The minimal amount of code required to create a window on MacOS from Rust

main.rs

use objc::runtime::{Object, NO};
use objc::*;

// We must link with AppKit for all application and windowing related matters.
#[link(name = "AppKit", kind = "framework")]
extern "C" {}

// https://developer.apple.com/documentation/objectivec/nsuinteger
#[cfg(target_pointer_width = "32")]
pub type NSUInteger = u32;
#[cfg(target_pointer_width = "64")]
pub type NSUInteger = u64;

// https://developer.apple.com/documentation/coregraphics/cgfloat?language=objc
#[cfg(target_pointer_width = "32")]
pub type CGFloat = f32;
#[cfg(target_pointer_width = "64")]
pub type CGFloat = f64;

// https://developer.apple.com/documentation/coregraphics/cgpoint?language=objc
#[repr(C)]
pub struct CGPoint {
    pub x: CGFloat,
    pub y: CGFloat,
}

// https://developer.apple.com/documentation/coregraphics/cgsize?language=objc
#[repr(C)]
pub struct CGSize {
    pub width: CGFloat,
    pub height: CGFloat,
}

// https://developer.apple.com/documentation/coregraphics/cgrect?language=objc
#[repr(C)]
pub struct CGRect {
    pub origin: CGPoint,
    pub size: CGSize,
}

// https://developer.apple.com/documentation/foundation/nsrect?language=objc
pub type NSRect = CGRect;

// https://developer.apple.com/documentation/objectivec/nil
pub const nil: *mut Object = 0 as *mut Object;

// https://developer.apple.com/documentation/appkit/nsapplicationactivationpolicy/nsapplicationactivationpolicyregular?language=objc
pub const NSApplicationActivationPolicyRegular: NSUInteger = 0;

// https://developer.apple.com/documentation/appkit/nsbackingstoretype/nsbackingstorebuffered?language=objc
pub const NSBackingStoreBuffered: NSUInteger = 2;

// https://developer.apple.com/documentation/appkit/nswindowstylemask
pub const NSWindowStyleMaskTitled: NSUInteger = 1;
pub const NSWindowStyleMaskClosable: NSUInteger = 1 << 1;
pub const NSWindowStyleMaskMiniaturizable: NSUInteger = 1 << 2;
pub const NSWindowStyleMaskResizable: NSUInteger = 1 << 3;

unsafe fn setup_application() {
    // https://developer.apple.com/documentation/appkit/nsapplication
    // Retrieve the global 'sharedApplication'
    let ns_application: *mut Object = msg_send![class!(NSApplication), sharedApplication];

    // https://developer.apple.com/documentation/appkit/nsapplicationactivationpolicy/nsapplicationactivationpolicyregular?language=objc
    // "The application is an ordinary app that appears in the Dock and may have a user interface."
    // Apple claims this is the default, but without manually setting it the application does not appear.
    //
    // Objective-C works with a message passing system. The 'msg_send!' macro abstracts some of the details of the
    // message passing system and requires a peculiar syntax.
    // The first item in msg_send! is the item being sent the message.
    // The following items are the names of the parameters followed by the argument.
    // A quirk of the `objc` crate is that it needs to know the return type of the msg_send!
    // call. So even though nothing is returned that must be specified with `let () = `.
    let () = msg_send![
        ns_application,
        setActivationPolicy: NSApplicationActivationPolicyRegular
    ];

    // Allocate the window before we initialize it.
    let ns_window: *mut Object = msg_send![class!(NSWindow), alloc];

    // The size and location of the window we're about to create.
    let window_rectangle = NSRect {
        origin: CGPoint { x: 0., y: 0. },
        size: CGSize {
            width: 500.,
            height: 500.,
        },
    };

    // We'd like a window that is resizable, has a title, is closable, and can be minimized.
    let style = NSWindowStyleMaskResizable
        | NSWindowStyleMaskTitled
        | NSWindowStyleMaskClosable
        | NSWindowStyleMaskMiniaturizable;

    // https://developer.apple.com/documentation/appkit/nswindow/1419477-initwithcontentrect?language=objc
    // Initialize the window with our settings.
    let () = msg_send![
        ns_window,
        initWithContentRect: window_rectangle
        styleMask: style
        backing:NSBackingStoreBuffered // The Apple recommended setting. See above documentation link for more info.
        defer:NO // Create the window immediately.
    ];

    // https://developer.apple.com/documentation/appkit/nswindow/1419208-makekeyandorderfront?language=objc
    // This call actually displays the window. Without it the window won't appear
    let () = msg_send![ns_window, makeKeyAndOrderFront: nil];

    // Run the application forever
    let () = msg_send![ns_application, run];
}

fn main() {
    unsafe {
        setup_application();
    }
}