[vm/ffi] Deprecate `Pointer.cast()`?

[dcharkes]

Now that we no longer reify the type arguments of Pointer

• https://github.com/dart-lang/sdk/issues/49935

we should consider whether .cast() can be replaced by simply as checks.

Currently, the runtime still have a type arguments vector at runtime

• https://github.com/dart-lang/sdk/issues/50777

So if we use a normal cast, we could accidentally introduce runtime type checks if the compiler can't figure out that pointers are always instantiated with Never as type argument.

Conceptually speaking Pointer<T> should maybe be an extension type on PointerNever (now that we stopped reifying the type argument). But, we've introduced Pointers before extension types were added to Dart. Its probably not possible to now change Pointer to an extension type. 😄

Thanks @HosseinYousefi for the suggestion!

[HosseinYousefi]

Its probably not possible to now change Pointer to an extension type.

• Pointer is a final class so nothing can extend or implement it
• Checking is Pointer<T> already returns true for all pointers
• runtimeType.toString() would be "PointerNever" instead of "Pointer<Never>" but I don't think people should depend on that to begin with.

So maybe this change won't be breaking afterall!

[dcharkes]

If you want to get your hands dirty, feel free to give it a spin in the Dart SDK! 😄