dtolnay/cxx

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

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#1,299 opened on Jan 3, 2024

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help wanted

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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

Research direction
Investigate how #[cxx::bridge] processes enum variants and ensure #[cfg(not(feature))] is respected.
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Rustcxx
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55