How do I convert a Rust closure to a C-style callback?

Question

I'm trying to write a Rusty wrapper for a piece of C API. There is one C construct I struggle with:

typedef bool (*listener_t) (int, int);
bool do_it(int x1, int y1, int x2, int y2, listener_t listener)

The function does its job for a range of numbers unless the listener returns false. In that case it aborts computation. I want to have a Rust wrapper like this:

fn do_with_callback<F>(start: (i32, i32), end: (i32, i32), callback: F)
    where F: Fn(i32, i32) -> bool

rust-bindgen created this for me, slightly edited for clarity:

pub type listener_t = Option<extern "C" fn(x: c_int, y: c_int) -> c_bool>;

pub fn TCOD_line(xFrom: c_int, yFrom: c_int,
                 xTo: c_int, yTo: c_int,
                 listener: listener_t) -> c_bool;

How should I convert a closure or a trait reference to a C-style callback in my do_with functions:

pub fn do_with_callback<F>(start: (i32, i32), end: (i32, i32), callback: F) -> Self
    where F: Fn(i32, i32) -> bool
{
    let wrapper = ???;
    unsafe {
        ffi::do_it(start.0, start.1, end.0, end.1, Some(wrapper))
    };
}

Answer 1

The first snippet from Vladimir Matveev no longer works as written. The size of &mut FnMut(i32) -> bool and *mut c_void is different and such casts lead to a crash. Corrected example (playpen):

extern crate libc;

use std::mem::*;

use libc::c_void;

pub fn run<F>(mut callback: F) -> bool
    where F: FnMut(i32) -> bool
{
    let mut cb: &mut FnMut(i32) -> bool = &mut callback;
    println!("sizeof(cb/*-ptr): {}/{}",
             size_of::<*mut FnMut(i32) -> bool>(),
             size_of::<*mut c_void>());

    let ctx = &mut cb as *mut &mut FnMut(i32) -> bool as *mut c_void;
    println!("ctx: {:?}", ctx);
    //----------------------------------------------------------
    // Convert backward
    let cb2: *mut *mut FnMut(i32) -> bool = unsafe { transmute(ctx) };
    println!("cb2: {:?}", cb2);

    // this is more useful, but can't be printed, because not implement Debug
    let closure: &mut &mut FnMut(i32) -> bool = unsafe { transmute(ctx) };

    closure(0xDEAD)
}

fn main() {
    println!("answer: {}",
             run(|x| {
                 println!("What can change nature of a man?");
                 x > 42
             }));
}

Answer 2

In C, a function pointer does not have associated context, which is why usually a C callback function usually carry an extra void* argument pass the context...

typedef bool (*listener_t)(int, int, void* user_data);
bool do_it(void* user_data, int x1, int y1, int x2, int y2, listener_t listener)

... or have an API to let to store the user data...

void api_set_user_data(void* user_data);   // <-- caller set the context
void* api_get_user_data();   // <-- callback use this to retrieve context.

---

If the library you want to wrap does not provide any of the above, you will need to pass the context via other channels, e.g. via a global variable, though that context will be shared across the whole process:

lazy_static! {
    static ref REAL_CALLBACK: Mutex<Option<Box<FnMut(c_int, c_int) -> bool + Send>>> = Default::default();
}

extern "C" fn callback(x: c_int, y: c_int) -> bool {
    if let Some(ref mut real_callback) = *REAL_CALLBACK.lock().unwrap() {
        real_callback(x, y)
    } else {
        panic!("<handle error here>");
    }
}

fn main() {
    *REAL_CALLBACK.lock().unwrap() = Some(Box::new(move |x, y| {
        println!("...");
        true
    }));
    unsafe {
        do_it(callback);
    }
}

It is also possible to create a trampoline function to stick the context directly in the function, but it is extremely difficult and unsafe.

_{Answer manually migrated from https://stackoverflow.com/a/42597209/224671}