replace .toIfc() and .unsafeFromIfc(...) with singletons and overloads

[S11001001]

Design from https://github.com/digital-asset/daml/issues/13668#issuecomment-1106899636 . For this part:

• [x] define codegen.InterfaceCompanion
• [x] generate named INTERFACE final subclass of InterfaceCompanion as static inner class of each interface, package-private ctor, singleton instance also named INTERFACE
• [x] replace .toIfc() with .toInterface(pkg.Ifc.INTERFACE marker)
• [x] rename .unsafeFromIfc(...) to .unsafeFromInterface(...)

[stefanobaghino-da]

Since we are renaming and we can still break: prepending unsafe to a method that could throw is not a common pattern in Java. I propose to:

• create an UnimplementedInterfaceException which can be used to report information about the error
• turn unsafeFromIfc(...) into fromInterface(...) throws UnimplementedInterfaceException

[S11001001]

@stefanobaghino-da unsafeFromInterface does not exist in cases where a template doesn't implement an interface, so there is never cause to throw such an exception. This is "unsafe" because we are coercing from interface-contract-ID to template-contract-ID, which we cannot check locally.

[stefanobaghino-da]

@stefanobaghino-da unsafeFromInterface does not exist in cases where a template doesn't implement an interface, so there is never cause to throw such an exception. This is "unsafe" because we are coercing from interface-contract-ID to template-contract-ID, which we cannot check locally.

Does that throw an exception? Under what circumstances?

[S11001001]

Does that throw an exception? Under what circumstances?

No, it doesn't.

Foo.ContractId z = // ...
var bcid = Bar.ContractId.unsafeFromInterface(
    z.toInterface(some.SharedIface.INTERFACE));
bcid.exerciseSomeBarChoice(...); // submitting this exercise will fail

[stefanobaghino-da]

I see, thanks!