Improve .Distinct().ToList() and .Union(e).ToList()

src/libraries/System.Linq/src/System/Linq/Distinct.SpeedOpt.cs

@@ -11,7 +11,7 @@ private sealed partial class DistinctIterator<TSource> : IIListProvider<TSource>
         {
             public TSource[] ToArray() => Enumerable.HashSetToArray(new HashSet<TSource>(_source, _comparer));
 
-            public List<TSource> ToList() => Enumerable.HashSetToList(new HashSet<TSource>(_source, _comparer));
+            public List<TSource> ToList() => new List<TSource>(new HashSet<TSource>(_source, _comparer));
 
             public int GetCount(bool onlyIfCheap) => onlyIfCheap ? -1 : new HashSet<TSource>(_source, _comparer).Count;
         }

src/libraries/System.Linq/src/System/Linq/ToCollection.cs

@@ -253,27 +253,5 @@ private static TSource[] HashSetToArray<TSource>(HashSet<TSource> set)
             set.CopyTo(result);
             return result;
         }
-
-        private static List<TSource> HashSetToList<TSource>(HashSet<TSource> set)
-        {
-            int count = set.Count;
-
-            var result = new List<TSource>(count);
-            if (count > 0)
-            {
-                Span<TSource> span = SetCountAndGetSpan(result, count);
-
-                int index = 0;
-                foreach (TSource item in set)
-                {
-                    span[index] = item;
-                    ++index;
-                }
-
-                Debug.Assert(index == span.Length, "All list elements were not initialized.");
-            }
-
-            return result;
-        }
     }
 }

src/libraries/System.Linq/src/System/Linq/Union.SpeedOpt.cs

@@ -26,7 +26,7 @@ private HashSet<TSource> FillSet()
 
             public TSource[] ToArray() => Enumerable.HashSetToArray(FillSet());
 
-            public List<TSource> ToList() => Enumerable.HashSetToList(FillSet());
+            public List<TSource> ToList() => new List<TSource>(FillSet());
 
             public int GetCount(bool onlyIfCheap) => onlyIfCheap ? -1 : FillSet().Count;
         }