What about those callbacks?

[shepmaster]

How can a callback be passed to C when it might run on another thread?

How can user data be passed? Is there a magic way to pass userdata when there's no userdata parameter?

[OptimisticPeach]

Hmm, perhaps your code must be written with Arc<Mutex<T>>s beforehand and passed a pointer to Fn* dynamic object which would then be passed back to rust which could successfully call it? Or perhaps creating a special struct which inherits Fn* and could provide a ffi to the other language.
For the first idea:

#[no_mangle]
pub extern fn get_callback() -> *mut Fn()->() {
    || {println!("HI")}
}
#[no_mangle]
pub extern fn run_callback(cback: *mut Fn() -> ()) -> bool {
    (*cback)();
    true
}

and for the other side, I'm not familiar with c externs so I'll post a c# example:

using System;
using System.Runtime.InteropServices;

class FnFFI
{
    bool initialized;
    (UIntPtr, UIntPtr) callback;
    public FnFFI() {
         callback = GetCallBack();
         initialized = true;
    }
    public void Run() {
        if this.initialized {
            if !RunCallBack(this.callback) {
                throw new Exception("This shouldn't happen :/");
            }
        }
    }
    [DllImport("fn_ffi", EntryPoint="get_callback")]
    public static extern (UIntPtr, UIntPtr) GetCallBack();
    
    [DllImport("fn_ffi", EntryPoint="run_callback")]
    public static extern bool RunCallBack((UIntPtr, UIntPtr) callback);

    static public void Main()
    {
        FnFFI callback = new FnFFI();
        callback.Run();
    }
}