Add IsUnmanaged, ScopedRef, and Nullable{Context/PublicOnly} attributes

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

@@ -813,6 +813,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\InterpolatedStringHandlerAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\InterpolatedStringHandlerArgumentAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DefaultInterpolatedStringHandler.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IsUnmanagedAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IteratorStateMachineAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ITuple.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\LoadHint.cs" />
@@ -821,6 +822,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MethodImplOptions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ModuleInitializerAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MetadataUpdateOriginalTypeAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\NullableAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\NullableContextAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\NullablePublicOnlyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ReferenceAssemblyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilderT.cs" />
@@ -832,6 +836,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RuntimeFeature.NonNativeAot.cs" Condition="'$(FeatureNativeAot)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RuntimeHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\RuntimeWrappedException.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ScopedRefAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\SkipLocalsInitAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\SpecialNameAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\StateMachineAttribute.cs" />

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

@@ -0,0 +1,9 @@
+// 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
+{
+    public sealed class IsUnmanagedAttribute : Attribute
+    {
+    }
+}

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

@@ -0,0 +1,21 @@
+// 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
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Event | AttributeTargets.Parameter | AttributeTargets.ReturnValue | AttributeTargets.GenericParameter, AllowMultiple = false, Inherited = false)]
+    public sealed class NullableAttribute : Attribute
+    {
+        public readonly byte[] NullableFlags;
+
+        public NullableAttribute(byte value)
+        {
+            NullableFlags = new[] { value };
+        }
+
+        public NullableAttribute(byte[] value)
+        {
+            NullableFlags = value;
+        }
+    }
+}

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/NullableContextAttribute.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.
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Method | AttributeTargets.Interface | AttributeTargets.Delegate, AllowMultiple = false, Inherited = false)]
+    public sealed class NullableContextAttribute : Attribute
+    {
+        public readonly byte Flag;
+
+        public NullableContextAttribute(byte value)
+        {
+            Flag = value;
+        }
+    }
+}

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/NullablePublicOnlyAttribute.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.
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Module, AllowMultiple = false, Inherited = false)]
+    public sealed class NullablePublicOnlyAttribute : Attribute
+    {
+        public readonly bool IncludesInternals;
+
+        public NullablePublicOnlyAttribute(bool value)
+        {
+            IncludesInternals = value;
+        }
+    }
+}

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

@@ -0,0 +1,10 @@
+// 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
+{
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
+    public sealed class ScopedRefAttribute : Attribute
+    {
+    }
+}

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

@@ -12781,6 +12781,9 @@ public partial interface IStrongBox
     {
         object? Value { get; set; }
     }
+    public sealed partial class IsUnmanagedAttribute : Attribute
+    {
+    }
     public static partial class IsVolatile
     {
     }
@@ -12834,6 +12837,26 @@ public sealed partial class ModuleInitializerAttribute : System.Attribute
     {
         public ModuleInitializerAttribute() { }
     }
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Event | AttributeTargets.Parameter | AttributeTargets.ReturnValue | AttributeTargets.GenericParameter, AllowMultiple = false, Inherited = false)]
+    public sealed partial class NullableAttribute : Attribute
+    {
+        public readonly byte[] NullableFlags;
+        public NullableAttribute(byte value) { }
+
+        public NullableAttribute(byte[] value) { }
+    }
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Method | AttributeTargets.Interface | AttributeTargets.Delegate, AllowMultiple = false, Inherited = false)]
+    public sealed partial class NullableContextAttribute : Attribute
+    {
+        public readonly byte Flag;
+        public NullableContextAttribute(byte value) { }
+    }
+    [AttributeUsage(AttributeTargets.Module, AllowMultiple = false, Inherited = false)]
+    public sealed partial class NullablePublicOnlyAttribute : Attribute
+    {
+        public readonly bool IncludesInternals;
+        public NullablePublicOnlyAttribute(bool value) { }
+    }
     public partial struct PoolingAsyncValueTaskMethodBuilder
     {
         private object _dummy;
@@ -12946,6 +12969,10 @@ public RuntimeWrappedException(object thrownObject) { }
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
         public override void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
     }
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
+    public sealed partial class ScopedRefAttribute : Attribute
+    {
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Constructor | System.AttributeTargets.Event | System.AttributeTargets.Interface | System.AttributeTargets.Method | System.AttributeTargets.Module | System.AttributeTargets.Property | System.AttributeTargets.Struct, Inherited=false)]
     public sealed partial class SkipLocalsInitAttribute : System.Attribute
     {