address more review comments

src/librustc/plugin/registry.rs

@@ -137,7 +137,7 @@ impl<'a> Registry<'a> {
     }
 
 
-    /// Register an attribute with an attribute type
+    /// Register an attribute with an attribute type.
     ///
     /// Registered attributes will bypass the `custom_attribute` feature gate.
     /// `Whitelisted` attributes will additionally not trigger the `unused_attribute`

src/librustc_lint/builtin.rs

@@ -643,11 +643,8 @@ impl LintPass for UnusedAttributes {
 
         let plugin_attributes = cx.sess().plugin_attributes.borrow_mut();
         for &(ref name, ty) in plugin_attributes.iter() {
-            match ty {
-                AttributeType::Whitelisted if attr.check_name(&*name) => {
-                    break;
-                },
-                _ => ()
+            if ty == AttributeType::Whitelisted && attr.check_name(&*name) {
+                break;
             }
         }
 

src/libsyntax/feature_gate.rs

@@ -389,6 +389,8 @@ impl<'a> Context<'a> {
         for &(ref n, ref ty) in self.plugin_attributes.iter() {
             if &*n == name {
                 // Plugins can't gate attributes, so we don't check for it
+                // unlike the code above; we only use this loop to
+                // short-circuit to avoid the checks below
                 debug!("check_attribute: {:?} is registered by a plugin, {:?}", name, ty);
                 return;
             }
@@ -403,7 +405,10 @@ impl<'a> Context<'a> {
                               "attributes of the form `#[derive_*]` are reserved \
                                for the compiler");
         } else {
-            // Only do the custom attribute lint post-expansion
+            // Only run the custom attribute lint during regular
+            // feature gate checking. Macro gating runs
+            // before the plugin attributes are registered
+            // so we skip this then
             if !is_macro {
                 self.gate_feature("custom_attribute", attr.span,
                            &format!("The attribute `{}` is currently \