Don't use a GVM in Linq Select with NAOT by default

src/libraries/System.Linq/src/System.Linq.csproj

@@ -64,6 +64,7 @@
     <Compile Include="System\Linq\Index.cs" />
     <Compile Include="System\Linq\Intersect.cs" />
     <Compile Include="System\Linq\Iterator.cs" />
+    <Compile Include="System\Linq\IteratorOptions.cs" />
     <Compile Include="System\Linq\Join.cs" />
     <Compile Include="System\Linq\Last.cs" />
     <Compile Include="System\Linq\Lookup.cs" />

src/libraries/System.Linq/src/System/Linq/IteratorOptions.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Linq
+{
+    /// <summary>
+    /// Defines options for how iterators behave.
+    /// </summary>
+    internal static class IteratorOptions
+    {
+        [FeatureSwitchDefinition("System.Linq.ValueTypeTrimFriendlySelect")]
+        public static bool ValueTypeTrimFriendlySelect { get; } = AppContext.TryGetSwitch("System.Linq.ValueTypeTrimFriendlySelect", out bool isEnabled) ? isEnabled : true;
+    }
+}

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

@@ -172,7 +172,7 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
                         new IEnumerableWhereSelectIterator<object, TResult2>(objectSource, isTResult, localSelector);
                 }
 
-                return base.Select(selector);
+                return SelectImplementation(selector, this);
             }
         }
     }

src/libraries/System.Linq/src/System/Linq/Select.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using static System.Linq.Utilities;
 
 namespace System.Linq
@@ -25,7 +26,7 @@ public static IEnumerable<TResult> Select<TSource, TResult>(
 
             if (source is Iterator<TSource> iterator)
             {
-                return iterator.Select(selector);
+                return SelectImplementation(selector, iterator);
             }
 
             if (source is IList<TSource> ilist)
@@ -51,6 +52,26 @@ public static IEnumerable<TResult> Select<TSource, TResult>(
             return new IEnumerableSelectIterator<TSource, TResult>(source, selector);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static IEnumerable<TResult> SelectImplementation<TSource, TResult>(Func<TSource, TResult> selector, Iterator<TSource> iterator)
+        {
+            // In Native AOT, GVMs with Value types are not trim friendly.
+            // If the option is enabled, we don't call the GVM for value types. We don't do the
+            // same for reference types because reference types are trim friendly with GVMs
+            // and it is preferable to call the GVM as it allows the select implementation to be
+            // specialized.
+            if (/*IteratorOptions.ValueTypeTrimFriendlySelect && */typeof(TResult).IsValueType)
+            {
+#if OPTIMIZE_FOR_SIZE
+                return new IEnumerableSelectIterator<TSource, TResult>(iterator, selector);
+#else
+                return new IteratorSelectIterator<TSource, TResult>(iterator, selector);
+#endif
+            }
+
+            return iterator.Select(selector);
+        }
+
         public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
         {
             if (source is null)

src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets

@@ -38,6 +38,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <EnableUnsafeBinaryFormatterSerialization Condition="'$(EnableUnsafeBinaryFormatterSerialization)' == ''">false</EnableUnsafeBinaryFormatterSerialization>
     <EnableUnsafeUTF7Encoding Condition="'$(EnableUnsafeUTF7Encoding)' == ''">false</EnableUnsafeUTF7Encoding>
     <BuiltInComInteropSupport Condition="'$(BuiltInComInteropSupport)' == ''">false</BuiltInComInteropSupport>
+    <ValueTypeTrimFriendlySelect Condition="'$(ValueTypeTrimFriendlySelect)' == ''">true</ValueTypeTrimFriendlySelect>
     <!-- GeneratedComInterface/ComImport interop requires built-in COM -->
     <EnableGeneratedComInterfaceComImportInterop Condition="'$(BuiltInComInteropSupport)' == 'false'">false</EnableGeneratedComInterfaceComImportInterop>
     <EnableGeneratedComInterfaceComImportInterop Condition="'$(EnableGeneratedComInterfaceComImportInterop)' == ''">false</EnableGeneratedComInterfaceComImportInterop>