Add FrozenSet.Create collection builder

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

@@ -70,8 +70,11 @@ void System.IDisposable.Dispose() { }
     }
     public static partial class FrozenSet
     {
+        public static System.Collections.Frozen.FrozenSet<T> Create<T>(params System.ReadOnlySpan<T> source) { throw null; }
+        public static System.Collections.Frozen.FrozenSet<T> Create<T>(System.Collections.Generic.IEqualityComparer<T>? equalityComparer, params System.ReadOnlySpan<T> source) { throw null; }
         public static System.Collections.Frozen.FrozenSet<T> ToFrozenSet<T>(this System.Collections.Generic.IEnumerable<T> source, System.Collections.Generic.IEqualityComparer<T>? comparer = null) { throw null; }
     }
+    [System.Runtime.CompilerServices.CollectionBuilder(typeof(System.Collections.Frozen.FrozenSet), nameof(System.Collections.Frozen.FrozenSet.Create))]
     public abstract partial class FrozenSet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable
     {
         internal FrozenSet() { }

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenSet.cs

@@ -5,6 +5,7 @@
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 namespace System.Collections.Frozen
@@ -14,6 +15,40 @@ namespace System.Collections.Frozen
     /// </summary>
     public static class FrozenSet
     {
+        /// <summary>Creates a <see cref="FrozenSet{T}"/> with the specified values.</summary>
+        /// <param name="source">The values to use to populate the set.</param>
+        /// <typeparam name="T">The type of the values in the set.</typeparam>
+        /// <returns>A frozen set.</returns>
+        public static FrozenSet<T> Create<T>(params ReadOnlySpan<T> source) => Create(null, source);
+
+        /// <summary>Creates a <see cref="FrozenSet{T}"/> with the specified values.</summary>
+        /// <param name="source">The values to use to populate the set.</param>
+        /// <param name="equalityComparer">The comparer implementation to use to compare values for equality. If null, <see cref="EqualityComparer{T}.Default"/> is used.</param>
+        /// <typeparam name="T">The type of the values in the set.</typeparam>
+        /// <returns>A frozen set.</returns>
+        public static FrozenSet<T> Create<T>(IEqualityComparer<T>? equalityComparer, params ReadOnlySpan<T> source)
+        {
+            if (source.Length == 0)
+            {
+                return equalityComparer is null || ReferenceEquals(equalityComparer, FrozenSet<T>.Empty.Comparer) ?
+                    FrozenSet<T>.Empty :
+                    new EmptyFrozenSet<T>(equalityComparer);
+            }
+
+            HashSet<T> set =
+#if NET
+                new(source.Length, equalityComparer); // we assume there are few-to-no duplicates when using this API
+#else
+                new(equalityComparer);
+#endif
+            foreach (T item in source)
+            {
+                set.Add(item);
+            }
+
+            return ToFrozenSet(set, equalityComparer);
+        }
+
         /// <summary>Creates a <see cref="FrozenSet{T}"/> with the specified values.</summary>
         /// <param name="source">The values to use to populate the set.</param>
         /// <param name="comparer">The comparer implementation to use to compare values for equality. If null, <see cref="EqualityComparer{T}.Default"/> is used.</param>
@@ -199,6 +234,7 @@ private static FrozenSet<T> CreateFromSet<T>(HashSet<T> source)
     /// the remainder of the life of the application. <see cref="FrozenSet{T}"/> should only be initialized
     /// with trusted elements, as the details of the elements impacts construction time.
     /// </remarks>
+    [CollectionBuilder(typeof(FrozenSet), nameof(FrozenSet.Create))]
     [DebuggerTypeProxy(typeof(ImmutableEnumerableDebuggerProxy<>))]
     [DebuggerDisplay("Count = {Count}")]
     public abstract class FrozenSet<T> : ISet<T>,

src/libraries/System.Collections.Immutable/tests/Frozen/FrozenSetTests.cs

@@ -80,6 +80,16 @@ public void EmptySource_ProducedFrozenSetEmpty()
 
                 Assert.NotSame(FrozenSet<T>.Empty, source.ToFrozenSet(NonDefaultEqualityComparer<T>.Instance));
             }
+
+            Assert.Same(FrozenSet<T>.Empty, FrozenSet.Create(ReadOnlySpan<T>.Empty));
+            Assert.Same(FrozenSet<T>.Empty, FrozenSet.Create<T>());
+
+            foreach (IEqualityComparer<T> comparer in new IEqualityComparer<T>[] { null, EqualityComparer<T>.Default })
+            {
+                Assert.Same(FrozenSet<T>.Empty, FrozenSet.Create(comparer));
+            }
+
+            Assert.NotSame(FrozenSet<T>.Empty, FrozenSet.Create(NonDefaultEqualityComparer<T>.Instance));
         }
 
         [Fact]
@@ -387,7 +397,7 @@ public class FrozenSet_Generic_Tests_string_OrdinalIgnoreCase : FrozenSet_Generi
         [Fact]
         public void TryGetValue_FindsExpectedResult()
         {
-            FrozenSet<string> frozen = new[] { "abc" }.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
+            FrozenSet<string> frozen = FrozenSet.Create(StringComparer.OrdinalIgnoreCase, "abc");
 
             Assert.False(frozen.TryGetValue("ab", out string actualValue));
             Assert.Null(actualValue);
@@ -498,7 +508,7 @@ public void Sparse_LookupItems_AlltemsFoundAsExpected()
                 foreach (int skip in new[] { 2, 3, 5 })
                 {
                     var original = new HashSet<int>(Enumerable.Range(-3, size).Where(i => i % skip == 0));
-                    FrozenSet<int> frozen = original.ToFrozenSet();
+                    FrozenSet<int> frozen = [.. original];
 
                     for (int i = -10; i <= size + 66; i++)
                     {
@@ -539,7 +549,7 @@ public void ClosedRange_Lookup_AllItemsFoundAsExpected()
                         original.Add(i);
                     }
 
-                    FrozenSet<long> frozen = original.ToFrozenSet();
+                    FrozenSet<long> frozen = [.. original];
 
                     min = start > long.MinValue ? start - 10 : start;
                     max = start + size - 1 < long.MaxValue ? start + size + 9 : start + size - 1;