Add support for custom string interpolation handlers.

README.md

@@ -103,7 +103,8 @@ PolyKit provides support for the following features across all [compatible targe
 - [System.HashCode](https://docs.microsoft.com/en-us/dotnet/api/system.hashcode) (see note #2);
 - [SkipLocalsInit](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/general#skiplocalsinit-attribute);
 - [ValidatedNotNull](https://docs.microsoft.com/en-us/dotnet/api/microsoft.validatednotnullattribute) (see note #3);
-- [StackTraceHidden](https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.stacktracehiddenattribute) (see note #4).
+- [StackTraceHidden](https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.stacktracehiddenattribute) (see note #4);
+- support for writing [custom string interpolation handlers](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/interpolated-string-handler).
 
 **Note #1:** This feature depends on `System.ValueTuple`, which is not present in .NET Framework versions prior to 4.7. If you reference the `PolyKit.Embedded` package in a project targeting .NET Framework 4.6.2 or 4.6.3, you must also add a package reference to [`System.ValueTuple`](https://www.nuget.org/packages/System.ValueTuple); otherwise, compilation will not fail, but features dependent on ValueTuple will not be present in the compiled assembly.
 

src/PolyKit.Polyfills/System/Runtime/CompilerServices/InterpolatedStringHandlerArgumentAttribute.cs

@@ -0,0 +1,57 @@
+#if NET6_0_OR_GREATER
+
+#if POLYKIT_PUBLIC
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.InterpolatedStringHandlerArgumentAttribute))]
+#endif
+
+#else
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Runtime.CompilerServices;
+
+// https://github.com/dotnet/runtime/blob/v6.0.8/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/InterpolatedStringHandlerArgumentAttribute.cs
+
+/// <summary>
+/// Indicates which arguments to a method involving an interpolated string handler should be passed to that handler.
+/// </summary>
+[AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
+[ExcludeFromCodeCoverage, DebuggerNonUserCode]
+#if POLYKIT_PUBLIC
+public
+#else
+internal
+#endif
+sealed class InterpolatedStringHandlerArgumentAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="InterpolatedStringHandlerArgumentAttribute"/> class.
+    /// </summary>
+    /// <param name="argument">The name of the argument that should be passed to the handler.</param>
+    /// <remarks><see langword="null"/> may be used as the name of the receiver in an instance method.</remarks>
+#pragma warning disable CA1019 // Define accessors for attribute arguments - Preserving original code.
+    public InterpolatedStringHandlerArgumentAttribute(string argument)
+#pragma warning restore CA1019 // Define accessors for attribute arguments
+    {
+        Arguments = new string[] { argument };
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="InterpolatedStringHandlerArgumentAttribute"/> class.
+    /// </summary>
+    /// <param name="arguments">The names of the arguments that should be passed to the handler.</param>
+    /// <remarks><see langword="null"/> may be used as the name of the receiver in an instance method.</remarks>
+    public InterpolatedStringHandlerArgumentAttribute(params string[] arguments)
+    {
+        Arguments = arguments;
+    }
+
+    /// <summary>Gets the names of the arguments that should be passed to the handler.</summary>
+    /// <remarks><see langword="null"/> may be used as the name of the receiver in an instance method.</remarks>
+    public string[] Arguments { get; }
+}
+
+#endif

src/PolyKit.Polyfills/System/Runtime/CompilerServices/InterpolatedStringHandlerAttribute.cs

@@ -0,0 +1,38 @@
+#if NET6_0_OR_GREATER
+
+#if POLYKIT_PUBLIC
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.InterpolatedStringHandlerAttribute))]
+#endif
+
+#else
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Runtime.CompilerServices;
+
+// https://github.com/dotnet/runtime/blob/v6.0.8/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/InterpolatedStringHandlerAttribute.cs
+
+/// <summary>
+/// Indicates the attributed type is to be used as an interpolated string handler.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
+[ExcludeFromCodeCoverage, DebuggerNonUserCode]
+#if POLYKIT_PUBLIC
+public
+#else
+internal
+#endif
+sealed class InterpolatedStringHandlerAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="InterpolatedStringHandlerAttribute"/> class.
+    /// </summary>
+    public InterpolatedStringHandlerAttribute()
+    {
+    }
+}
+
+#endif