From 64779737db21f9e506f4562d9923e7037e15ecfb Mon Sep 17 00:00:00 2001
From: David Barsky <me@davidbarsky.com>
Date: Fri, 23 Feb 2024 15:34:23 -0500
Subject: [PATCH] internal: fix deadlock introduced by #16643

---
 crates/salsa/src/derived.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crates/salsa/src/derived.rs b/crates/salsa/src/derived.rs
index bf532bdccf6..153df999f53 100644
--- a/crates/salsa/src/derived.rs
+++ b/crates/salsa/src/derived.rs
@@ -146,11 +146,14 @@ where
         revision: Revision,
     ) -> bool {
         debug_assert!(revision < db.salsa_runtime().current_revision());
-        let read = &self.slot_map.read();
+        let read = self.slot_map.read();
         let Some((key, slot)) = read.get_index(index as usize) else {
             return false;
         };
-        slot.maybe_changed_after(db, revision, key)
+        let (key, slot) = (key.clone(), slot.clone());
+        // note: this drop is load-bearing. removing it would causes deadlocks.
+        drop(read);
+        slot.maybe_changed_after(db, revision, &key)
     }
 
     fn fetch(&self, db: &<Q as QueryDb<'_>>::DynDb, key: &Q::Key) -> Q::Value {