extern crate libc;

use std::marker::PhantomData;
use libc::{malloc, free, c_void};

trait Trace {}

struct MyTrace<'b> {
    #[allow(dead_code)]
    buf: *mut c_void,
    _phantom: PhantomData<&'b c_void>,
}

impl<'b> MyTrace<'b> {
    fn new() -> Self {
        Self{buf: unsafe{malloc(128)}, _phantom: PhantomData}
    }
}

impl<'b> Trace for MyTrace<'b> {}

impl<'b> Drop for MyTrace<'b> {
    fn drop(&mut self) {
        unsafe{free(self.buf)};
    }
}

trait Tracer {
    fn start(&mut self);
    fn stop(&mut self) -> Box<Trace>;
}

struct MyTracer<'b> {
    trace: Option<MyTrace<'b>>,
}

impl<'b> MyTracer<'b> {
    fn new() -> Self {
        Self{trace: None}
    }
}

impl<'b> Tracer for MyTracer<'b> {
    fn start(&mut self) {
        self.trace = Some(MyTrace::new());
        // Pretend the buffer is mutated in C here...
    }

    fn stop(&mut self) -> Box<Trace> {
        Box::new(self.trace.take().unwrap())
    }
}

fn main() {
    let mut tracer = MyTracer::new();
    tracer.start();
    let _trace = tracer.stop();
    println!("Hello, world!");
}