Add System.Private.CoreLib shared sources (#12162)

Taken from corefx commit 5956ad35864352b38c53aa8c2d1ce012c96b168d. Subsequent updates will happen via a mirroring bot.

1 files changed, 21 insertions, 0 deletions

diff --git a/netcore/System.Private.CoreLib/shared/System/Runtime/CompilerServices/AsyncMethodBuilderAttribute.cs b/netcore/System.Private.CoreLib/shared/System/Runtime/CompilerServices/AsyncMethodBuilderAttribute.cs
new file mode 100644
index 00000000000..688a3a01ba7
--- /dev/null
+++ b/netcore/System.Private.CoreLib/shared/System/Runtime/CompilerServices/AsyncMethodBuilderAttribute.cs
@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Indicates the type of the async method builder that should be used by a language compiler to
+    /// build the attributed type when used as the return type of an async method.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Delegate | AttributeTargets.Enum, Inherited = false, AllowMultiple = false)]
+    public sealed class AsyncMethodBuilderAttribute : Attribute
+    {
+        /// <summary>Initializes the <see cref="AsyncMethodBuilderAttribute"/>.</summary>
+        /// <param name="builderType">The <see cref="Type"/> of the associated builder.</param>
+        public AsyncMethodBuilderAttribute(Type builderType) => BuilderType = builderType;
+
+        /// <summary>Gets the <see cref="Type"/> of the associated builder.</summary>
+        public Type BuilderType { get; }
+    }
+}