gpui_macos: Migrate PlatformWindow::prompt to objc2 on MacOS (#59572)

# Objective

Rust 1.96 emits a warning when compiling the `block` crate, that it will
no longer compile past some future version of Rust. The `block` crate
provides `ConcreteBlock` and `RcBlock`, which are stack- and heap-based
functions often passed as callbacks to MacOS. The old `objc` crate
family is deprecated and no longer maintained.

The goal of the PR is to replace one call to `block::ConcreteBlock` by
migrating a standalone portion of code to the objc2 crate family. A full
migration to `objc2` by Zed has been long on the horizon (#22408)

Additional benefits: The new code is completely typed, uses automatic
memory management, and reduces lines of unsafe Rust from 50+ to 1. The
lingering unsafe converts a raw pointer stored by `self` to the
respective `objc2` type, since the rest of the window implementation
does not use the new crates yet.

## Solution

Migrate `PlatformWindow::prompt`'s MacOS implementation to `objc2`.

## Testing

Direct platform code is quite fragile and hard to test, however you can
exercise the codepath by running the following `gpui` example and
clicking on the prompt buttons.

```sh
cargo run -p gpui --example window
```

I've also built Zed with these changes and tried codepaths which
activate prompts, such as deleting a file.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- N/A or Added/Fixed/Improved ...

Cargo.lock

@@ -11791,6 +11791,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d49e936b501e5c5bf01fda3a9452ff86dc3ea98ad5f283e1455153142d97518c"
 dependencies = [
  "bitflags 2.13.1",
+ "block2 0.6.2",
  "objc2 0.6.3",
  "objc2-core-foundation",
  "objc2-foundation 0.3.2",

Cargo.toml

@@ -693,12 +693,15 @@ num-format = "0.4.4"
 objc = "0.2"
 objc2 = "0.6"
 objc2-app-kit = { version = "0.3.2", default-features = false, features = [
+    "NSAlert",
+    "NSApplication",
     "NSButton",
     "NSControl",
     "NSGraphics",
     "NSResponder",
     "NSView",
     "NSWindow",
+    "block2",
     "objc2-core-foundation",
 ] }
 objc2-foundation = { version = "=0.3.2", default-features = false, features = [

crates/gpui_macos/src/platform.rs

@@ -44,6 +44,7 @@ use objc::{
     runtime::{Class, Object, Sel},
     sel, sel_impl,
 };
+use objc2::MainThreadMarker;
 use parking_lot::Mutex;
 use ptr::null_mut;
 use semver::Version;
@@ -163,7 +164,7 @@ unsafe fn build_classes() {
     }
 }
 
-pub struct MacPlatform(Mutex<MacPlatformState>);
+pub struct MacPlatform(Mutex<MacPlatformState>, MainThreadMarker);
 
 pub(crate) struct MacPlatformState {
     background_executor: BackgroundExecutor,
@@ -195,6 +196,7 @@ pub(crate) struct MacPlatformState {
 
 impl MacPlatform {
     pub fn new(headless: bool) -> Self {
+        let marker = MainThreadMarker::new().expect("Mac platform not created on main thread");
         let dispatcher = Arc::new(MacDispatcher::new());
 
         #[cfg(feature = "font-kit")]
@@ -213,7 +215,7 @@ impl MacPlatform {
         let keyboard_layout = MacKeyboardLayout::new();
         let keyboard_mapper = Rc::new(MacKeyboardMapper::new(keyboard_layout.id()));
 
-        Self(Mutex::new(MacPlatformState {
+        let state = Mutex::new(MacPlatformState {
             headless,
             text_system,
             background_executor: BackgroundExecutor::new(dispatcher.clone()),
@@ -238,7 +240,8 @@ impl MacPlatform {
             keyboard_mapper,
             cursor_visible: Arc::new(AtomicBool::new(true)),
             system_notifications: crate::system_notifications::SystemNotificationState::new(),
-        }))
+        });
+        Self(state, marker)
     }
 
     unsafe fn create_menu_bar(
@@ -674,6 +677,7 @@ impl Platform for MacPlatform {
             foreground_executor,
             background_executor,
             renderer_context,
+            self.1,
         )))
     }
 

crates/gpui_macos/src/window.rs

@@ -7,6 +7,7 @@ use crate::{
 #[cfg(any(test, feature = "test-support"))]
 use anyhow::Result;
 use block::ConcreteBlock;
+use block2::RcBlock;
 use cocoa::{
     appkit::{
         NSApplication, NSBackingStoreBuffered, NSColor, NSEvent, NSEventModifierFlags, NSEventType,
@@ -49,10 +50,10 @@ use objc::{
     runtime::{BOOL, Class, NO, Object, Protocol, Sel, YES},
     sel, sel_impl,
 };
-use objc2::{rc::Retained, runtime::AnyObject as Objc2Object};
+use objc2::{MainThreadMarker, rc::Retained, runtime::AnyObject as Objc2Object};
 use objc2_app_kit::{
-    NSBeep, NSButton as Objc2NSButton, NSView as Objc2NSView, NSWindow as Objc2NSWindow,
-    NSWindowButton as Objc2NSWindowButton,
+    NSAlert, NSAlertStyle, NSBeep, NSButton as Objc2NSButton, NSView as Objc2NSView,
+    NSWindow as Objc2NSWindow, NSWindowButton as Objc2NSWindowButton,
 };
 use objc2_foundation::{NSPoint as Objc2NSPoint, NSRect as Objc2NSRect};
 use parking_lot::Mutex;
@@ -878,7 +879,7 @@ impl MacWindowState {
 
 unsafe impl Send for MacWindowState {}
 
-pub(crate) struct MacWindow(Arc<Mutex<MacWindowState>>);
+pub(crate) struct MacWindow(Arc<Mutex<MacWindowState>>, MainThreadMarker);
 
 impl MacWindow {
     pub fn open(
@@ -902,6 +903,7 @@ impl MacWindow {
         foreground_executor: ForegroundExecutor,
         background_executor: BackgroundExecutor,
         renderer_context: renderer::Context,
+        marker: MainThreadMarker,
     ) -> Self {
         unsafe {
             let pool = NSAutoreleasePool::new(nil);
@@ -1011,7 +1013,7 @@ impl MacWindow {
             let native_view = NSView::initWithFrame_(native_view, NSView::bounds(content_view));
             assert!(!native_view.is_null());
 
-            let mut window = Self(Arc::new(Mutex::new(MacWindowState {
+            let state = Arc::new(Mutex::new(MacWindowState {
                 handle,
                 foreground_executor,
                 background_executor,
@@ -1065,7 +1067,8 @@ impl MacWindow {
                 closed: Arc::new(AtomicBool::new(false)),
                 accesskit_adapter: None,
                 sheet_parent: None,
-            })));
+            }));
+            let mut window = Self(state, marker);
 
             (*native_window).set_ivar(
                 WINDOW_STATE_IVAR,
@@ -1511,6 +1514,8 @@ impl PlatformWindow for MacWindow {
         detail: Option<&str>,
         answers: &[PromptButton],
     ) -> Option<oneshot::Receiver<usize>> {
+        use objc2_foundation::{NSInteger, NSString};
+
         // NSAlert's first button keeps Return and Cancel keeps Escape, but the keyboard
         // focus (and therefore Space) defaults to Cancel, leaving the middle button of
         // prompts like "Save / Don't Save / Cancel" unreachable from the keyboard. Move
@@ -1523,69 +1528,67 @@ impl PlatformWindow for MacWindow {
             .map(|(ix, _)| ix)
             .filter(|&ix| ix > 0);
 
-        unsafe {
-            let alert: id = msg_send![class!(NSAlert), alloc];
-            let alert: id = msg_send![alert, init];
-            let alert_style = match level {
-                PromptLevel::Info => 1,
-                PromptLevel::Warning => 0,
-                PromptLevel::Critical => 2,
-            };
-            let _: () = msg_send![alert, setAlertStyle: alert_style];
-            let _: () = msg_send![alert, setMessageText: ns_string(msg)];
-            if let Some(detail) = detail {
-                let _: () = msg_send![alert, setInformativeText: ns_string(detail)];
-            }
+        let alert = NSAlert::new(self.1);
+        alert.setAlertStyle(match level {
+            PromptLevel::Critical => NSAlertStyle::Critical,
+            PromptLevel::Warning => NSAlertStyle::Warning,
+            PromptLevel::Info => NSAlertStyle::Informational,
+        });
+        let message = NSString::from_str(msg);
+        alert.setMessageText(message.as_ref());
 
-            let mut initial_focus_button: Option<id> = None;
-            for (ix, answer) in answers.iter().enumerate() {
-                let button: id = msg_send![alert, addButtonWithTitle: ns_string(answer.label())];
-                let _: () = msg_send![button, setTag: ix as NSInteger];
-
-                if answer.is_cancel() {
-                    if let Some(key) = std::char::from_u32(crate::events::ESCAPE_KEY as u32) {
-                        let _: () =
-                            msg_send![button, setKeyEquivalent: ns_string(&key.to_string())];
-                    }
-                } else if Some(ix) == initial_focus_ix {
-                    initial_focus_button = Some(button);
-                }
-            }
-
-            if let Some(button) = initial_focus_button {
-                let alert_window: id = msg_send![alert, window];
-                let _: () = msg_send![alert_window, setInitialFirstResponder: button];
-            }
-
-            let (done_tx, done_rx) = oneshot::channel();
-            let done_tx = Cell::new(Some(done_tx));
-            let block = ConcreteBlock::new(move |answer: NSInteger| {
-                let _: () = msg_send![alert, release];
-                if let Some(done_tx) = done_tx.take() {
-                    let _ = done_tx.send(answer.try_into().unwrap());
-                }
-            });
-            let block = block.copy();
-            let lock = self.0.lock();
-            let native_window = lock.native_window;
-            let closed = lock.closed.clone();
-            let executor = lock.foreground_executor.clone();
-            executor
-                .spawn(async move {
-                    if !closed.load(Ordering::Acquire) {
-                        let _: () = msg_send![
-                            alert,
-                            beginSheetModalForWindow: native_window
-                            completionHandler: block
-                        ];
-                    } else {
-                        let _: () = msg_send![alert, release];
-                    }
-                })
-                .detach();
-
-            Some(done_rx)
+        if let Some(detail) = detail {
+            let detail_text = NSString::from_str(detail);
+            alert.setInformativeText(detail_text.as_ref());
         }
+
+        let mut initial_focus_button: Option<Retained<Objc2NSButton>> = None;
+        for (ix, answer) in answers.iter().enumerate() {
+            let title = NSString::from_str(answer.label());
+            let button = alert.addButtonWithTitle(&title);
+            button.setTag(ix as NSInteger);
+
+            if answer.is_cancel() {
+                if let Some(key) = core::char::from_u32(crate::events::ESCAPE_KEY as u32) {
+                    let key = NSString::from_str(&key.to_string());
+                    button.setKeyEquivalent(&key);
+                }
+            } else if Some(ix) == initial_focus_ix {
+                initial_focus_button = Some(button);
+            }
+        }
+
+        if let Some(button) = initial_focus_button {
+            alert.window().setInitialFirstResponder(Some(&button));
+        }
+
+        let (done_tx, done_rx) = oneshot::channel();
+        let done_tx = Cell::new(Some(done_tx));
+
+        let block = RcBlock::new(move |answer: NSInteger| {
+            if let Some(done_tx) = done_tx.take() {
+                let _ = done_tx.send(answer.try_into().unwrap());
+            }
+        });
+
+        let lock = self.0.lock();
+        let native_window = lock.native_window;
+        let closed = lock.closed.clone();
+        let executor = lock.foreground_executor.clone();
+        executor
+            .spawn(async move {
+                if !closed.load(Ordering::Acquire) {
+                    // SAFETY: `native_window` is an Objective-C `NSWindow` pointer
+                    // owned by the platform window; bridge it into objc2.
+                    let sheet_window: &Objc2NSWindow =
+                        unsafe { &*(native_window as *const Objc2NSWindow) };
+
+                    alert.beginSheetModalForWindow_completionHandler(sheet_window, Some(&block));
+                }
+            })
+            .detach();
+
+        Some(done_rx)
     }
 
     fn activate(&self) {