Referencing type from ILLink.Descriptors doesn't preserve its static abstracts #128120

NOTE: when fixed, undo the workaround from #128020

The test now passes (it doesn't with Mono, but that's a Mono problem with marshalling DateTime in general).

However, I'm not convinced this is the right fix, I'll need to study what exactly is the reflection-like mechanism that #126911 introduced here.

So the problem reproes with e.g. this (even in .NET 10):

typeof(IX).GetMethod(nameof(IX.Call)).MakeGenericMethod(A.GetA()).Invoke(null, []);

interface IX
{
    static abstract void Method();

    public static void Call<T>() where T : IX => T.Method();
}

class A : IX
{
    public static Type GetA() => MethodBase.GetCurrentMethod().DeclaringType;

    public static void Method() => Console.WriteLine("Hello");
}

Trimming will strip the static abstracts implementation from A because it will assume nobody can get at A. However I get at it with trim unsafe code. Since this is only broken with trim unsafe code, so far so good.

However, the specific case in interop does mention the equivalent of A in ILLink.Descriptors.xml (it instructs to keep some unrelated methods on the DateMarshaler type). So if I were to add A.GetA to ILLink.Descriptors.xml, can I have a reasonable assumption that the MethodBase.GetCurrentMethod() call is now safe to suppress (it is reflection visible after all)? If so, the bug here is in ILLink, because the lines:

    <type fullname="System.StubHelpers.DateMarshaler">
      <method name="ConvertToNative" />
      <method name="ConvertToManaged" />
    </type>

didn't make it so that a DateMarshaler Type can be passed to MakeGenericMethod with something that is constrained with static abstracts that are called in the program.

Originally posted by @MichalStrehovsky in #128020 (comment)