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

I recently had the same problem. Based on the answers here and other places, I wrote a simple function for heap allocation:

pub fn unsafe_allocate() -> Box {
    let mut grid_box: Box;
    unsafe {
        use std::alloc::{alloc, dealloc, Layout};
        let layout = Layout::new::();
        let ptr = alloc(layout) as *mut T;
        grid_box = Box::from_raw(ptr);
    }
    return grid_box;
}

This will create a region in memory automatically sized after T and unsafely convince Rust that that memory region is an actual T value. The memory may contain arbitrary data; you should not assume all values are 0.

Example use:

let mut triangles: Box<[Triangle; 100000]> = unsafe_allocate::<[Triangle; 100000]>();