How to generate headers for third party crate?

Here is a rust example to illustrate the problem

my current crate

use crate::canvas::Canvas;
// app_surface is third party crate
pub use app_surface::{AppSurface, IOSViewObj};

#[no_mangle]
pub extern "C" fn create_canvas(ios_obj: IOSViewObj) -> *mut libc::c_void {
    println!(
        "create_canvas, maximum frames: {}",
        ios_obj.maximum_frames
    );
    
    let obj = Canvas::new(AppSurface::new(ios_obj), 0_i32);
    
    let box_obj = Box::new(obj);
    // 将 box_obj 对象的内存管理权转交给调用方
    Box::into_raw(box_obj) as *mut libc::c_void
}

third party crate

#[repr(C)]
pub struct IOSViewObj {
    pub view: *mut Object,
    pub metal_layer: *mut c_void,
    pub maximum_frames: i32,
    pub callback_to_swift: extern "C" fn(arg: i32),
}

No header found for IOSViewObj, got warning:

WARN: Can't find IOSViewObj. This usually means that this type was incompatible or not found.

Start by reproducing the warning with the two Rust snippets and inspect how cbindgen resolves the pub use of IOSViewObj from app_surface. Check whether third-party #[repr(C)] types containing an extern "C" callback are supported, and identify the configuration or behavior needed for the header to include IOSViewObj.