Yeah, Rust closures that capture data are fat pointers { fn*, data* }, so you need an awkward dance to make them thin pointers for C.

    let mut state = 1;
    let mut fat_closure = || state += 1;
    let (fnptr, userdata) = make_trampoline(&mut &mut fat_closure);

    unsafe {
        fnptr(userdata);
    }

    assert_eq!(state, 2);

    use std::ffi::c_void;
    fn make_trampoline<C: FnMut()>(closure: &mut &mut C) -> (unsafe fn(*mut c_void), *mut c_void) {
        let fnptr = |userdata: *mut c_void| {
            let closure: *mut &mut C = userdata.cast();
            (unsafe { &mut *closure })()
        };
        (fnptr, closure as *mut _ as *mut c_void)
    }
    

> Rust closures that capture data are fat pointers { fn, data }

This isn’t fully accurate. In your example, `&mut C` actually has the same layout as usize. It’s not a fat pointer. `C` is a concrete type and essentially just an anonymous struct with FnMut implemented for it.

You’re probably thinking of `&mut dyn FnMut` which is a fat pointer that pairs a pointer to the data with a pointer to a VTable.

So in your specific example, the double indirection is unnecessary.

The following passes miri: https://play.rust-lang.org/?version=nightly&mode=debug&editi...

(did this on mobile, so please excuse any messiness).

I know about this technique but it uses too much unsafe for my taste. Not that it's bad or anything, just a personal preference.