Fix build failures: restore #if NET guards for allows ref struct in shared files

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IAsyncEnumerable.cs

@@ -8,7 +8,9 @@ namespace System.Collections.Generic
     /// <summary>Exposes an enumerator that provides asynchronous iteration over values of a specified type.</summary>
     /// <typeparam name="T">The type of values to enumerate.</typeparam>
     public interface IAsyncEnumerable<out T>
+#if NET
         where T : allows ref struct
+#endif
     {
         /// <summary>Returns an enumerator that iterates asynchronously through the collection.</summary>
         /// <param name="cancellationToken">A <see cref="CancellationToken"/> that may be used to cancel the asynchronous iteration.</param>

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IAsyncEnumerator.cs

@@ -8,7 +8,9 @@ namespace System.Collections.Generic
     /// <summary>Supports a simple asynchronous iteration over a generic collection.</summary>
     /// <typeparam name="T">The type of objects to enumerate.</typeparam>
     public interface IAsyncEnumerator<out T> : IAsyncDisposable
+#if NET
          where T : allows ref struct
+#endif
     {
         /// <summary>Advances the enumerator asynchronously to the next element of the collection.</summary>
         /// <returns>

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConfiguredCancelableAsyncEnumerable.cs

@@ -10,7 +10,9 @@ namespace System.Runtime.CompilerServices
     /// <summary>Provides an awaitable async enumerable that enables cancelable iteration and configured awaits.</summary>
     [StructLayout(LayoutKind.Auto)]
     public readonly struct ConfiguredCancelableAsyncEnumerable<T>
+#if NET
         where T : allows ref struct
+#endif
     {
         private readonly IAsyncEnumerable<T> _enumerable;
         private readonly CancellationToken _cancellationToken;

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskAsyncEnumerableExtensions.cs

@@ -23,7 +23,9 @@ public static ConfiguredAsyncDisposable ConfigureAwait(this IAsyncDisposable sou
         /// <returns>The configured enumerable.</returns>
         public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(
             this IAsyncEnumerable<T> source, bool continueOnCapturedContext)
+#if NET
             where T : allows ref struct
+#endif
             => new ConfiguredCancelableAsyncEnumerable<T>(source, continueOnCapturedContext, cancellationToken: default);
 
         /// <summary>Sets the <see cref="CancellationToken"/> to be passed to <see cref="IAsyncEnumerable{T}.GetAsyncEnumerator(CancellationToken)"/> when iterating.</summary>
@@ -33,7 +35,9 @@ public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(
         /// <returns>The configured enumerable.</returns>
         public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(
             this IAsyncEnumerable<T> source, CancellationToken cancellationToken)
+#if NET
             where T : allows ref struct
+#endif
             => new ConfiguredCancelableAsyncEnumerable<T>(source, continueOnCapturedContext: true, cancellationToken);
     }
 }