From 11df3a0dec58b3210bf858c200ef4ba60a7157fd Mon Sep 17 00:00:00 2001
From: Lukas Wirth <me@lukaswirth.dev>
Date: Sun, 28 Dec 2025 12:41:09 +0100
Subject: [PATCH] Drop AstIdMap asynchronously

---
 .../rust-analyzer/crates/span/src/ast_id.rs   | 42 +++++++++++++++++++
 .../rust-analyzer/crates/span/src/map.rs      | 33 ++++-----------
 2 files changed, 49 insertions(+), 26 deletions(-)

diff --git a/src/tools/rust-analyzer/crates/span/src/ast_id.rs b/src/tools/rust-analyzer/crates/span/src/ast_id.rs
index 3bf14dea75b3..da537ba9900c 100644
--- a/src/tools/rust-analyzer/crates/span/src/ast_id.rs
+++ b/src/tools/rust-analyzer/crates/span/src/ast_id.rs
@@ -777,6 +777,48 @@ impl AstIdMap {
     }
 }
 
+#[cfg(not(no_salsa_async_drops))]
+impl Drop for AstIdMap {
+    fn drop(&mut self) {
+        let arena = std::mem::take(&mut self.arena);
+        let ptr_map = std::mem::take(&mut self.ptr_map);
+        let id_map = std::mem::take(&mut self.id_map);
+        static AST_ID_MAP_DROP_THREAD: std::sync::OnceLock<
+            std::sync::mpsc::Sender<(
+                Arena<(SyntaxNodePtr, ErasedFileAstId)>,
+                hashbrown::HashTable<ArenaId>,
+                hashbrown::HashTable<ArenaId>,
+            )>,
+        > = std::sync::OnceLock::new();
+        AST_ID_MAP_DROP_THREAD
+            .get_or_init(|| {
+                let (sender, receiver) = std::sync::mpsc::channel::<(
+                    Arena<(SyntaxNodePtr, ErasedFileAstId)>,
+                    hashbrown::HashTable<ArenaId>,
+                    hashbrown::HashTable<ArenaId>,
+                )>();
+                std::thread::Builder::new()
+                    .name("AstIdMapDropper".to_owned())
+                    .spawn(move || {
+                        loop {
+                            // block on a receive
+                            _ = receiver.recv();
+                            // then drain the entire channel
+                            while let Ok(_) = receiver.try_recv() {}
+                            // and sleep for a bit
+                            std::thread::sleep(std::time::Duration::from_millis(100));
+                        }
+                        // why do this over just a `receiver.iter().for_each(drop)`? To reduce contention on the channel lock.
+                        // otherwise this thread will constantly wake up and sleep again.
+                    })
+                    .unwrap();
+                sender
+            })
+            .send((arena, ptr_map, id_map))
+            .unwrap();
+    }
+}
+
 #[inline]
 fn hash_ptr(ptr: &SyntaxNodePtr) -> u64 {
     FxBuildHasher.hash_one(ptr)
diff --git a/src/tools/rust-analyzer/crates/span/src/map.rs b/src/tools/rust-analyzer/crates/span/src/map.rs
index f5c083a9f650..75a1b0029cbc 100644
--- a/src/tools/rust-analyzer/crates/span/src/map.rs
+++ b/src/tools/rust-analyzer/crates/span/src/map.rs
@@ -153,26 +153,22 @@ impl SpanMap {
 #[cfg(not(no_salsa_async_drops))]
 impl Drop for SpanMap {
     fn drop(&mut self) {
-        struct SendPtr(*mut [()]);
-        unsafe impl Send for SendPtr {}
+        let spans = std::mem::take(&mut self.spans);
         static SPAN_MAP_DROP_THREAD: std::sync::OnceLock<
-            std::sync::mpsc::Sender<(SendPtr, fn(SendPtr))>,
+            std::sync::mpsc::Sender<Vec<(TextSize, Span)>>,
         > = std::sync::OnceLock::new();
+
         SPAN_MAP_DROP_THREAD
             .get_or_init(|| {
-                let (sender, receiver) = std::sync::mpsc::channel::<(SendPtr, fn(SendPtr))>();
+                let (sender, receiver) = std::sync::mpsc::channel::<Vec<(TextSize, Span)>>();
                 std::thread::Builder::new()
                     .name("SpanMapDropper".to_owned())
                     .spawn(move || {
                         loop {
                             // block on a receive
-                            if let Ok((b, drop)) = receiver.recv() {
-                                drop(b);
-                            }
+                            _ = receiver.recv();
                             // then drain the entire channel
-                            while let Ok((b, drop)) = receiver.try_recv() {
-                                drop(b);
-                            }
+                            while let Ok(_) = receiver.try_recv() {}
                             // and sleep for a bit
                             std::thread::sleep(std::time::Duration::from_millis(100));
                         }
@@ -182,22 +178,7 @@ impl Drop for SpanMap {
                     .unwrap();
                 sender
             })
-            .send((
-                unsafe {
-                    SendPtr(std::mem::transmute::<*mut [(TextSize, Span)], *mut [()]>(Box::<
-                        [(TextSize, Span)],
-                    >::into_raw(
-                        std::mem::take(&mut self.spans).into_boxed_slice(),
-                    )))
-                },
-                |b: SendPtr| {
-                    _ = unsafe {
-                        Box::from_raw(std::mem::transmute::<*mut [()], *mut [(TextSize, Span)]>(
-                            b.0,
-                        ))
-                    }
-                },
-            ))
+            .send(spans)
             .unwrap();
     }
 }