A lot of our container mapping depends on the `is_owned` flag
which we have for in-crate mapped objects to map references and
non-references into the same container type. Transaction was
mapped to two completely different types (a slice and a Vec type),
which led to a number of edge cases in the bindings generation.
Specifically, I spent a few days trying to map
`[(A, &Transaction)]` properly and came up empty - we map slices
into the same types as Vecs (and rely on the `is_owned` flag to
avoid double-free) and the lack of one for `Transaction` would have
required a special-case in numerous functions.
Instead, we just add a flag in `Transaction` to mirror what we do
for in-crate types and check it before free-ing any underlying
memory.
Note that, sadly, because the c_types objects aren't mapped as a
part of our C++ bindings generation, you have to manually call
`Transaction_free()` even in C++.
When mapping an `Option<T>` where T is a mapped trait, we need to
move out of the `*mut T`, however the current generation results in
a `*const T` and a conversion that just takes a reference to the
pointed-to object. This is because the only place this code was
previously used was for slices, which *do* need a reference.
Additionally, we need to know how to convert `Option` containers
which do not contain an opaque type.
Sadly, the easiest way to get the desired result is to add another
special case in container mapping, keeping the current behavior for
slices, but moving out of the pointed-to object for other types.
When we have a `Trait` wrapped as an `Option<Trait>`, we called
`<*const Trait>.is_null()` which resulted in rustc trying to take
the most braindead option of dereferencing the whole thing and
hitting a recursive dereference since we
`impl Deref<Target=Trait> for Trait` for all our traits.
Instead, we can be explicit and just compare the pointer directly
with `std::ptr::null()` which avoids this.
A few places got a None in the previous commit to avoid increasing
the diff size. However, it makes sense to have GenericTypes contexts
there, so we pipe them through the neccessary places.
This pushes down the logic to check if a given Path is, in fact,
a reference to a generic into the common maybe_resolve_path instead
of doing it redundantly in a few different places. Net loss of LoC.
In general, it maps:
* Traits to a struct with a void* and a list of function pointers,
emulating what the compiler will do for a dyn trait anyway,
* Structs as a struct with a single opaque pointer to the
underlying type and a flag to indicate ownership. While this is
a bit less effecient than just a direct pointer, it neatly lets
us expose in the public interface the concept of ownership by
setting a flag in the generated struct.
* Unit enums as enums with each type copied over and conversion
functions,
* Non-unit enums have each field converted back and forth with a
type flag and a union across all the C-mapped fields.
