LazyStaticType::get_or_init returns an *mut instead of a & ref

[scalexm]

Previously, we were returning a &ffi::PyTypeObject. However, we were often casting this immutable reference to a *mut ffi::PyTypeObject and then passing that pointer to FFI functions.

But we don't know whether the FFI functions mutate the PyTypeObject behind our back or not, and if they do, I believe that this is Undefined Behavior because the pointer comes from a shared reference.

Unfortunately, these things are very badly documented: I'm only aware of this because I followed many Unsafe Code Guideline discussions. Best source that I could get is the following comment from @RalfJung, who is very knowledgeable on the subject. The details are probably written in the Stacked Borrows paper, to be fair.

Even if it's not UB right now because of ill-specification or because of the compiler not taking advantage of it, we'd better be safe.

[davidhewitt]

This seems like a sensible idea to me!

[davidhewitt]

I wonder about a possible further refactoring: PyTypeInfo could be PyTypeInfo: PyTypeObject, and we could add an .as_raw() method to pyo3::types::PyType which returns *mut ffi::PyTypeObject.

[kngwyu]

I wonder about a possible further refactoring: PyTypeInfo could be PyTypeInfo: PyTypeObject, and we could add an .as_raw() method to pyo3::types::PyType which returns *mut ffi::PyTypeObject.

Since impl<T: PyTypeInfo> PyTypeObject for T uses type_object_raw, I don't think it's possible.

[kngwyu]

Thanks!

[RalfJung]

Unfortunately, these things are very badly documented: I'm only aware of this because I followed many Unsafe Code Guideline discussions. Best source that I could get is the following comment from @RalfJung, who is very knowledgeable on the subject. The details are probably written in the Stacked Borrows paper, to be fair.

Yeah, the details of our reference rules are pretty fuzzy, and Stacked Borrows is a promising next step but it is not an official model yet.

However, this page is clear at least about some things. It is UB to do the following:

Mutating immutable data. All data inside a const item is immutable. Moreover, all data reached through a shared reference or data owned by an immutable binding is immutable, unless that data is contained within an UnsafeCell.