From 246a6a6589b118c0a08e449f981bb4ae07bc089a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20K=C3=A5re=20Alsaker?= <john.kare.alsaker@gmail.com>
Date: Wed, 23 Aug 2023 20:16:22 +0200
Subject: [PATCH] Extend comment on assertion

---
 compiler/rustc_span/src/hygiene.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 37ec2f0801c..ab57d5c6803 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1410,8 +1410,12 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
             &mut hygiene_data.syntax_context_data[ctxt.as_u32() as usize],
             ctxt_data,
         );
-        // Make sure nothing weird happening while `decode_data` was running
         if cfg!(not(parallel_compiler)) {
+            // Make sure nothing weird happened while `decode_data` was running.
+            // We used `kw::Empty` for the dummy value and we expect nothing to be
+            // modifying the dummy entry.
+            // This does not hold for the parallel compiler as another thread may
+            // have inserted the fully decoded data.
             assert_eq!(dummy.dollar_crate_name, kw::Empty);
         }
     });