEnumerable<T> other)
                 return true;
             }
 
-            var otherSet = new SortedSet<T>(other, this.KeyComparer);
-            if (this.Count != otherSet.Count)
+            if (other is ICollection<T> otherAsICollection && otherAsICollection.Count < this.Count)
             {
                 return false;
             }
 
-            int matches = 0;
-            foreach (T item in otherSet)
+            else if (other is IReadOnlyCollection<T> otherAsIReadOnlyCollection && otherAsIReadOnlyCollection.Count < this.Count)
             {
-                if (!this.Contains(item))
+                return false;
+            }
+
+            if (other is ImmutableSortedSet<T> otherAsImmutableSortedSet && otherAsImmutableSortedSet.KeyComparer == this.KeyComparer)
+            {
+                if (otherAsImmutableSortedSet.Count != this.Count)
                 {
                     return false;
                 }
+                using var otherAsImmutableSortedSetEnumerator = otherAsImmutableSortedSet.GetEnumerator();
+                using (var thisEnumerator = this.GetEnumerator())
+                {
+                    while (otherAsImmutableSortedSetEnumerator.MoveNext() && thisEnumerator.MoveNext())
+                    {
+                    if (this.KeyComparer.Compare(otherAsImmutableSortedSetEnumerator.Current, thisEnumerator.Current) != 0)
+                        {
+                            return false;
+                        }
+                    }
+                }
+                return true;
+            }
 
-                matches++;
+            else if (other is SortedSet<T> otherAsSortedSet && otherAsSortedSet.Comparer == this.KeyComparer)
+            {
+                if (otherAsSortedSet.Count != this.Count)
+                {
+                    return false;
+                }
+                using var otherAsSortedSetEnumerator = otherAsSortedSet.GetEnumerator();
+                using (var thisEnumerator = this.GetEnumerator())
+                {
+                    while (otherAsSortedSetEnumerator.MoveNext() && thisEnumerator.MoveNext())
+                    {
+                    if (this.KeyComparer.Compare(otherAsSortedSetEnumerator.Current, thisEnumerator.Current) != 0)
+                        {
+                            return false;
+                        }
+                    }
+                }
+                return true;
             }
 
-            return matches == this.Count;
+            var otherSet = new SortedSet<T>(other, this.KeyComparer);
+            if (this.Count != otherSet.Count)
+            {
+                return false;
+            }
+            using var otherSetEnumerator = otherSet.GetEnumerator();
+            using (var thisEnumerator = this.GetEnumerator())
+            {
+                while (otherSetEnumerator.MoveNext() && thisEnumerator.MoveNext())
+                {
+                    if (this.KeyComparer.Compare(otherSetEnumerator.Current, thisEnumerator.Current) != 0)
+                    {
+                        return false;
+                    }
+                }
+            }
+            return true;
         }
 
         /// <summary>