"works on my machine" Rust -> C -> Rust `Box<dyn Any>` ffi

boxany.c

void free_box_any(void *dat, void *tbl);

void take_boxptr(void *boxptr, void *tblptr) {
    free_box_any(boxptr, tblptr);
}

build.rs

extern crate cc;

fn main() {
    cc::Build::new().file("boxany.c").compile("boxany");
    println!("cargo::rustc-link-lib=boxany");
}

Cargo.toml

[package]
name = "boxany"
version = "0.1.0"
edition = "2024"

[dependencies]

[build-dependencies]
cc = "1.0"

main.rs

use std::any::Any;

unsafe extern "C" {
    fn take_boxptr(boxptr: *mut (), tblptr: *mut ());
}

#[unsafe(no_mangle)]
pub extern "C" fn free_box_any(datptr: *mut (), tblptr: *mut ()) {
    let boxany: Box<dyn Any> = unsafe {
        std::mem::transmute((datptr, tblptr))
    };
    std::mem::drop(boxany);
}

pub unsafe fn send_box(boxany: Box<dyn Any>) {
    unsafe {
        let (boxptr, tblptr): (*mut (), *mut ()) =
            std::mem::transmute(boxany);
        take_boxptr(boxptr, tblptr);
    }
}

struct Foo(String);

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Foo::drop()");
    }
}

fn main() {
    let boxany: Box<dyn Any> = Box::new(Foo(String::from("test string")));
    unsafe { send_box(boxany) }
}