Unusual return value from FnOnce closure

[tomjakubowski]

#![feature(unboxed_closures, unboxed_closure_sugar)]

use std::ops::FnOnce;

fn main() {
    let task: Box<|: int| -> int> = box |: x| x;
    let (vtable, data): (*mut (), *mut ()) = unsafe { std::mem::transmute_copy(&task) };
    println!("vtable {}", vtable);
    println!("data   {}", data);
    println!("return 0x{:x}", task.call_once((0i, )));
}

prints something like:

vtable 0x7f9fa1b750c0
data   0x7f9fa1b7bc40
return 0x7f9fa1b7bc40

task.call_once() with a string slice tends to cause a segfault.

[tomjakubowski]

Updated with a much shorter example.

[huonw]

It seems to be returning the data pointer of the Box trait object. (It's also a little weird that there is a data pointer, since the closure is theoretically zero sized, as it has no captures. It's doing the right thing: all zero sized allocations point to the same (non-null) address.)

[pcwalton]

I think we need an unboxing shim here