PyO3/pyo3

Define C-variadic FFI functions on `nightly` feature

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#5777 opened on Feb 4, 2026

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FFIGood First Issue

Description

It struck me that FFI definitions of variadic functions like PyArg_ParseTupleAndKeywords could be implemented with the nightly feature - this would probably be a good way to help the c_variadic feature gain feedback upstream.

Contributor guide

Tech stack
rustpythonc
Domain
api
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
RustFFI conceptsPython C APInightly Rust
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the current implementation of FFI definitions in PyO3, particularly for variadic functions. Examine the Rust nightly c variadic feature tracking issue (https://github.com/rust lang/rust/issues/44930) and existing usage in PyO3. Propose a concrete implementation by defining a macro or attribute that maps to the c variadic ABI on nightly, with a fallback for stable.