rust-cpython: add macro for sharing references...
rust-cpython: add macro for sharing references
Following an experiment done by Georges Racinet, we now have a working way of
sharing references between Python and Rust. This is needed in many points of
the codebase, for example every time we need to expose an iterator to a
Rust-backed Python class.
In a few words, references are (unsafely) marked as `'static` and coupled
with manual reference counting; we are doing manual borrow-checking.
This changes introduces two declarative macro to help reduce boilerplate.
While it is better than not using macros, they are not perfect. They need to:
- Integrate with the garbage collector for container types (not needed
as of yet), as stated in the docstring
- Allow for leaking multiple attributes at the same time
- Inject the `py_shared_state` data attribute in `py_class`-generated
structs
- Automatically namespace the functions and attributes they generate
For at least the last two points, we will need to write a procedural macro
instead of a declarative one.
While this reference-sharing mechanism is being ironed out I thought it best
not to implement it yet.
Lastly, and implementation detail renders our Rust-backed Python iterators too
strict to be proper drop-in replacements, as will be illustrated in a future
patch: if the data structure referenced by a non-depleted iterator is mutated,
an `AlreadyBorrowed` exception is raised, whereas Python would allow it, only
to raise a `RuntimeError` if `next` is called on said iterator. This will have
to be addressed at some point.
Differential Revision:
https://phab.mercurial-scm.org/D6631
