From the ABI Stability manifesto:
The runtime is also responsible for lazily creating new type metadata entries at run time, either for generic type instantiations or for resilient constructs. Library evolution in general introduces a whole new category of needs from the runtime by making data and metadata more opaque, requiring interaction to be done through runtime APIs. Additionally, ownership semantics may require new runtime APIs or modifications to existing APIs. These new runtime needs are still under investigation.
This issue tracks the completion of those investigations.