Add MTLCopyAllDevices fallback for iOS, tvOS and visionOS

This function is only available on these platforms on very recent OS
versions, so let's add a fallback implementation so that users of
objc2-metal doesn't have to do it themselves.

crates/objc2/src/topics/about_generated/CHANGELOG.md

@@ -28,6 +28,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   - `QuickLookUI` / `objc2-quick-look-ui`.
   - `SensorKit` / `objc2-sensor-kit`.
   - `UserNotificationsUI` / `objc2-user-notifications-ui`.
+* Added fallback for `MTLCopyAllDevices` on non-macOS platforms.
 
 
 ## 0.3.0 - 2025-01-22

framework-crates/objc2-metal/src/device.rs

@@ -0,0 +1,47 @@
+use crate::MTLDevice;
+use objc2::rc::Retained;
+use objc2::runtime::ProtocolObject;
+use objc2_foundation::NSArray;
+
+/// Returns all Metal devices in the system.
+///
+/// On macOS and macCatalyst, this API will not cause the system to switch
+/// devices and leaves the decision about which GPU to use up to the
+/// application based on whatever criteria it deems appropriate.
+///
+/// On iOS, tvOS and visionOS, this API returns an array containing the same
+/// device that MTLCreateSystemDefaultDevice would have returned, or an empty
+/// array if it would have failed.
+#[inline]
+pub extern "C-unwind" fn MTLCopyAllDevices() -> Retained<NSArray<ProtocolObject<dyn MTLDevice>>> {
+    // MTLCopyAllDevices is always available on macOS and Mac Catalyst, but
+    // only available recently on iOS 18.0 / tvOS 18.0 / visionOS 2.0.
+    //
+    // Instead, we do the fallback to MTLCreateSystemDefaultDevice on those
+    // platforms that they do themselves on newer systems.
+    //
+    // TODO: Add `target_abi = "macabi" once in MSRV.
+    // TODO: Use something like <https://github.com/rust-lang/rfcs/pull/3750>
+    // to call the actual API when available.
+    #[cfg(target_os = "macos")]
+    {
+        extern "C-unwind" {
+            fn MTLCopyAllDevices() -> *mut NSArray<ProtocolObject<dyn MTLDevice>>;
+        }
+
+        let ret = unsafe { MTLCopyAllDevices() };
+        // SAFETY: Marked NS_RETURNS_RETAINED (and has `Copy` in the name).
+        unsafe { Retained::from_raw(ret) }
+            .expect("function was marked as returning non-null, but actually returned NULL")
+    }
+    #[cfg(not(target_os = "macos"))]
+    {
+        let device = crate::MTLCreateSystemDefaultDevice();
+        let slice = if let Some(device) = device.as_deref() {
+            &[device]
+        } else {
+            &[]
+        };
+        NSArray::from_slice(slice)
+    }
+}

framework-crates/objc2-metal/src/lib.rs

@@ -55,6 +55,8 @@ extern crate std;
 mod capture;
 #[cfg(feature = "MTLCounters")]
 mod counters;
+#[cfg(feature = "MTLDevice")]
+mod device;
 mod generated;
 #[cfg(feature = "MTLAccelerationStructureTypes")]
 mod packed;
@@ -72,6 +74,8 @@ mod types;
 
 #[cfg(feature = "MTLCounters")]
 pub use self::counters::*;
+#[cfg(feature = "MTLDevice")]
+pub use self::device::*;
 #[allow(unused_imports, unreachable_pub)]
 pub use self::generated::*;
 #[cfg(feature = "MTLAccelerationStructureTypes")]

framework-crates/objc2-metal/tests/runs_with_core_graphics.rs

@@ -11,7 +11,6 @@ fn test_create_default() {
 }
 
 #[test]
-#[cfg(target_os = "macos")]
 fn get_all() {
     let _ = objc2_metal::MTLCopyAllDevices();
 }

framework-crates/objc2-metal/translation-config.toml

@@ -28,6 +28,9 @@ struct._MTLPackedFloat3.skipped = true
 typedef.MTLPackedFloat3.skipped = true
 fn.MTLPackedFloat3Make.skipped = true
 
+# Manually defined to allow it to work on older OSes.
+fn.MTLCopyAllDevices.skipped = true
+
 ###
 ### Safety
 ###