Prepare for making the crate no_std

Cargo.toml

@@ -11,6 +11,11 @@ keywords = ["window", "metal", "graphics"]
 categories = ["game-engines", "graphics"]
 exclude = [".github/*"]
 
+[features]
+default = ["std"]
+std = ["alloc"]
+alloc = []
+
 [target.'cfg(target_vendor = "apple")'.dependencies]
 objc2 = "0.5.2"
 objc2-foundation = { version = "0.2.2", features = [

src/lib.rs

@@ -97,6 +97,7 @@
 //!
 //! Option 3 seems like the most robust solution, so this is what this crate does.
 
+#![no_std]
 #![cfg(target_vendor = "apple")]
 #![cfg_attr(docsrs, feature(doc_auto_cfg, doc_cfg_hide), doc(cfg_hide(doc)))]
 #![deny(unsafe_op_in_unsafe_fn)]
@@ -113,6 +114,12 @@ use objc2::{msg_send_id, ClassType};
 use objc2_foundation::{MainThreadMarker, NSObject, NSObjectProtocol};
 use objc2_quartz_core::{CALayer, CAMetalLayer};
 
+#[cfg(not(feature = "alloc"))]
+compile_error!("The `alloc` feature must currently be enabled.");
+
+#[cfg(not(feature = "std"))]
+compile_error!("The `std` feature must currently be enabled.");
+
 /// A wrapper around [`CAMetalLayer`].
 #[doc(alias = "CAMetalLayer")]
 #[derive(Debug, Clone)]