Add polyfills for .NET 11 attributes.

[teo-tsirpanis]

Note

Depends on the release of .NET 11 RTM.

API breakdown (for new features)

Polyfills for the following attributes were added:

• IsClosedTypeAttribute
• RequiresUnsafeAttribute

Additional context (optional)

The list of attributes was populated by searching for the regex : .+Attribute in files matching release-notes/11.0/11.0.0/api-diff/Microsoft.NETCore.App in the branch of dotnet/core#10484, and excluding the attributes in OOB packages.

.NET 11 added the following properties to existing attributes that we polyfill:

namespace System.Diagnostics.CodeAnalysis
{
    public sealed class StringSyntaxAttribute : System.Attribute
    {
+       public const string CSharp = "C#";
+       public const string FSharp = "F#";
+       public const string VisualBasic = "Visual Basic";
    }
}

Following the precedent set by the .NET 10 attributes in #131, and because it would lead to an inconsistent experience, where the polyfills will be added when targeting say .NET Standard, but not a .NET version earlier than 11, these changes are not polyfilled at this moment.

The following attributes were not polyfilled:

• UnionAttribute — will be added in Add polyfills for C# 15 union types (UnionAttribute and IUnion) #140
• MemorySafetyRulesAttribute — the compiler will synthesize it if it's missing

Also, the following runtime-supported attributes were not polyfilled:

• ExtendedLayoutAttribute — would require polyfilling an enum, which has not been done so far

Note

I'm starting to believe that polyfilling runtime-supported attributes has not been a good idea in general.