Add ExtensionMarkerAttribute (#118188)

Author: jcouv

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -853,6 +853,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DiscardableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\EnumeratorCancellationAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ExtensionAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ExtensionMarkerAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\FixedAddressValueTypeAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\FixedBufferAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\FormattableStringFactory.cs" />

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ExtensionMarkerAttribute.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; }
+    }
+}

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -13495,6 +13495,13 @@ public sealed partial class ExtensionAttribute : System.Attribute
     {
         public ExtensionAttribute() { }
     }
+    [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+    [System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Struct | System.AttributeTargets.Enum | System.AttributeTargets.Method | System.AttributeTargets.Property | System.AttributeTargets.Field | System.AttributeTargets.Event | System.AttributeTargets.Interface | System.AttributeTargets.Delegate, Inherited = false)]
+    public sealed partial class ExtensionMarkerAttribute : System.Attribute
+    {
+        public ExtensionMarkerAttribute(string name) { }
+        public string Name { get { throw null; } }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Field)]
     public sealed partial class FixedAddressValueTypeAttribute : System.Attribute
     {

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/AttributesTests.cs

@@ -235,6 +235,15 @@ public static void RefSafetyRulesAttributeTests(int version)
             Assert.Equal(version, attr.Version);
         }
 
+        [Theory]
+        [InlineData("1")]
+        [InlineData("2")]
+        public static void ExtensionMarkerAttributeTests(string name)
+        {
+            var attr = new ExtensionMarkerAttribute(name);
+            Assert.Equal(name, attr.Name);
+        }
+
         [Fact]
         public static void ReferenceAssemblyAttributeTests()
         {