Class constructor trips `misaligned pointer dereference`

Bug Description

Using 0.27.2, the constructor of a #[pyclass] immediately trips a rustc-injected check at pyo3-0.27.2/src/pyclass_init.rs:185:17: when compiled in debug mode. The problem can be reproduced locally (3.14) and on CI (3.9, 3.13, 3.14).

misaligned pointer dereference: address must be a multiple of 0x40 but is 0x105219430

I'm somewhat lost what to do here, since the constructor-call that is causing the problem is entirely pyo3-derived. I've pushed a branch that demonstrates the problem.

Steps to Reproduce

1. Checkout https://github.com/lukaslueg/pyhyperminhash/tree/6f8774e373b86928433154f701f0acfac4d3f5b4
2. Build using maturin in development mode
3. Observe pytest -v or import pyhyperminhash; pyhyperminhash.Entry()

Backtrace

> pytest -v

tests/test_pyhyperminhash.py::test_from_iter PASSED                                                                                                                                                                    [ 73%]
tests/test_pyhyperminhash.py::test_entry_unhashable Fatal Python error: Aborted

Current thread 0x000000020683e080 (most recent call first):
  File "/Users/llueg/dev/pyhyperminhash/tests/test_pyhyperminhash.py", line 148 in test_entry_unhashable
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/python.py", line 166 in pytest_pyfunc_call
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/python.py", line 1720 in runtest
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 179 in pytest_runtest_call
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 245 in <lambda>
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 353 in from_call
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 244 in call_and_report
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 137 in runtestprotocol
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/runner.py", line 118 in pytest_runtest_protocol
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/main.py", line 396 in pytest_runtestloop
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/main.py", line 372 in _main
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/main.py", line 318 in wrap_session
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/main.py", line 365 in pytest_cmdline_main
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/config/__init__.py", line 199 in main
  File "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/_pytest/config/__init__.py", line 223 in console_main
  File "/Users/llueg/dev/pyhyperminhash/.venv/bin/pytest", line 7 in <module>

Current thread's C stack trace (most recent call first):
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _Py_DumpStack+0x44 [0x10103c1fc]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at faulthandler_dump_c_stack+0x58 [0x10104ec9c]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at faulthandler_fatal_error+0x160 [0x10104eb60]
  Binary file "/usr/lib/system/libsystem_platform.dylib", at _sigtramp+0x38 [0x199e7b744]
  Binary file "/usr/lib/system/libsystem_pthread.dylib", at pthread_kill+0x128 [0x199e71888]
  Binary file "/usr/lib/system/libsystem_c.dylib", at abort+0x7c [0x199d76850]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std3sys3pal4unix4sync5mutex5Mutex4lock4fail17hfd38f3b0af37e16dE+0x0 [0x106e46e18]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std9panicking11panic_count17is_zero_slow_path17h0e49bc980a448302E+0x0 [0x106e476ec]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std9panicking15panic_with_hook17h93c775fc227522ddE+0x310 [0x106e20180]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std9panicking13panic_handler28_$u7b$$u7b$closure$u7d$$u7d$17h8561e58c9391724aE+0x90 [0x106e1fd78]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std3sys9backtrace4lock17hbd44f6947d8c1202E+0x0 [0x106e1e74c]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _RNvCsiGVaDesi5rv_7___rustc18___rdl_alloc_zeroed+0x0 [0x106e15900]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4core9panicking18panic_nounwind_fmt17hbccf1478dcdd0d51E+0x34 [0x106e4b4f4]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4core9panicking5panic17h4a3c64c65dc98033E+0x0 [0x106e4b6fc]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo312pyclass_init27PyClassInitializer$LT$T$GT$27create_class_object_of_type17h5926d8e37bfd3ec2E+0x198 [0x106dcd724]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo35impl_9pymethods11tp_new_impl17h98257976ae80db68E+0x24 [0x106dbbd10]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN14pyhyperminhash5Entry20__pymethod___new____17hcea4db44adc7e8cdE+0x178 [0x106dbe4cc]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo35impl_10trampoline7newfunc28_$u7b$$u7b$closure$u7d$$u7d$17h3889011bc89d48b8E+0x34 [0x106dc59b8]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo35impl_10trampoline10trampoline28_$u7b$$u7b$closure$u7d$$u7d$17h4526ddc3c5e82b96E+0x28 [0x106dc52c4]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std9panicking12catch_unwind7do_call17hd7f47029c2ca2a12E+0x44 [0x106dc95c0]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at __rust_try+0x20 [0x106dca42c]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN3std5panic12catch_unwind17he952f5781ccd8351E+0x54 [0x106dca098]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo35impl_10trampoline10trampoline17hee2fb9e98ad73a59E+0xf0 [0x106dc4f44]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN4pyo35impl_10trampoline7newfunc17h535810fe2a1cc4c5E+0x44 [0x106db98d8]
  Binary file "/Users/llueg/dev/pyhyperminhash/.venv/lib/python3.14/site-packages/pyhyperminhash/pyhyperminhash.abi3.so", at _ZN14pyhyperminhash162_$LT$impl$u20$pyo3..impl_..pyclass..PyMethods$LT$pyhyperminhash..Entry$GT$$u20$for$u20$pyo3..impl_..pyclass..PyClassImplCollector$LT$pyhyperminhash..Entry$GT$$GT$10py_methods5ITEMS10trampoline17h1db992949ce13519E+0x30 [0x106dbd4e8]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at type_call+0x64 [0x100f0eba0]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _PyObject_MakeTpCall+0x78 [0x100e75400]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _PyEval_EvalFrameDefault+0x5b58 [0x100fac700]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _PyEval_Vector+0x10c [0x100fa68a4]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _PyObject_VectorcallDictTstate+0xc8 [0x100e752a4]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at call_method+0xcc [0x100f1848c]
  Binary file "/opt/homebrew/Cellar/python@3.14/3.14.2/Frameworks/Python.framework/Versions/3.14/Python", at _PyObject_MakeTpCall+0x78 [0x100e75400]
  <truncated rest of calls>
[1]    3627 abort      RUST_BACKTRACE=full pytest -v

> python3 -c "import pyhyperminhash; pyhyperminhash.Entry()"
thread '<unnamed>' (8810098) panicked at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/pyclass_init.rs:185:17:
misaligned pointer dereference: address must be a multiple of 0x40 but is 0x105219430
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_nounwind_fmt::runtime
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/panicking.rs:122:22
   2: core::panicking::panic_nounwind_fmt
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/intrinsics/mod.rs:2435:9
   3: core::fmt::rt::<impl core::fmt::Arguments>::new_v1_formatted
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/fmt/rt.rs:236:9
   4: core::panicking::panic_misaligned_pointer_dereference
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/panicking.rs:290:9
   5: pyo3::pyclass_init::PyClassInitializer<T>::create_class_object_of_type
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/pyclass_init.rs:185:17
   6: pyo3::impl_::pymethods::tp_new_impl
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/impl_/pymethods.rs:716:14
   7: pyhyperminhash::Entry::__pymethod___new____
             at ./src/lib.rs:46:1
   8: pyo3::impl_::trampoline::newfunc::{{closure}}
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/impl_/trampoline.rs:44:37
   9: pyo3::impl_::trampoline::trampoline::{{closure}}
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/impl_/trampoline.rs:190:54
  10: std::panicking::catch_unwind::do_call
             at /Users/llueg/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:590:40
  11: ___rust_try
  12: std::panicking::catch_unwind
             at /Users/llueg/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:553:19
  13: std::panic::catch_unwind
             at /Users/llueg/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/panic.rs:359:14
  14: pyo3::impl_::trampoline::trampoline
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/impl_/trampoline.rs:190:9
  15: pyo3::impl_::trampoline::newfunc
             at /Users/llueg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.27.2/src/impl_/trampoline.rs:44:21
  16: pyhyperminhash::<impl pyo3::impl_::pyclass::PyMethods<pyhyperminhash::Entry> for pyo3::impl_::pyclass::PyClassImplCollector<pyhyperminhash::Entry>>::py_methods::ITEMS::trampoline
             at ./src/lib.rs:46:1
  17: _type_call
  18: __PyObject_MakeTpCall
  19: __PyEval_EvalFrameDefault
  20: _PyEval_EvalCode
  21: _run_mod
  22: __PyRun_StringFlagsWithName
  23: __PyRun_SimpleStringFlagsWithName
  24: _Py_RunMain
  25: _pymain_main
  26: _Py_BytesMain
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread caused non-unwinding panic. aborting.
[1]    3419 abort      RUST_BACKTRACE=1 python3 -c "import pyhyperminhash; pyhyperminhash.Entry()"

Your operating system and version

macOS Tahoe

Your Python version (python --version)

3.14.2

Your Rust version (rustc --version)

1.92

Your PyO3 version

0.27.2

How did you install python? Did you use a virtualenv?

brew, gh-actions

Additional Info

No response

Reproduce the failure from the linked pyhyperminhash branch with maturin in development mode, then run tests/test_pyhyperminhash.py or import pyhyperminhash and construct Entry(). Read pyo3-0.27.2/src/pyclass_init.rs around line 185 and compare the constructor backtrace with the failing debug build. Done means the Entry constructor no longer aborts with a misaligned pointer dereference across the reported Python versions.