Allow LINQ optimizations for FirstOrNone/LastOrNone

FrameworkFeatureConstants.props

@@ -4,7 +4,7 @@
 		<DefineConstants>$(DefineConstants);INDEX_OF_CHAR_COMPARISONTYPE_SUPPORTED;TIMESPAN_MULTIPLY_SUPPORTED;SPLIT_ACCEPTS_STRING_SEPARATOR;LAZY_RETURN_CONSTRUCTOR;QUEUE_TRY_OVERLOADS;OPTIMIZED_ELEMENT_AT;RANGE_SUPPORTED;READ_ONLY_SPAN_SUPPORTED;INDEX_TYPE;JOIN_TO_STRING_CHAR_SEPARATOR;REMOVE_EXTENSION</DefineConstants>
 	</PropertyGroup>
 	<PropertyGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netcoreapp3.0'))">
-		<DefineConstants>$(DefineConstants);SYSTEM_INDEX_SUPPORTED;IP_END_POINT_TRY_PARSE_SUPPORTED</DefineConstants>
+		<DefineConstants>$(DefineConstants);SYSTEM_INDEX_SUPPORTED;IP_END_POINT_TRY_PARSE_SUPPORTED;OPTIMIZED_FIRST_LAST_OR_DEFAULT</DefineConstants>
 	</PropertyGroup>
 	<PropertyGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net5.0'))">
 		<DefineConstants>$(DefineConstants);SET_CURRENT_STACK_TRACE_SUPPORTED;DYNAMICALLY_ACCESSED_MEMBERS_ATTRIBUTE_SUPPORTED</DefineConstants>

Funcky.Test/Extensions/EnumerableExtensions/FirstSingleLastOrNoneTest.cs

@@ -1,5 +1,7 @@
 #pragma warning disable SA1010 // StyleCop support for collection expressions is missing
 using System.Collections;
+using Funcky.Test.TestUtils;
+using Xunit.Sdk;
 
 namespace Funcky.Test.Extensions.EnumerableExtensions;
 
@@ -24,6 +26,21 @@ public void GivenAnEnumerableSingleOrNoneGivesTheCorrectOption(List<int> valueEn
         ExpectedSingleOrNoneBehaviour(valueEnumerable, () => referenceEnumerable.SingleOrNone().Match(none: false, some: True));
     }
 
+#if OPTIMIZED_FIRST_LAST_OR_DEFAULT
+    [Fact]
+#else
+    [Fact(Skip = ".NET Framework 4.8 doesn't optimize these methods")]
+#endif
+    public void DoesNotEnumerateListsWhenCalledWithoutPredicate()
+    {
+        var list = new FailOnEnumerateListWrapper<string>(["foo"]);
+        _ = list.FirstOrNone();
+        _ = list.LastOrNone();
+
+        // SingleOrNone does not specialize for `Select`ed lists.
+        Assert.Throws<XunitException>(() => _ = list.SingleOrNone());
+    }
+
     public static TheoryData<List<int>, List<string>> ValueReferenceEnumerables()
         => new()
         {

Funcky.Test/TestUtils/FailOnEnumerateCollectionWrapper.cs

@@ -0,0 +1,25 @@
+using System.Collections;
+using Xunit.Sdk;
+
+namespace Funcky.Test.TestUtils;
+
+internal class FailOnEnumerateCollectionWrapper<T>(ICollection<T> collection) : ICollection<T>
+{
+    public bool IsReadOnly => true;
+
+    public int Count => collection.Count;
+
+    public IEnumerator<T> GetEnumerator() => throw new XunitException("Should not be enumerated");
+
+    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+    public void Add(T item) => throw new NotSupportedException();
+
+    public void Clear() => throw new NotSupportedException();
+
+    public bool Contains(T item) => collection.Contains(item);
+
+    public void CopyTo(T[] array, int arrayIndex) => collection.CopyTo(array, arrayIndex);
+
+    public bool Remove(T item) => throw new NotSupportedException();
+}

Funcky.Test/TestUtils/FailOnEnumerateListWrapper.cs

@@ -0,0 +1,16 @@
+namespace Funcky.Test.TestUtils;
+
+internal sealed class FailOnEnumerateListWrapper<T>(IList<T> list) : FailOnEnumerateCollectionWrapper<T>(list), IList<T>
+{
+    public T this[int index]
+    {
+        get => list[index];
+        set => throw new NotSupportedException();
+    }
+
+    public int IndexOf(T item) => list.IndexOf(item);
+
+    public void Insert(int index, T item) => throw new NotSupportedException();
+
+    public void RemoveAt(int index) => throw new NotSupportedException();
+}

Funcky/Extensions/EnumerableExtensions/FirstOrNone.cs

@@ -9,7 +9,9 @@ public static partial class EnumerableExtensions
     [Pure]
     public static Option<TSource> FirstOrNone<TSource>(this IEnumerable<TSource> source)
         where TSource : notnull
-        => source.FirstOrNone(True);
+        => source
+            .Select(Option.Some)
+            .FirstOrDefault();
 
     /// <summary>
     /// Returns the first element of the sequence as an <see cref="Option{T}" /> that satisfies a condition or a <see cref="Option{T}.None" /> value if no such element is found.

Funcky/Extensions/EnumerableExtensions/LastOrNone.cs

@@ -9,7 +9,9 @@ public static partial class EnumerableExtensions
     [Pure]
     public static Option<TSource> LastOrNone<TSource>(this IEnumerable<TSource> source)
         where TSource : notnull
-        => source.LastOrNone(True);
+        => source
+            .Select(Option.Some)
+            .LastOrDefault();
 
     /// <summary>
     /// Returns the last element of a sequence that satisfies a condition as an <see cref="Option{T}" />  or a <see cref="Option{T}.None" /> value if no such element is found.

Funcky/Monads/Option/OptionExtensions.cs

@@ -1,6 +1,5 @@
 using System.ComponentModel;
 
-#pragma warning disable RS0026
 namespace Funcky.Monads;
 
 public static partial class OptionExtensions