Add UnscopedRefAttribute (#72499)

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

@@ -266,6 +266,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\DynamicDependencyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\ExcludeFromCodeCoverageAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\NullableAttributes.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\UnscopedRefAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\RequiresAssemblyFilesAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\RequiresDynamicCodeAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\RequiresUnreferencedCodeAttribute.cs" />

src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/UnscopedRefAttribute.cs

@@ -0,0 +1,41 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Used to indicate a byref escapes and is not scoped.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// There are several cases where the C# compiler treats a <see langword="ref"/> as implicitly
+    /// <see langword="scoped"/> - where the compiler does not allow the <see langword="ref"/> to escape the method.
+    /// </para>
+    /// <para>
+    /// For example:
+    /// <list type="number">
+    ///     <item><see langword="this"/> for <see langword="struct"/> instance methods.</item>
+    ///     <item><see langword="ref"/> parameters that refer to <see langword="ref"/> <see langword="struct"/> types.</item>
+    ///     <item><see langword="out"/> parameters.</item>
+    /// </list>
+    /// </para>
+    /// <para>
+    /// This attribute is used in those instances where the <see langword="ref"/> should be allowed to escape.
+    /// </para>
+    /// <para>
+    /// Applying this attribute, in any form, has impact on consumers of the applicable API. It is necessary for
+    /// API authors to understand the lifetime implications of applying this attribute and how it may impact their users.
+    /// </para>
+    /// </remarks>
+    [AttributeUsageAttribute(
+        AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Parameter,
+        AllowMultiple = false,
+        Inherited = false)]
+    public sealed class UnscopedRefAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnscopedRefAttribute"/> class.
+        /// </summary>
+        public UnscopedRefAttribute() { }
+    }
+}

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

@@ -8299,6 +8299,11 @@ public sealed partial class NotNullWhenAttribute : System.Attribute
         public NotNullWhenAttribute(bool returnValue) { }
         public bool ReturnValue { get { throw null; } }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Method | System.AttributeTargets.Property | System.AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
+    public sealed class UnscopedRefAttribute : System.Attribute
+    {
+        public UnscopedRefAttribute() { }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Constructor | System.AttributeTargets.Event | System.AttributeTargets.Method | System.AttributeTargets.Property, Inherited=false, AllowMultiple=false)]
     public sealed partial class RequiresAssemblyFilesAttribute : System.Attribute
     {