[vm/ffi] ☂️ Exploration of dart:ffi2 using new language features

[dcharkes]

Dart has gotten a fair amount of new language features since the dart:ffi API was designed. The most notable recent feature is extension types. We should explore whether it would be worth introducing a new FFI API that would leverage new language features.

Things to explore:

• Pointers as extension types on int.
• Structs and unions as extension types on Pointer and TypedData.
• Integer native types as views on int, and floating native types as views on double.
• Only writing a single signature for FFI calls by leveraging native types being views on Dart types.

There are a lot of technical complexities to consider as well:

• Pointer addresses on 32 bit are not 64 bit integers.
• Structs and Unions rely on using a TypedDataBase which is not visible in the user type hierarchy.
• Structs and Unions can have constructors allocating a typed data that uses its size in bytes.
• FFI calls can do typed data unwrapping, but expressing a type hierarchy between typed data and pointers with extension types seems not feasible (especially if pointers are already an extension type on int)

Related issues:

• 'Pointer<NativeFunction<T Function()>> ptr; T Function() func = ptr.asFunction(); ' When T must be 'Pointer' to pass compilation checking. #54467 (comment)
• [vm/ffi] Make FfiNatives more concise #50097
• [ffi] Use inline classes for pointer arithmetic? #52320
• [vm/ffi] Disallow user methods on Structs #40291
• Internal doc exploring using extension types and value types

[mkustermann]

Structs and Unions rely on using a TypedDataBase which is not visible in the user type hierarchy.

As discussed offline, most likely we'd make the views on top of typed data and require converting the pointers (via Pointer.asTypedList()). So we'd avoid the allocation of struct/union/array objects at the expense of allocating a typed data view object.

The FooStruct foo = Pointer<FooStruct>.ref would then internally create the typed list and return the view on it.