dtolnay/cxx

#[cxx::bridge] doesn't apply #[cfg(not(feature))]

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#1299 opened on Jan 3, 2024

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help wanted

Description

It looks like #[cfg(not(feature))] is ignored by #[cxx::bridge], so the marked field would always exist:

mod ffi {
    #[repr(u32)]
    enum Variant {
        #[cfg(not(feature = "v2"))]
        A,
        B,
    }
}

See this demo repo for a reproducible example.

#[cfg(not(feature))] is useful when a newer version of a C++ library dropped something, while we would like to support both new and old versions with Cargo feature flags.

Contributor guide

Tech stack
rust
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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.
clear
Prerequisites
Rust macro knowledgeUnderstanding of #[cfg]
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue demonstrates that #[cfg(not(feature))] is ignored by #[cxx::bridge]. The likely fix is in the macro expansion code, possibly in the `bridge` module or the enum processing logic. Investigate how the macro handles enum variants and ensure that cfg attributes are properly evaluated. The demo repo at https://github.com/hombit/cxx not feature can be used for testing.