add IEnumerable<T>.ToHashSet (#125)

src/Polyfill/Polyfill_IEnumerable.cs

@@ -286,4 +286,21 @@ public static IEnumerable<TSource> SkipLast<TSource>(
         int count) =>
         target.Reverse().Skip(count).Reverse();
 #endif
+
+#if NET471 || NET46X || NETSTANDARD2_0
+
+    /// <summary>
+    /// Creates a HashSet<T> from an IEnumerable<T> using the comparer to compare keys.
+    /// </summary>
+    /// <param name="source">An IEnumerable<T> to create a HashSet<T> from.</param>
+    /// <param name="comparer">An IEqualityComparer<T> to compare keys.</param>
+    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
+    /// <returns>A HashSet<T> that contains values of type TSource selected from the input sequence.</returns>
+    [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tohashset#system-linq-enumerable-tohashset-1(system-collections-generic-ienumerable((-0))-system-collections-generic-iequalitycomparer((-0)))")]
+    public static HashSet<TSource> ToHashSet<TSource>(
+        this IEnumerable<TSource> target,
+        IEqualityComparer<TSource>? comparer = null) =>
+        new HashSet<TSource>(target, comparer);
+
+#endif
 }

src/Tests/PolyfillTests_IEnumerable.cs

@@ -39,6 +39,16 @@ public void IEnumerableSkipLast()
         Assert.IsTrue(enumerable.SkipLast(1).SequenceEqual(new List<string> { "a" }));
     }
 
+    [Test]
+    public void ToHashSet ()
+    {
+        var enumerable = (IEnumerable<string>)new List<string> { "a", "b" };
+
+        var hashSet = enumerable.ToHashSet();
+        Assert.IsTrue(hashSet.Contains("a"));
+        Assert.IsTrue(hashSet.Contains("b"));
+    }
+
     [Test]
     public void Chunk_SizeOf3()
     {