01-error.txt

C:/Users/demurgos/.cargo/bin/cargo.exe run --package hello-win --bin hello-win
    Updating git repository `https://github.com/retep998/winapi-rs`
   Compiling hello-win v0.1.0 (file:///E:/projects/various/hello-win)
error[E0432]: unresolved import `winapi::um::winuser`
  --> src\main.rs:10:19
   |
10 |   use winapi::um::winuser::{MB_OK, MessageBoxW};
   |                   ^^^^^^^ Could not find `winuser` in `um`

error: aborting due to previous error

error: Could not compile `hello-win`.

To learn more, run the command again with --verbose.

Process finished with exit code 101

02-Cargo.toml

[package]
name = "hello-win"
version = "0.1.0"
authors = ["demurgos"]

[target.'cfg(windows)'.dependencies]
winapi = { git = "https://github.com/retep998/winapi-rs" }

03-src_main.rs

#[cfg(windows)] extern crate winapi;
use std::io::Error;

#[cfg(windows)]
fn print_message(msg: &str) -> Result<i32, Error> {
  use std::ffi::OsStr;
  use std::iter::once;
  use std::os::windows::ffi::OsStrExt;
  use std::ptr::null_mut;
  use winapi::um::winuser::{MB_OK, MessageBoxW};
  let wide: Vec<u16> = OsStr::new(msg).encode_wide().chain(once(0)).collect();
  let ret = unsafe {
    MessageBoxW(null_mut(), wide.as_ptr(), wide.as_ptr(), MB_OK)
  };
  if ret == 0 { Err(Error::last_os_error()) }
    else { Ok(ret) }
}
#[cfg(not(windows))]
fn print_message(msg: &str) -> Result<(), Error> {
  println!("{}", msg);
  Ok(())
}
fn main() {
  print_message("Hello, world!").unwrap();
}