Support higher rank function pointers

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

Tech stack

rust

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.

over 1 week

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Rust lifetime systemcxx code generationhigher ranked types

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

20

Research direction

Investigate the code generation logic in cxx for function pointers, specifically how lifetime parameters are handled. Look at the error message source location. Implement parsing and code generation for higher ranked lifetimes in function pointer types. Ensure compatibility with existing code.