Add public RefSafetyRulesAttribute (#80379)

Author: stephentoub

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

@@ -806,6 +806,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilderT.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PreserveBaseOverridesAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RefSafetyRulesAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RequiredMemberAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RuntimeCompatibilityAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RuntimeFeature.cs" />

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

@@ -0,0 +1,17 @@
+// 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 the language version of the ref safety rules used when the module was compiled.</summary>
+    [AttributeUsage(AttributeTargets.Module, AllowMultiple = false, Inherited = false)]
+    public sealed class RefSafetyRulesAttribute : Attribute
+    {
+        /// <summary>Initializes a new instance of the <see cref="RefSafetyRulesAttribute"/> class.</summary>
+        /// <param name="version">The language version of the ref safety rules used when the module was compiled.</param>
+        public RefSafetyRulesAttribute(int version) => Version = version;
+
+        /// <summary>Gets the language version of the ref safety rules used when the module was compiled.</summary>
+        public int Version { get; }
+    }
+}

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

@@ -12549,6 +12549,12 @@ public sealed partial class PreserveBaseOverridesAttribute : System.Attribute
     {
         public PreserveBaseOverridesAttribute() { }
     }
+    [System.AttributeUsage(System.AttributeTargets.Module, AllowMultiple=false, Inherited=false)]
+    public sealed partial class RefSafetyRulesAttribute : System.Attribute
+    {
+        public RefSafetyRulesAttribute(int version) { }
+        public int Version { get { throw null; } }
+    }
     [System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Struct | System.AttributeTargets.Field | System.AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
     public sealed class RequiredMemberAttribute : System.Attribute
     {

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

@@ -192,6 +192,15 @@ public static void MethodImplAttributeTests()
             Assert.Equal(MethodImplOptions.Unmanaged, attr3.Value);
         }
 
+        [Theory]
+        [InlineData(-1)]
+        [InlineData(42)]
+        public static void RefSafetyRulesAttributeTests(int version)
+        {
+            var attr = new RefSafetyRulesAttribute(version);
+            Assert.Equal(version, attr.Version);
+        }
+
         [Fact]
         public static void ReferenceAssemblyAttributeTests()
         {