From 91a0e09d3e328dc0eaacf3de733beb0c0347c665 Mon Sep 17 00:00:00 2001
From: Zalathar <Zalathar@users.noreply.github.com>
Date: Wed, 21 Jan 2026 18:09:51 +1100
Subject: [PATCH 1/2] Derive `Default` for `QueryArenas`

---
 compiler/rustc_middle/src/query/plumbing.rs | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/compiler/rustc_middle/src/query/plumbing.rs b/compiler/rustc_middle/src/query/plumbing.rs
index df333e68add1a..be7a459d4bb1b 100644
--- a/compiler/rustc_middle/src/query/plumbing.rs
+++ b/compiler/rustc_middle/src/query/plumbing.rs
@@ -342,6 +342,7 @@ macro_rules! define_callbacks {
             })*
         }
 
+        #[derive(Default)]
         pub struct QueryArenas<'tcx> {
             $($(#[$attr])* pub $name: query_if_arena!([$($modifiers)*]
                 (TypedArena<<$V as $crate::query::arena_cached::ArenaCached<'tcx>>::Allocated>)
@@ -349,17 +350,6 @@ macro_rules! define_callbacks {
             ),)*
         }
 
-        impl Default for QueryArenas<'_> {
-            fn default() -> Self {
-                Self {
-                    $($name: query_if_arena!([$($modifiers)*]
-                        (Default::default())
-                        ()
-                    ),)*
-                }
-            }
-        }
-
         #[derive(Default)]
         pub struct QueryCaches<'tcx> {
             $($(#[$attr])* pub $name: queries::$name::Storage<'tcx>,)*

From 2495e5834bd166fc5b2fab99f3bcae7ca96fbcd8 Mon Sep 17 00:00:00 2001
From: Zalathar <Zalathar@users.noreply.github.com>
Date: Wed, 21 Jan 2026 18:10:53 +1100
Subject: [PATCH 2/2] Add some comments to `QueryArenas`

---
 compiler/rustc_middle/src/query/plumbing.rs | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/compiler/rustc_middle/src/query/plumbing.rs b/compiler/rustc_middle/src/query/plumbing.rs
index be7a459d4bb1b..16121c38d1a9b 100644
--- a/compiler/rustc_middle/src/query/plumbing.rs
+++ b/compiler/rustc_middle/src/query/plumbing.rs
@@ -342,12 +342,18 @@ macro_rules! define_callbacks {
             })*
         }
 
+        /// Holds per-query arenas for queries with the `arena_cache` modifier.
         #[derive(Default)]
         pub struct QueryArenas<'tcx> {
-            $($(#[$attr])* pub $name: query_if_arena!([$($modifiers)*]
-                (TypedArena<<$V as $crate::query::arena_cached::ArenaCached<'tcx>>::Allocated>)
-                ()
-            ),)*
+            $(
+                $(#[$attr])*
+                pub $name: query_if_arena!([$($modifiers)*]
+                    // Use the `ArenaCached` helper trait to determine the arena's value type.
+                    (TypedArena<<$V as $crate::query::arena_cached::ArenaCached<'tcx>>::Allocated>)
+                    // No arena for this query, so the field type is `()`.
+                    ()
+                ),
+            )*
         }
 
         #[derive(Default)]