Optimize HashSet.UnionWith to copy data from another HashSet when empty

src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.cs

@@ -907,5 +907,69 @@ static void TestComparerSerialization<TCompared>(IEqualityComparer<TCompared> eq
         }
 
         #endregion
+
+        #region UnionWith
+
+        [Theory]
+        [MemberData(nameof(ValidCollectionSizes))]
+        public void HashSet_Generic_UnionWith_EmptySetWithHashSetSameComparer_CopiesData(int count)
+        {
+            HashSet<T> source = (HashSet<T>)GenericISetFactory(count);
+            HashSet<T> destination = new HashSet<T>(source.Comparer);
+
+            destination.UnionWith(source);
+
+            Assert.Equal(source.Count, destination.Count);
+            Assert.True(source.SetEquals(destination));
+        }
+
+        [Theory]
+        [MemberData(nameof(ValidCollectionSizes))]
+        public void HashSet_Generic_UnionWith_EmptySetWithSparselyFilledHashSet_CopiesData(int count)
+        {
+            HashSet<T> source = (HashSet<T>)CreateEnumerable(EnumerableType.HashSet, null, count, 0, 0);
+            List<T> sourceElements = source.ToList();
+
+            foreach (int i in NonSquares(count))
+                source.Remove(sourceElements[i]);
+
+            HashSet<T> destination = new HashSet<T>(source.Comparer);
+
+            destination.UnionWith(source);
+
+            Assert.Equal(source.Count, destination.Count);
+            Assert.True(source.SetEquals(destination));
+        }
+
+        [Fact]
+        public void HashSet_Generic_UnionWith_EmptySetWithEmptyHashSet()
+        {
+            HashSet<T> source = new HashSet<T>();
+            HashSet<T> destination = new HashSet<T>();
+
+            destination.UnionWith(source);
+
+            Assert.Equal(0, destination.Count);
+        }
+
+        [Theory]
+        [MemberData(nameof(ValidCollectionSizes))]
+        public void HashSet_Generic_UnionWith_NonEmptySet_DoesNotCopyButAddsElements(int count)
+        {
+            if (count > 0)
+            {
+                HashSet<T> source = (HashSet<T>)GenericISetFactory(count);
+                HashSet<T> destination = (HashSet<T>)GenericISetFactory(1);
+                T existingItem = destination.First();
+
+                destination.UnionWith(source);
+
+                Assert.True(destination.Count >= source.Count);
+                Assert.True(source.IsSubsetOf(destination));
+                Assert.True(destination.Contains(existingItem));
+            }
+        }
+
+        #endregion
     }
 }

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs

@@ -806,6 +806,14 @@ public void UnionWith(IEnumerable<T> other)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
             }
 
+            // If this set is empty and other is a HashSet with the same effective comparer,
+            // we can copy the data directly instead of adding each element individually.
+            if (Count == 0 && other is HashSet<T> otherAsSet && EffectiveEqualityComparersAreEqual(this, otherAsSet))
+            {
+                ConstructFrom(otherAsSet);
+                return;
+            }
+
             foreach (T item in other)
             {
                 AddIfNotPresent(item, out _);