Add skeleton for Metal Allocator

Cargo.toml

@@ -33,6 +33,12 @@ ash = { version = ">=0.34, <=0.37", optional = true, default-features = false, f
 egui = { version = "0.24", optional = true, default-features = false }
 egui_extras = { version = "0.24", optional = true, default-features = false }
 
+[target.'cfg(any(target_os = "macos", target_os = "ios"))'.dependencies]
+metal = { version = "0.27.0", default-features = false, features = [
+    "link",
+    "dispatch",
+] }
+
 [target.'cfg(windows)'.dependencies]
 # Only needed for public-winapi interop helpers
 winapi = { version = "0.3.9", features = ["d3d12", "winerror", "impl-default", "impl-debug"], optional = true }
@@ -88,3 +94,6 @@ d3d12 = ["windows"]
 public-winapi = ["dep:winapi"]
 
 default = ["d3d12", "vulkan"]
+
+[patch.crates-io]
+metal = { git = "https://github.com/Traverse-Research/metal-rs", rev = "d40f71a" }

src/lib.rs

@@ -170,6 +170,10 @@ pub mod vulkan;
 #[cfg(all(windows, feature = "d3d12"))]
 pub mod d3d12;
 
+// todo(lily): add a feature toggle for metal?
+#[cfg(all(any(target_os = "macos", target_os = "ios")))]
+pub mod metal;
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum MemoryLocation {
     /// The allocated resource is stored at an unknown memory location; let the driver decide what's the best location

src/metal/mod.rs

@@ -0,0 +1,37 @@
+#![deny(clippy::unimplemented, clippy::unwrap_used, clippy::ok_expect)]
+use crate::Result;
+use log::Level;
+
+pub struct Allocation {}
+pub struct AllocationCreateDesc {}
+pub struct Allocator {}
+pub struct AllocatorCreateDesc {}
+pub struct ResourceCreateDesc {}
+pub struct Resource {}
+
+impl Allocator {
+    pub fn device(&self) -> &metal::Device {
+        todo!()
+    }
+    pub fn new(_desc: &AllocatorCreateDesc) -> Result<Self> {
+        todo!()
+    }
+    pub fn allocate(&mut self, _desc: &AllocationCreateDesc) -> Result<Allocation> {
+        todo!()
+    }
+    pub fn free(&mut self, _allocation: Allocation) -> Result<()> {
+        todo!()
+    }
+    pub fn rename_allocation(&mut self, _allocation: &mut Allocation, _name: &str) -> Result<()> {
+        todo!()
+    }
+    pub fn report_memory_leaks(&self, _log_level: Level) {
+        todo!()
+    }
+    pub fn create_resource(&mut self, _desc: &ResourceCreateDesc) -> Result<Resource> {
+        todo!()
+    }
+    pub fn free_resource(&mut self, mut _resource: Resource) -> Result<()> {
+        todo!()
+    }
+}