Edit Cargo.toml:#

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[package]
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name = "message_box"
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version = "0.1.0"
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edition = "2024"
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[profile.dev]
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panic = "abort"
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[profile.release]
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opt-level = "z"
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lto = true
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codegen-units = 1
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panic = "abort"
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strip = true

• opt-level = "z" optimizes for size rather than speed, the z flag aggressively eliminates anything that adds bytes.
• lto = true enables Link Time Optimization, allowing the linker to remove unused code across crates and produce a leaner final binary.
• codegen-units = 1 forces the compiler to treat the entire crate as a single unit, enabling better dead code elimination at the cost of longer compile times.
• panic = "abort" replaces the default panic handler with a simple abort, removing a significant chunk of runtime code. Applied to both dev and release profiles to keep behavior consistent.
• strip = true strips debug symbols from the final binary.

Create .cargo/config.toml:#

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[build]
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target = "x86_64-pc-windows-msvc"
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rustflags = [
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  "-C", "link-arg=/ENTRY:mainCRTStartup",
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  "-C", "link-arg=/SUBSYSTEM:WINDOWS",
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  "-C", "link-arg=/NODEFAULTLIB",
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  "-C", "link-arg=/MERGE:.rdata=.text",
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  "-C", "link-arg=/MERGE:.pdata=.text",
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]

• target = "x86_64-pc-windows-msvc" sets the default compilation target, so you don’t need to pass —target on every cargo build.
• /ENTRY:mainCRTStartup tells the linker which function serves as the binary’s entry point.
• /SUBSYSTEM:WINDOWS defines the PE subsystem. Using WINDOWS instead of CONSOLE tells Windows this is a GUI application.
• /NODEFAULTLIB instructs the linker to not automatically link any default libraries, only what you explicitly declare gets linked.
• /MERGE:.rdata=.text merges the .rdata section into .text, reducing the total number of PE sections and trimming binary size.
• /MERGE:.pdata=.text does the same for .pdata, which holds exception handling data for stack unwinding. Since we use panic = “abort”, this section is unused and can be safely merged away.

Edit src/main.rs#

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#![no_std]
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#![no_main]
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use core::panic::PanicInfo;
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use core::ptr;
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use core::ffi::c_void;
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#[link(name = "user32")]
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unsafe extern "system" {
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    fn MessageBoxA(
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        hwnd: *mut c_void,
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        text: *const u8,
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        title: *const u8,
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        flags: u32,
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    ) -> i32;
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}
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#[link(name = "kernel32")]
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unsafe extern "system" {
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    fn ExitProcess(uExitCode: u32) -> !;
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}
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#[panic_handler]
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fn panic(_: &PanicInfo) -> ! {
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    unsafe {
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        ExitProcess(1);
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    }
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}
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#[unsafe(no_mangle)]
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pub extern "system" fn mainCRTStartup() -> ! {
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    static TITLE: &[u8] = b"Title\0";
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    static BODY: &[u8] = b"Hello, world!\0";
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    unsafe {
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        MessageBoxA(
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            ptr::null_mut(),
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            BODY.as_ptr(),
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            TITLE.as_ptr(),
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            0x00000030,
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        );
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        ExitProcess(0);
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    }
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}

• #![no_std] disables the Rust standard library entirely, no heap allocator, no runtime, no OS abstractions.
• #![no_main] tells the compiler you are not using the standard main entry point, allowing you to define your own.
• #[link(name = "...")] instructs the linker to include a specific Windows library.
• #[panic_handler] is mandatory in no_std, without the standard library, you must define what happens on a panic yourself.
• #[unsafe(no_mangle)] prevents Rust from mangling the function name, ensuring the linker can find mainCRTStartup as the entry point we declared in config.toml.

Build and Run#

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cargo build --release
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.\target\x86_64-pc-windows-msvc\release\message_box.exe

If everything is set up correctly, a MessageBox should appear. And the final binary will weigh around 2KB!

Article source:#