No dtype-aware replacement for deprecated `downcast::<PyArrayDyn<T>>()` in PyO3 0.28

[asjad2401]

Problem

PyO3 0.28 deprecated PyAnyMethods::downcast in favour of Bound::cast / Bound::cast_exact. However, these replacements perform Python type checks only — they do not check the NumPy element dtype. This breaks the standard pattern for runtime dtype dispatch in rust-numpy:

if let Ok(arr) = x.downcast::<PyArrayDyn<f64>>() {
    // f64 path
} else if let Ok(arr) = x.downcast::<PyArrayDyn<f32>>() {
    // f32 path
}

Using cast_exact instead causes the f64 branch to always succeed regardless of the actual dtype, because both PyArrayDyn and PyArrayDyn share the same Python type (numpy.ndarray). The dtype check is performed inside rust-numpy's downcast hook, not by PyO3 itself.

Current workaround
#![allow(deprecated)] // still using downcast — cast/cast_exact do not check element dtype

Question
What is the intended migration path for dtype-aware dispatch in rust-numpy 0.28? Is there a FromPyObject-based or extract-based pattern that performs the dtype check, or is this a known gap pending a fix?

Environment

• rust-numpy: 0.28.0
• pyo3: 0.28.3
• numpy (Python): 1.26.4
• Python: 3.12

---

[Icxolu]

The type check for both dtype and shape is performed within the PyTypeInfo impl for PyArray<T, D>. This impl is used by PyO3 to perform the type check. Both the deprecated downcast and the new Bound::cast have the same implementation inside of PyO3, the only difference is in the error type, the check itself is the same.

I've also verified locally that they behave the same. I noticed that cast_exact is indeed broken, but so was/is downcast_exact.
numpy currently does not overwrite PyTypeInfo::is_exact_type_of (which is used by the "exact" variants), so the default is used, which is just a pointer comparison of the type object, which is not sufficient here as it does not take into account shape or dtype.

So the migration path is what PyO3 suggests and the fix is for rust-numpy to provide the proper implementation for is_exact_type_of I think.