Make ObsoleteAttributeData public

[ericstj]

Background and Motivation

We have tools that operate on API and use the roslyn symbol model to analyze it. We'd like those tools to treat experimental and obsolete API differently. To do so today, we'd need to recalculate this, with similar rules to what roslyn has today.

See dotnet/sdk#50637

Proposed API

namespace Microsoft.CodeAnalysis
{

-    internal enum ObsoleteAttributeKind
+    public enum ObsoleteAttributeKind
    {
        None,
        Uninitialized,
        Obsolete,
        Deprecated,
        WindowsExperimental,
        Experimental,
    }
-    internal sealed class ObsoleteAttributeData { ... }
+    public sealed class ObsoleteAttributeData { ... }

// plus additional plumbing to expose from internal symbol model to public symbol model

Usage Examples

if (someSymbol?.ObsoleteAttributeData?.Kind == ObsoleteAttributeKind.Experimental && skipComparingExperimentalAPI)
{
    return; 
}

Alternative Designs

New api on symbol, or symbol helper static/extension API that could just provide an answer to the question, would avoid the need for any new types and could be added entirely on the side.

  public static class ObsoleteHelperExtensions
  {
      public bool IsExperimental(this ISymbol symbol) {}
      public bool IsObsolete(this ISymbol symbol) {}
  }

Risks

Low risk - just exposing existing types / data.

[333fred]

It feels like such an API would be incomplete; I find it somewhat likely that the next thing you'll ask for is the error code, message, and other data from the attribute; and at that point, just finding the attribute isn't onerous.