Rust allocations

main.rs

// This is an example of making a c-style struct ending in a dynamically sized array in stable rust.
use std::alloc::{GlobalAlloc, Layout, System};

struct Blah {
    height: usize,
    arr: [u8; 0],
}

impl Blah {
    fn get_layout_at(height: usize) -> Layout {
        Layout::from_size_align(
                std::mem::size_of::<Blah>() + height,
                std::mem::align_of::<Blah>()).unwrap()
    }

    fn alloc(height: usize) -> *mut Blah {
        unsafe {
            let b = System.alloc(Blah::get_layout_at(height)) as *mut Blah;
            (*b).height = height;
            b
        }
    }

    unsafe fn free(p: *mut Blah) {
        System.dealloc(p as *mut u8, Blah::get_layout_at((*p).height));
    }

    fn get_mut_slice(&mut self) -> &mut [u8] {
        unsafe {
            std::slice::from_raw_parts_mut(self.arr.as_mut_ptr(), self.height)
        }
    }
}

// Safe boxed version
struct BlahBox(*mut Blah);

impl BlahBox {
    fn new(height: usize) -> BlahBox {
        BlahBox(Blah::alloc(height))
    }

    fn get_mut_slice(&mut self) -> &mut [u8] {
        unsafe { (*self.0).get_mut_slice() }
    }
}

impl Drop for BlahBox {
    fn drop(&mut self) {
        unsafe { Blah::free(self.0); }
    }
}

fn main() {
    let mut blah = BlahBox::new(100);

    let slice = blah.get_mut_slice();
    slice[2] = 7;
    println!("{}", slice[2]);
}