Support higher rank function pointers

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4,472 stars (4,472 stars)

PR merge metrics

Avg merge 8m (Avg merge 8m)3 merged PRs in 30d (3 merged PRs in 30d)

Description

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        type Thing<'a>;
    }

    unsafe extern "C++" {
        fn f(callback: for<'a> fn(&mut Thing<'a>, &mut Thing<'a>));
    }
}

error: function pointer with lifetime parameters is not supported yet
 --> src/main.rs:8:24
  |
8 |         fn f(callback: for<'a> fn(&mut Thing<'a>, &mut Thing<'a>));
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Contributor guide

Research direction

Investigate the cxx bridge macro's parsing of function pointer types, particularly where it rejects lifetime parameters (likely in the `parse ty` or `parse fn ptr` functions in the cxxbridge macro crate). Understand how lifetime bounds are currently handled and extend the parser to accept higher ranked lifetime bounds (`for<'a> fn(...)`) on function pointer arguments. Also verify that code generation correctly uses the lifetimes when bridging the function pointer.

Tech stack

rust

Domain

apibackend

Issue type

Feature

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.

1-2 days

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

Rustlifetimes

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

40