How to allocate structs on the heap without taking up space on the stack in stable Rust?

Question

In this code...

struct Test { a: i32, b: i64 }
    
fn foo() -> Box {              // Stack frame:
            


        

Answer 1

As of Rust 1.39, there seems to be only one way in stable to allocate memory on the heap directly - by using std::alloc::alloc (note that the docs state that it is expected to be deprecated). It's reasonably unsafe.

Example:

#[derive(Debug)]
struct Test {
    a: i64,
    b: &'static str,
}

fn main() {
    use std::alloc::{alloc, dealloc, Layout};

    unsafe {
        let layout = Layout::new::();
        let ptr = alloc(layout) as *mut Test;

        (*ptr).a = 42;
        (*ptr).b = "testing";

        let bx = Box::from_raw(ptr);

        println!("{:?}", bx);
    }
}

This approach is used in the unstable method Box::new_uninit.

It turns out there's even a crate for avoiding memcpy calls (among other things): copyless. This crate also uses an approach based on this.