Clarify `SyntaxExtensionKind::LegacyDerive`.

diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index bdcd53349492ccb98945ee4a41a8fe5c39e8adda..8955abebf1e0f0d116d100e84ba1f0b1e9c12e3d 100644 (file)
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -676,8 +676,13 @@ pub enum SyntaxExtensionKind {
 
     /// A token-based derive macro.
     Derive(
-        /// An expander with signature TokenStream -> TokenStream (not yet).
+        /// An expander with signature TokenStream -> TokenStream.
         /// The produced TokenSteam is appended to the input TokenSteam.
+        ///
+        /// FIXME: The text above describes how this should work. Currently it
+        /// is handled identically to `LegacyDerive`. It should be migrated to
+        /// a token-based representation like `Bang` and `Attr`, instead of
+        /// using `MultiItemModifier`.
         Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
     ),