PyO3/pyo3 · error

size_t should fit the flag bits

Error message

size_t should fit the flag bits

What it means

The CPython <3.12 variant of PY_VECTORCALL_ARGUMENTS_OFFSET is computed as 1 << (size_t bits - 1) and guarded by an expect panicking 'size_t should fit the flag bits'. It's a defensive check that the shift is valid for the platform's size_t; on standard platforms it never fires.

Source

Thrown at pyo3-ffi/src/cpython/abstract_.rs:48

        callable: *mut PyObject,
        result: *mut PyObject,
        where_: *const c_char,
    ) -> *mut PyObject;

    #[cfg(not(PyPy))]
    fn _PyObject_MakeTpCall(
        tstate: *mut PyThreadState,
        callable: *mut PyObject,
        args: *const *mut PyObject,
        nargs: Py_ssize_t,
        keywords: *mut PyObject,
    ) -> *mut PyObject;
}

#[cfg(not(Py_3_12))]
const PY_VECTORCALL_ARGUMENTS_OFFSET: size_t = (1 as size_t)
    .checked_shl((8 * core::mem::size_of::<size_t>() - 1) as u32)
    .expect("size_t should fit the flag bits");

#[cfg(Py_3_12)] // public API from 3.12
use crate::PY_VECTORCALL_ARGUMENTS_OFFSET;

#[inline(always)]
pub unsafe fn PyVectorcall_NARGS(n: size_t) -> Py_ssize_t {
    let n = n & !PY_VECTORCALL_ARGUMENTS_OFFSET;
    n.try_into().expect("cannot fail due to mask")
}

#[cfg(any(PyPy, Py_3_11))]
extern_libpython! {
    #[cfg_attr(PyPy, link_name = "PyPyVectorcall_Function")]
    pub fn PyVectorcall_Function(callable: *mut PyObject) -> Option<vectorcallfunc>;
}

#[cfg(not(any(PyPy, Py_3_11)))]
#[inline(always)]

View on GitHub (pinned to ac9b6899d3)

Solutions

1. Use a supported CPython/target combination
2. Fix the target specification/toolchain so size_t has a standard width
3. Report to pyo3 if a real-world target triggers this

// ensure size_t can represent the shifted flag
assert!(8 * std::mem::size_of::<usize>() >= 16);

When it happens

Trigger: Building for a target where (8 * size_of::<size_t>()) yields an invalid shift amount for size_t — only possible on exotic/nonstandard ABIs.

Common situations: Cross-compilation to unusual embedded targets; toolchain misconfiguration giving a bogus size_t size.

Related errors

• size_t should fit the flag bits
• cannot fail due to mask
• string contains nul bytes
• Neither abi3 or abi3t features are enabled
• Cannot target an abi3t version below {MINIMUM_SUPPORTED_VERS

AI-assisted analysis of PyO3/pyo3@ac9b6899d3 (2026-09-05). Data as JSON: /api/errors/93628c5748cbe499. Report an issue: GitHub.