You can display a MessageBox in Rust by following these steps.

Install Rust. See Getting Started with Rust.
Run cargo new --bin MessageBox in the command prompt.
Navigate to the MessageBox directory.
Open Cargo.toml and modify it as follows:

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[package]
name = "hello_world"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
winapi = "0.2.7"
user32-sys = "0.2.0"

Open src\main.rs and modify it as follows:

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extern crate user32;
extern crate winapi;

use std::ffi::CString;
use user32::MessageBoxA;
use winapi::winuser::{MB_OK, MB_ICONINFORMATION};

fn main() {
    let lp_text = CString::new("Hello, world!").unwrap();
    let lp_caption = CString::new("MessageBox Example").unwrap();

    unsafe {
        MessageBoxA(
            std::ptr::null_mut(),
            lp_text.as_ptr(),
            lp_caption.as_ptr(),
            MB_OK | MB_ICONINFORMATION
        );
    }
}

Run cargo run in the command prompt.
If you want to build a release version, run cargo build --release.

References

Hello World MesssageBox example in Rust