Add ParamCollectionAttribute (#99385)

* Add ParamCollectionAttribute

In support of new C# params collection feature

* Update System.Runtime.cs

Author: stephentoub

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

@@ -874,6 +874,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\NullableContextAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\NullablePublicOnlyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ReferenceAssemblyAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ParamCollectionAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilderT.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PreserveBaseOverridesAttribute.cs" />
@@ -2762,4 +2763,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
-</Project>
+</Project>

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

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Indicates that a method will allow a variable number of arguments in its invocation.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
+    public sealed class ParamCollectionAttribute : Attribute
+    {
+    }
+}

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

@@ -13059,6 +13059,11 @@ public sealed partial class NullablePublicOnlyAttribute : System.Attribute
         public readonly bool IncludesInternals;
         public NullablePublicOnlyAttribute(bool value) { }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
+    public sealed partial class ParamCollectionAttribute : System.Attribute
+    {
+        public ParamCollectionAttribute() { }
+    }
     public partial struct PoolingAsyncValueTaskMethodBuilder
     {
         private object _dummy;

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

@@ -268,6 +268,7 @@
     <Compile Include="System\Runtime\CompilerServices\DefaultInterpolatedStringHandlerTests.cs" />
     <Compile Include="System\Runtime\CompilerServices\FormattableStringFactoryTests.cs" />
     <Compile Include="System\Runtime\CompilerServices\StrongBoxTests.cs" />
+    <Compile Include="System\Runtime\CompilerServices\ParamCollectionAttributeTests.cs" />
     <Compile Include="System\Runtime\CompilerServices\RuntimeHelpersTests.cs" />
     <Compile Include="System\Runtime\ConstrainedExecution\PrePrepareMethodAttributeTests.cs" />
     <Compile Include="System\Runtime\ExceptionServices\HandleProcessCorruptedStateExceptions.cs" />

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

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.Runtime.CompilerServices.Tests
+{
+    public static class ParamCollectionAttributeTests
+    {
+        [Fact]
+        public static void Ctor()
+        {
+            var attribute = new ParamCollectionAttribute();
+        }
+    }
+}