diff --git a/cidre/src/cg/direct_display.rs b/cidre/src/cg/direct_display.rs
index 2d2e8175..a29693ff 100644
--- a/cidre/src/cg/direct_display.rs
+++ b/cidre/src/cg/direct_display.rs
@@ -1,3 +1,6 @@
+#[cfg(feature = "mtl")]
+use crate::{arc, mtl};
+
 pub type Id = u32;
 
 pub type RefreshRate = f64;
@@ -9,9 +12,29 @@ pub fn main_display_id() -> Id {
     unsafe { CGMainDisplayID() }
 }
 
-extern "C" {
+#[doc(alias = "CGDirectDisplayCopyCurrentMetalDevice")]
+#[cfg(all(target_os = "macos", feature = "mtl"))]
+pub fn direct_display_current_mtl_device(display: Id) -> Option<arc::R<mtl::Device>> {
+    unsafe { CGDirectDisplayCopyCurrentMetalDevice(display) }
+}
+
+extern "C-unwind" {
     #[cfg(target_os = "macos")]
     fn CGMainDisplayID() -> Id;
+
+    #[cfg(all(target_os = "macos", feature = "mtl"))]
+    fn CGDirectDisplayCopyCurrentMetalDevice(display: Id) -> Option<arc::R<mtl::Device>>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{direct_display_current_mtl_device, main_display_id};
+
+    #[test]
+    fn basics() {
+        let display = main_display_id();
+        let _device = direct_display_current_mtl_device(display).expect("Failed to get device");
+    }
 }
 
 // typedef uint32_t CGDirectDisplayID;
diff --git a/cidre/src/mtl/function_constant_values.rs b/cidre/src/mtl/function_constant_values.rs
index 383a45b8..b068be3f 100644
--- a/cidre/src/mtl/function_constant_values.rs
+++ b/cidre/src/mtl/function_constant_values.rs
@@ -15,12 +15,12 @@ impl FnConstValues {
     pub fn set_value_at(&mut self, val: *const c_void, type_: mtl::DType, at_index: ns::UInteger);
 
     pub fn set_bool_at(&mut self, val: bool, at_index: usize) {
-        debug_assert!(at_index < u16::MAX as _);
+        debug_assert!(at_index <= u16::MAX as _);
         self.set_value_at(&val as *const bool as *const _, mtl::DType::Bool, at_index);
     }
 
     pub fn set_u8_at(&mut self, val: u8, at_index: usize) {
-        debug_assert!(at_index < u16::MAX as _);
+        debug_assert!(at_index <= u16::MAX as _);
         self.set_value_at(&val as *const u8 as *const _, mtl::DType::U8, at_index);
     }