(Almost) How to desugar dynsized values in rust

dyn.rs

#![feature(ptr_metadata)]
// use libc; // 0.2.159
use std::alloc::Layout;
use std::ptr::copy_nonoverlapping;
use std::mem::forget;
use std::ptr::metadata;
use std::ptr::DynMetadata;

fn main() {

    // let ptr: fn() -> impl Iterator<Item=usize> = ff;
    // let mut iter = ptr();
    // let item = iter.next().unwrap();
    
    let item = call_dynsized() ;
    println!("{item}!");
    assert!(item == 42);
}

fn call_dynsized() -> usize {
    let (desc, pro) = ff_equiv_rw_1();
    // let sptr1 = libc::alloca(mtd.size_of());
    let mut sptr1 = [0usize;32];
    let ptr = (&raw mut sptr1) as *mut usize;
    pro(ptr.cast());
    let iter: *mut dyn Iterator<Item=usize> = core::ptr::from_raw_parts_mut(ptr, desc);
    let item = unsafe { (&mut *iter).next().unwrap() };
    unsafe { iter.drop_in_place() };
    return item
}

fn ff_equiv_rw_1() -> (DynMetadata<dyn Iterator<Item=usize>>, impl FnOnce(*mut u8)) {
    let result = ff();
    let desc = metadata(&result as &dyn Iterator<Item=usize>);
    (desc, move |ptr| unsafe {
        copy_nonoverlapping((&raw const result) as *mut u8, ptr, desc.size_of());
        forget(result);
    })
}

fn ff() -> impl Iterator<Item=usize> {
    core::iter::from_fn(||{
        return Some(42usize);
    })
}