comments describing why objc::class!() and objc::selector!() only expose the static value

Author: jbatez

library/core/src/os/darwin/objc.rs

@@ -72,6 +72,9 @@ pub type SEL = *mut objc_selector;
 /// ```
 #[allow_internal_unstable(rustc_attrs)]
 pub macro class($classname:expr) {{
+    // Since static Objective-C class references actually end up with multiple definitions
+    // across dylib boundaries, we only expose the value of the static and don't provide a way to
+    // get the address of or a reference to the static.
     unsafe extern "C" {
         #[rustc_objc_class = $classname]
         safe static VAL: $crate::os::darwin::objc::Class;
@@ -93,6 +96,9 @@ pub macro class($classname:expr) {{
 /// ```
 #[allow_internal_unstable(rustc_attrs)]
 pub macro selector($methname:expr) {{
+    // Since static Objective-C selector references actually end up with multiple definitions
+    // across dylib boundaries, we only expose the value of the static and don't provide a way to
+    // get the address of or a reference to the static.
     unsafe extern "C" {
         #[rustc_objc_selector = $methname]
         safe static VAL: $crate::os::darwin::objc::SEL;