Dynamically loading Windows function with GetProcAddress

Cargo.toml

[package]
name = "procaddress"
version = "0.1.0"
authors = []

[dependencies]
libc = "0.2.33"

main.rs

extern crate libc;
use std::ptr;

#[link(name = "kernel32")]
#[link(name = "user32")]
extern "stdcall" {
    pub fn LoadLibraryA(lpFileName: *const u8) -> *const usize;
    pub fn GetProcAddress(hModule: *const usize, lpProcName: *const u8) -> *const usize;
    pub fn MessageBoxA(hWnd: *const usize, lpText: *const u8, lpCaption: *const u8, uType: u32)
        -> i32;
}

type FnMessageBox = extern "stdcall" fn(*const (), *const u8, *const u8, u32) -> i32;

fn main() {
    unsafe {
        MessageBoxA(
            ptr::null(),
            MESSAGE.as_ptr() as *const u8,
            TITLE.as_ptr() as *const u8,
            0,
        );

        let module = LoadLibraryA(USER32_DLL.as_ptr() as *const u8);
        println!("module {}", module as usize);
        let h_message_box = GetProcAddress(module, MESSAGE_BOX.as_ptr() as *const u8);
        println!("MessageBoxA {}", h_message_box as usize);
        let message_box = std::mem::transmute::<*const usize, FnMessageBox>(h_message_box);
        message_box(
            ptr::null(),
            MESSAGE.as_ptr() as *const u8,
            TITLE.as_ptr() as *const u8,
            0,
        );
    }
}

const USER32_DLL: &'static [u8] = b"user32\0";
const MESSAGE_BOX: &'static [u8] = b"MessageBoxA\0";
const TITLE: &'static [u8] = b"GetProcAddress\0";
const MESSAGE: &'static [u8] = b"Hello, World!\0";