Drop span maps on a background thread

Author: Veykril

crates/span/src/map.rs

@@ -156,6 +156,38 @@ where
     }
 }
 
+impl<S> Drop for SpanMap<S> {
+    fn drop(&mut self) {
+        struct SendPtr(*mut [()]);
+        unsafe impl Send for SendPtr {}
+        static SPAN_MAP_DROP_THREAD: std::sync::OnceLock<
+            std::sync::mpsc::Sender<(SendPtr, fn(SendPtr))>,
+        > = std::sync::OnceLock::new();
+        SPAN_MAP_DROP_THREAD
+            .get_or_init(|| {
+                let (sender, receiver) = std::sync::mpsc::channel::<(SendPtr, fn(SendPtr))>();
+                std::thread::Builder::new()
+                    .name("SpanMapDropper".to_owned())
+                    .spawn(move || receiver.iter().for_each(|(b, drop)| drop(b)))
+                    .unwrap();
+                sender
+            })
+            .send((
+                unsafe {
+                    SendPtr(std::mem::transmute(Box::<[(TextSize, SpanData<S>)]>::into_raw(
+                        std::mem::take(&mut self.spans).into_boxed_slice(),
+                    )))
+                },
+                |b: SendPtr| {
+                    _ = unsafe {
+                        Box::from_raw(std::mem::transmute::<_, *mut [(TextSize, SpanData<S>)]>(b.0))
+                    }
+                },
+            ))
+            .unwrap();
+    }
+}
+
 #[derive(PartialEq, Eq, Hash, Debug)]
 pub struct RealSpanMap {
     file_id: EditionedFileId,