Private ctors are trimmed despite use in Marshal.GetDelegateForFunctionPointer

[scalablecory]

I have a private SafeHandle ctor that is being trimmed, despite me using it in a delegate type passed to Marshal.GetDelegateForFunctionPointer, which has an out MySafeHandle parameter.

My code is failing on this line:

https://github.com/scalablecory/runtime/blob/720f3d7c3c60506f70123149c1a67a4a71aa81fc/src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/Internal/MsQuicApi.cs#L60-L62

With the delegate looking like:

https://github.com/scalablecory/runtime/blob/720f3d7c3c60506f70123149c1a67a4a71aa81fc/src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/Interop/MsQuicNativeMethods.cs#L76-L79

And the safe handle's private ctor:

https://github.com/scalablecory/runtime/blob/720f3d7c3c60506f70123149c1a67a4a71aa81fc/src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/Interop/SafeMsQuicRegistrationHandle.cs#L12-L14

Current workaround is to use DynamicDependency.

[scalablecory]

CC @eerhardt

[marek-safar]

@vitek-karas @MichalStrehovsky Is this lack of correct annotations on libraries side or we cannot do it yet?

[MichalStrehovsky]

Linker needs to learn how to analyze things for interop correctness. The SafeHandle case is sort of covered by this code in the p/invoke case:

https://github.com/mono/linker/blob/4f7f6614be59d84cbf6f52d895986c2116cbfeb8/src/linker/Linker.Steps/MarkStep.cs#L2648

...but there's nothing in IL linker making this work in the reverse p/invoke case. Basically the code that is running here for p/invokes should also run for Delegate.Invoke methods used in interop.

This is kind of the same bucket as #1555. Linker needs to embed more knowledge of how interop works.