Add a feature flag to not use GVM in Linq Select

src/libraries/System.Linq/src/System/Linq/Enumerable.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -63,5 +64,8 @@ internal static bool TryGetSpan<TSource>(this IEnumerable<TSource> source, out R
 
             return result;
         }
+
+        [FeatureSwitchDefinition("System.Linq.Enumerable.ValueTypeTrimFriendlySelect")]
+        internal static bool ValueTypeTrimFriendlySelect { get; } = AppContext.TryGetSwitch("System.Linq.Enumerable.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 new IteratorSelectIterator<TResult, TResult2>(this, selector);
             }
         }
     }

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)
+        {
+            // With native AOT, calling into the `Select` generic virtual method results in NxM
+            // expansion of native code. If the option is enabled, we don't call the generic virtual
+            // for value types. We don't do the same for reference types because reference type
+            // expansion can happen lazily at runtime and the AOT compiler does postpone it (we
+            // don't need more code, just more data structures describing the new types).
+            if (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/libraries/System.Linq/tests/SkipWhileTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 
 namespace System.Linq.Tests
@@ -62,14 +63,33 @@ public void RunOnce()
             Assert.Equal(Enumerable.Range(10, 10), Enumerable.Range(0, 20).RunOnce().SkipWhile((i, idx) => idx < 10));
         }
 
-        [Fact]
-        public void SkipErrorWhenSourceErrors()
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public static void SkipErrorWhenSourceErrors_TrimFriendlySelectTrue()
         {
-            var source = NumberRangeGuaranteedNotCollectionType(-2, 5).Select(i => (decimal)i).Select(m => 1 / m).Skip(4);
-            using(var en = source.GetEnumerator())
+            RemoteExecutor.Invoke(() =>
             {
-                Assert.Throws<DivideByZeroException>(() => en.MoveNext());
-            }
+                AppContext.SetSwitch("System.Linq.Enumerable.ValueTypeTrimFriendlySelect", true);
+
+                var source = NumberRangeGuaranteedNotCollectionType(-2, 5).Select(i => (decimal)i).Select(m => 1 / m).Skip(4);
+                var valuesFromEnumerable = source.ToList();
+                List<decimal> expectedValues = [(decimal)1/2];
+                Assert.Equal(expectedValues, valuesFromEnumerable);
+            }).Dispose();
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void SkipErrorWhenSourceErrors_TrimFriendlySelectFalse()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                AppContext.SetSwitch("System.Linq.Enumerable.ValueTypeTrimFriendlySelect", false);
+
+                var source = NumberRangeGuaranteedNotCollectionType(-2, 5).Select(i => (decimal)i).Select(m => 1 / m).Skip(4);
+                using (var en = source.GetEnumerator())
+                {
+                    Assert.Throws<DivideByZeroException>(() => en.MoveNext());
+                }
+            }).Dispose();
         }
 
         [Fact]

src/libraries/System.Linq/tests/System.Linq.Tests.csproj

@@ -1,5 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
+    <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
     <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
     <DebuggerSupport Condition="'$(DebuggerSupport)' == '' and '$(TargetOS)' == 'browser'">true</DebuggerSupport>
   </PropertyGroup>