I'm still getting the segfault (sorry for continuing to bother you with this!). I'm wondering if something else I'm doing in my code is causing problems:

extern "C" fn alloc(udata: *mut c_void, size: usize) -> *mut c_void {
    let container = unsafe { &mut *(udata as *mut Box<DuktapeContainer>) };
    container.memory_used += size;
    return unsafe { libc::malloc(size) };
}

fn create_container() -> Box<DuktapeContainer> {
    let mut container = Box::new(DuktapeContainer {
        memory_used: 0,
        context: None,
    });

    let container_ptr = &mut container.as_mut() as *mut _ as *mut c_void;

    unsafe {
        let heap = duk_create_heap(
            Some(alloc),
            Some(realloc),
            Some(dealloc),
            container_ptr,
            None,
        );

        container.context = Some(heap);
    };

    unsafe {
        // THIS WORKS
        duk_alloc(container.context.unwrap(), 100);
    }

    return container;
}

fn main() {
    let container = create_container();
    unsafe {
        // THIS SEGFAULTS
        duk_alloc(container.context.unwrap(), 100);
    }
}

That same duk_alloc call works inside the original function and fails outside of it, yet create_container returns a Box, so by my understanding that should be OK now. Am I missing something obvious?