From 1b92d592b5c7f8a3d399e5b9005f4b316b316ef1 Mon Sep 17 00:00:00 2001
From: Matthew Jasper <mjjasper1@gmail.com>
Date: Tue, 16 Jun 2020 19:29:53 +0100
Subject: [PATCH] Explain unused macro param

---
 src/librustc_arena/lib.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/librustc_arena/lib.rs b/src/librustc_arena/lib.rs
index 7f154052538ac..a21d30bf817e3 100644
--- a/src/librustc_arena/lib.rs
+++ b/src/librustc_arena/lib.rs
@@ -602,6 +602,10 @@ macro_rules! which_arena_for_type {
 
 #[macro_export]
 macro_rules! declare_arena {
+    // This macro has to take the same input as
+    // `impl_arena_allocatable_decoders` which requires a second version of
+    // each type. We ignore that type until we can fix
+    // `impl_arena_allocatable_decoders`.
     ([], [$($a:tt $name:ident: $ty:ty, $_gen_ty:ty;)*], $tcx:lifetime) => {
         #[derive(Default)]
         pub struct Arena<$tcx> {