comments explaining custom objc error messages

Author: jbatez

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -169,6 +169,9 @@ impl<S: Stage> SingleAttributeParser<S> for ObjcClassParser {
             return None;
         };
         let Some(classname) = nv.value_as_str() else {
+            // `#[rustc_objc_class = ...]` is expected to be used as an implementatioin detail
+            // inside a standard library macro, but `cx.expected_string_literal` exposes too much.
+            // Use a custom error message instead.
             cx.emit_err(ObjcClassExpectedStringLiteral { span: nv.value_span });
             return None;
         };
@@ -198,6 +201,9 @@ impl<S: Stage> SingleAttributeParser<S> for ObjcSelectorParser {
             return None;
         };
         let Some(methname) = nv.value_as_str() else {
+            // `#[rustc_objc_selector = ...]` is expected to be used as an implementatioin detail
+            // inside a standard library macro, but `cx.expected_string_literal` exposes too much.
+            // Use a custom error message instead.
             cx.emit_err(ObjcSelectorExpectedStringLiteral { span: nv.value_span });
             return None;
         };