chore: add a warning if failed to get allocator capsule

pyo3-polars/src/alloc.rs

@@ -1,6 +1,5 @@
 use std::alloc::{GlobalAlloc, Layout, System};
 use std::ffi::c_char;
-
 use once_cell::race::OnceRef;
 use pyo3::ffi::{PyCapsule_Import, Py_IsInitialized};
 use pyo3::Python;
@@ -67,16 +66,22 @@ impl PolarsAllocator {
         // Do not allocate in this function,
         // otherwise it will cause infinite recursion.
         self.0.get_or_init(|| {
-            (unsafe { Py_IsInitialized() } != 0)
+            let r = (unsafe { Py_IsInitialized() } != 0)
                 .then(|| {
                     Python::with_gil(|_| unsafe {
                         (PyCapsule_Import(ALLOCATOR_CAPSULE_NAME.as_ptr() as *const c_char, 0)
                             as *const AllocatorCapsule)
                             .as_ref()
                     })
                 })
-                .flatten()
-                .unwrap_or(&FALLBACK_ALLOCATOR_CAPSULE)
+                .flatten();
+            #[cfg(debug_assertions)]
+            if r.is_none() {
+                // Do not use eprintln; it may alloc
+                use std::io::Write;
+                std::io::stderr().write_all(b"failed to get allocator capsule\n").unwrap();
+            }
+            r.unwrap_or(&FALLBACK_ALLOCATOR_CAPSULE)
         })
     }