Add CompilerFeatureRequiredAttribute (#67924)

Closes #66167.

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/README.md

@@ -6,7 +6,7 @@ The goal is to have the majority of code located in this folder, as that code is
 
 ### File Naming Convention
 
-Any runtime-specific `partial` part which also has a shared part sholud use a runtime-specific file name suffix to ease the navigation.
+Any runtime-specific `partial` part which also has a shared part should use a runtime-specific file name suffix to ease the navigation.
 
 * `*.CoreCLR.cs` for CoreCLR runtime
 * `*.Mono.cs` for Mono runtime

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

@@ -709,6 +709,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CallingConventions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CompilationRelaxations.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CompilationRelaxationsAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CompilerFeatureRequiredAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CompilerGeneratedAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\CompilerGlobalScopeAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ConditionalWeakTable.cs" />
@@ -2411,4 +2412,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
-</Project>
+</Project>

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

@@ -0,0 +1,37 @@
+// 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 compiler support for a particular feature is required for the location where this attribute is applied.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+    public sealed class CompilerFeatureRequiredAttribute : Attribute
+    {
+        public CompilerFeatureRequiredAttribute(string featureName)
+        {
+            FeatureName = featureName;
+        }
+
+        /// <summary>
+        /// The name of the compiler feature.
+        /// </summary>
+        public string FeatureName { get; }
+
+        /// <summary>
+        /// If true, the compiler can choose to allow access to the location where this attribute is applied if it does not understand <see cref="FeatureName"/>.
+        /// </summary>
+        public bool IsOptional { get; init; }
+
+        /// <summary>
+        /// The <see cref="FeatureName"/> used for the ref structs C# feature.
+        /// </summary>
+        public const string RefStructs = nameof(RefStructs);
+
+        /// <summary>
+        /// The <see cref="FeatureName"/> used for the required members C# feature.
+        /// </summary>
+        public const string RequiredMembers = nameof(RequiredMembers);
+    }
+}

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

@@ -11275,6 +11275,15 @@ public CompilationRelaxationsAttribute(int relaxations) { }
         public CompilationRelaxationsAttribute(System.Runtime.CompilerServices.CompilationRelaxations relaxations) { }
         public int CompilationRelaxations { get { throw null; } }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.All, AllowMultiple=true, Inherited=false)]
+    public sealed partial class CompilerFeatureRequiredAttribute : System.Attribute
+    {
+        public const string RefStructs = "RefStructs";
+        public const string RequiredMembers = "RequiredMembers";
+        public CompilerFeatureRequiredAttribute(string featureName) { }
+        public string FeatureName { get { throw null; } }
+        public bool IsOptional { get { throw null; } init { } }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.All, Inherited=true)]
     public sealed partial class CompilerGeneratedAttribute : System.Attribute
     {

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

@@ -326,5 +326,17 @@ public static void RequiredMemberAttributeTests()
         {
             new RequiredMemberAttribute();
         }
+
+        [Fact]
+        public static void CompilerFeatureRequiredTests()
+        {
+            var attr1 = new CompilerFeatureRequiredAttribute("feature1");
+            Assert.Equal("feature1", attr1.FeatureName);
+            Assert.False(attr1.IsOptional);
+
+            var attr2 = new CompilerFeatureRequiredAttribute("feature2") { IsOptional = true };
+            Assert.Equal("feature2", attr2.FeatureName);
+            Assert.True(attr2.IsOptional);
+        }
     }
 }