Introduce CallConvSwift and special register types to represent the S…

…wift ABI calling convention (dotnet#95065)

* Add CallConvSwift attribute and introduce types to represent each of the special registers

---------

Co-authored-by: Aaron Robinson <arobins@microsoft.com>

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

@@ -1015,6 +1015,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\StringMarshalling.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\StructLayoutAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\SuppressGCTransitionAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Swift\SwiftTypes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\TypeIdentifierAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\UnknownWrapper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\UnmanagedCallConvAttribute.cs" />

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

@@ -15,6 +15,16 @@ public class CallConvStdcall
     {
         public CallConvStdcall() { }
     }
+    /// <summary>
+    /// Indicates that a method should using the <see href="https://github.com/apple/swift/blob/main/docs/ABIStabilityManifesto.md#calling-convention">Swift</see>calling convention.
+    /// </summary>
+    public class CallConvSwift
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CallConvSwift" /> class.
+        /// </summary>
+        public CallConvSwift() { }
+    }
 
     /// <summary>
     /// Indicates that a method should suppress the GC transition as part of the calling convention.

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Swift/SwiftTypes.cs

@@ -0,0 +1,71 @@
+// 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.InteropServices.Swift
+{
+    /// <summary>
+    /// Represents the Swift 'self' context, indicating that the argument is the self context.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This struct is used to pass the 'self' context to Swift functions in the context of interop with .NET.
+    /// </para>
+    /// <para>
+    /// Here's an example of how a SwiftSelf context can be declared:
+    /// <code lang="csharp">
+    /// [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
+    /// [DllImport("SwiftLibrary", EntryPoint = "export")]
+    /// public static extern void swiftFunction(SwiftSelf self);
+    /// </code>
+    /// </para>
+    /// </remarks>
+    [CLSCompliantAttribute(false)]
+    public readonly unsafe struct SwiftSelf
+    {
+        /// <summary>
+        /// Creates a new instance of the SwiftSelf struct with the specified pointer value.
+        /// </summary>
+        /// <param name="value">The pointer value representing the self context.</param>
+        public SwiftSelf(void* value)
+        {
+            Value = value;
+        }
+        /// <summary>
+        /// Gets the pointer of the self context.
+        /// </summary>
+        public void* Value { get; }
+    }
+
+    /// <summary>
+    /// Represents the Swift error context, indicating that the argument is the error context.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This struct is used to retrieve the 'error' context from Swift functions in the context of interop with .NET.
+    /// </para>
+    /// <para>
+    /// Here's an example of how a SwiftError can be declared:
+    /// <code lang="csharp">
+    /// [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
+    /// [DllImport("SwiftLibrary", EntryPoint = "export")]
+    /// public static extern void swiftFunction(SwiftError* error);
+    /// </code>
+    /// </para>
+    /// </remarks>
+    [CLSCompliantAttribute(false)]
+    public readonly unsafe struct SwiftError
+    {
+        /// <summary>
+        /// Creates a new instance of the SwiftError struct with the specified pointer value.
+        /// </summary>
+        /// <param name="value">The pointer value representing the error context.</param>
+        public SwiftError(void* value)
+        {
+            Value = value;
+        }
+        /// <summary>
+        /// Gets the pointer of the error context.
+        /// </summary>
+        public void* Value { get; }
+    }
+}

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

@@ -12616,6 +12616,10 @@ public partial class CallConvStdcall
     {
         public CallConvStdcall() { }
     }
+    public partial class CallConvSwift
+    {
+        public CallConvSwift() { }
+    }
     public partial class CallConvSuppressGCTransition
     {
         public CallConvSuppressGCTransition() { }
@@ -13813,6 +13817,22 @@ public void Free() { }
         }
     }
 }
+namespace System.Runtime.InteropServices.Swift
+{
+    [System.CLSCompliantAttribute(false)]
+    public readonly unsafe struct SwiftSelf
+    {
+        public SwiftSelf(void* value) { }
+        public void* Value { get; }
+    }
+
+    [System.CLSCompliantAttribute(false)]
+    public readonly unsafe struct SwiftError
+    {
+        public SwiftError(void* value) { }
+        public void* Value { get; }
+    }
+}
 namespace System.Runtime.Remoting
 {
     public partial class ObjectHandle : System.MarshalByRefObject