Add ExtensionMarkerAttribute

Author: nike4613

src/MonoMod.Backports/System/Runtime/CompilerServices/ExtensionMarkerAttribute,gte_core_10.0.cs

@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(ExtensionMarkerAttribute))]

src/MonoMod.Backports/System/Runtime/CompilerServices/ExtensionMarkerAttribute,lt_core_10.0,is_std,is_fx.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.ComponentModel;
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// This attribute is used to mark extension members and associate them with a specific marker type (which provides detailed information about an extension block and its receiver parameter).
+    /// This attribute should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Event | AttributeTargets.Interface | AttributeTargets.Delegate, Inherited = false)]
+    public sealed class ExtensionMarkerAttribute : Attribute
+    {
+        /// <summary>Initializes a new instance of the <see cref="ExtensionMarkerAttribute"/> class.</summary>
+        /// <param name="name">The name of the marker type this extension member is associated with.</param>
+        public ExtensionMarkerAttribute(string name)
+            => Name = name;
+
+        /// <summary>The name of the marker type this extension member is associated with.</summary>
+        public string Name { get; }
+    }
+}