Add ExperimentalAttribute polyfill

README.md

@@ -51,6 +51,7 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ModuleInitializer]` (needed to enable [custom module initializers](https://learn.microsoft.com/dotnet/csharp/language-reference/proposals/csharp-9.0/module-initializers))
 - `[RequiresLocation]` (needed to enable [ref readonly parameters](https://github.com/dotnet/csharplang/issues/6010))
 - `[CollectionBuilder]` (needed for [collection expressions](https://github.com/dotnet/csharplang/issues/5354))
+- `[Experimental]` (needed for [experimental features](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/experimental-attribute))
 
 To leverage them, make sure to bump your C# language version. You can do this by setting the `<LangVersion>` MSBuild property in your project. For instance, by adding `<LangVersion>11.0</LangVersion>` (or your desired C# version) to the first `<PropertyGroup>` of your .csproj file. For more info on this, [see here](https://sergiopedri.medium.com/enabling-and-using-c-9-features-on-older-and-unsupported-runtimes-ce384d8debb), but remember that you don't need to manually copy polyfills anymore: simply adding a reference to **PolySharp** will do this for you automatically.
 

src/PolySharp.Package/README.md

@@ -49,6 +49,7 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ModuleInitializer]` (needed to enable [custom module initializers](https://learn.microsoft.com/dotnet/csharp/language-reference/proposals/csharp-9.0/module-initializers))
 - `[RequiresLocation]` (needed to enable [ref readonly parameters](https://github.com/dotnet/csharplang/issues/6010))
 - `[CollectionBuilder]` (needed for [collection expressions](https://github.com/dotnet/csharplang/issues/5354))
+- `[Experimental]` (needed for [experimental features](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/experimental-attribute))
 
 To leverage them, make sure to bump your C# language version. You can do this by setting the `<LangVersion>` MSBuild property in your project. For instance, by adding `<LangVersion>11.0</LangVersion>` (or your desired C# version) to the first `<PropertyGroup>` of your .csproj file. For more info on this, [see here](https://sergiopedri.medium.com/enabling-and-using-c-9-features-on-older-and-unsupported-runtimes-ce384d8debb), but remember that you don't need to manually copy polyfills anymore: simply adding a reference to **PolySharp** will do this for you automatically.
 

src/PolySharp.SourceGenerators/EmbeddedResources/System.Diagnostics.CodeAnalysis.ExperimentalAttribute.cs

@@ -0,0 +1,62 @@
+// <auto-generated/>
+#pragma warning disable
+#nullable enable annotations
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Indicates that an API is experimental and it may change in the future.
+    /// </summary>
+    /// <remarks>
+    /// This attribute allows call sites to be flagged with a diagnostic that indicates that an experimental
+    /// feature is used. Authors can use this attribute to ship preview features in their assemblies.
+    /// </remarks>
+    [global::System.AttributeUsage(
+        global::System.AttributeTargets.Assembly |
+        global::System.AttributeTargets.Module |
+        global::System.AttributeTargets.Class |
+        global::System.AttributeTargets.Struct |
+        global::System.AttributeTargets.Enum |
+        global::System.AttributeTargets.Constructor |
+        global::System.AttributeTargets.Method |
+        global::System.AttributeTargets.Property |
+        global::System.AttributeTargets.Field |
+        global::System.AttributeTargets.Event |
+        global::System.AttributeTargets.Interface |
+        global::System.AttributeTargets.Delegate,
+        Inherited = false)]
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    internal sealed class ExperimentalAttribute : global::System.Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="global::System.Diagnostics.CodeAnalysis.ExperimentalAttribute"/> class,
+        /// specifying the ID that the compiler will use when reporting a use of the API the attribute applies to.
+        /// </summary>
+        /// <param name="diagnosticId">The ID that the compiler will use when reporting a use of the API the attribute applies to.</param>
+        public ExperimentalAttribute(string diagnosticId)
+        {
+            DiagnosticId = diagnosticId;
+        }
+
+        /// <summary>
+        /// Gets the ID that the compiler will use when reporting a use of the API the attribute applies to.
+        /// </summary>
+        /// <value>The unique diagnostic ID.</value>
+        /// <remarks>
+        /// The diagnostic ID is shown in build output for warnings and errors.
+        /// <para>This property represents the unique ID that can be used to suppress the warnings or errors, if needed.</para>
+        /// </remarks>
+        public string DiagnosticId { get; }
+
+        /// <summary>
+        /// Gets or sets the URL for corresponding documentation.
+        /// The API accepts a format string instead of an actual URL, creating a generic URL that includes the diagnostic ID.
+        /// </summary>
+        /// <value>The format string that represents a URL to corresponding documentation.</value>
+        /// <remarks>An example format string is <c>https://contoso.com/obsoletion-warnings/{0}</c>.</remarks>
+        public string? UrlFormat { get; set; }
+    }
+}

tests/PolySharp.Tests/LanguageFeatures.cs

@@ -101,6 +101,11 @@ public void TakeRegex([StringSyntax(StringSyntaxAttribute.Regex)] string pattern
     public static void InitializeModule()
     {
     }
+
+    [Experimental("PS0001")]
+    public void ExperimentalMethod()
+    {
+    }
 }
 
 internal class TestClassWithRequiredMembers