[ffi] `@Native` for global fixed-size arrays

[dcharkes]

https://dart-review.googlesource.com/c/sdk/+/338020 adds support for #50551, but does not add support for global fixed size arrays.

double foo[4];

double return_double() {
  return foo[1];
}

void set_double(double value) {
  foo[2] = value;
}

Could theoretically be bound to.

@Array(4)
@Native()
external Array<Double> foo;

It just so happens to be that currently the following works because the memory layout is identical.

@Native()
external Pointer<Double> foo;

The advantage of supporting the latter is that we can do bound checks on use.

@simolus3 Thanks for making the Array limitation explicit in https://dart-review.googlesource.com/c/sdk/+/338020. Here are some of my thoughts about it.

Re dart-lang/native#860, should we temporarily support generating Pointers for fixed-size global arrays? That would make it a breaking change later to make it Arrays, though. On the other hand, if users would like to use those fields than without generating Pointers, users can't use FFIgen at all.

[simolus3]

That would make it a breaking change later to make it Arrays, though.

Technically yes, but thanks to all the extensions in dart:ffi which provide operator[] and operator[]= on pointers too, migrating between pointers and arrays is unlikely to require users to change their code.

[simolus3]

Re dart-lang/native#860, should we temporarily support generating Pointers for fixed-size global arrays?

As it turns out, we're already generating pointers for global arrays in ffigen. That's understandable, since there is no public mechanism for getting from a pointer to an array instance, and so the generated code based on looking up bindings manually can't do any better.

This gives us a good opportunity to get this right with native fields though. I have opened https://dart-review.googlesource.com/c/sdk/+/341860 to support array fields in the SDK, and I've updated the ffigen PR to make use of that.