Why does the following not compile, illustrating a core rule that makes `ref struct` safe?
ref struct Span2<T> { /* holds a ref/Span<T> */ }
class Holder {
Span2<int> _field; // <-- error here
}
static async Task M() {
Span2<int> s = default;
await Task.Yield();
UseLater(s); // and this is also illegal
}