diff --git a/README.md b/README.md
index 6f3dba4..2c3f034 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,8 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ConstantExpected]` (see [proposal](https://github.com/dotnet/runtime/issues/33771))
 - `[CompilerLoweringPreserve]` (see [proposal](https://github.com/dotnet/runtime/issues/103430))
 - `[ExtensionMarker]` (needed for [extensions](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#extension-members))
+- `[IsClosedType]` (needed for [closed hierarchies](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-15.0/closed-hierarchies))
+- `[RequiresUnsafe]` (needed for [unsafe evolution](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unsafe-evolution))
 
 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>14.0</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.
 
diff --git a/src/PolySharp.Package/README.md b/src/PolySharp.Package/README.md
index 576713a..9e7a46f 100644
--- a/src/PolySharp.Package/README.md
+++ b/src/PolySharp.Package/README.md
@@ -55,6 +55,8 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ConstantExpected]` (see [proposal](https://github.com/dotnet/runtime/issues/33771))
 - `[CompilerLoweringPreserve]` (see [proposal](https://github.com/dotnet/runtime/issues/103430))
 - `[ExtensionMarker]` (needed for [extensions](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#extension-members))
+- `[IsClosedType]` (needed for [closed hierarchies](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-15.0/closed-hierarchies))
+- `[RequiresUnsafe]` (needed for [unsafe evolution](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unsafe-evolution))
 
 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>14.0</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.
 
diff --git a/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Diagnostics.CodeAnalysis.RequiresUnsafeAttribute.cs b/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Diagnostics.CodeAnalysis.RequiresUnsafeAttribute.cs
new file mode 100644
index 0000000..f415f47
--- /dev/null
+++ b/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Diagnostics.CodeAnalysis.RequiresUnsafeAttribute.cs
@@ -0,0 +1,29 @@
+// <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 the specified member requires the caller to be in an unsafe context.
+    /// </summary>
+    [global::System.AttributeUsage(
+        global::System.AttributeTargets.Constructor |
+        global::System.AttributeTargets.Event |
+        global::System.AttributeTargets.Method |
+        global::System.AttributeTargets.Property,
+        Inherited = false,
+        AllowMultiple = false)]
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    [global::Microsoft.CodeAnalysis.Embedded]
+    internal sealed class RequiresUnsafeAttribute : global::System.Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RequiresUnsafeAttribute"/> class.
+        /// </summary>
+        public RequiresUnsafeAttribute() { }
+    }
+}
\ No newline at end of file
diff --git a/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Runtime.CompilerServices.IsClosedTypeAttribute.cs b/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Runtime.CompilerServices.IsClosedTypeAttribute.cs
new file mode 100644
index 0000000..954ab37
--- /dev/null
+++ b/src/PolySharp.SourceGenerators/EmbeddedResources/LanguageSupport/System.Runtime.CompilerServices.IsClosedTypeAttribute.cs
@@ -0,0 +1,35 @@
+// <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.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved for use by a compiler for tracking metadata.
+    /// This attribute should not be used by developers in source code.
+    /// </summary>
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    [global::System.AttributeUsage(AttributeTargets.Class, Inherited = false)]
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    [global::Microsoft.CodeAnalysis.Embedded]
+    public sealed class IsClosedTypeAttribute : global::System.Attribute
+    {
+        private global::System.Type[] _derivedTypes = global::System.Type.EmptyTypes;
+
+        /// <summary>Initializes the attribute.</summary>
+        public IsClosedTypeAttribute()
+        {
+        }
+
+        /// <summary>Gets or sets the derived types of the closed type.</summary>
+        /// <value>An array of the derived types of the closed type. A <see langword="null" /> value is normalized to an empty array.</value>
+        public global::System.Type[] DerivedTypes
+        {
+            get => _derivedTypes;
+            set => _derivedTypes = value ?? global::System.Type.EmptyTypes;
+        }
+    }
+}
\ No newline at end of file