fix(vfs): report files deleted on macOS again (#1435)

fix(vfs): report files deleted on macOS again

Since the watcher queues raw notify events (#1412), deleting a file on macOS was never reported, so
its symbols stayed in the index and the tree kept it (the project smoke tests failed for every
language). FSEvents delivers a deleted file's coalesced history flags as create, remove, then
modify events. Create + remove merged into a transient path, the following modify turned it back
into a creation, and a creation of a path that no longer exists is dropped. A modify no longer
revives a transient path; only a create does, and the disk decides as before.

crates/ide-vfs/src/watch.rs

@@ -404,6 +404,9 @@ fn merge(prev: Option<Change>, next: Change) -> Change {
     match (prev, next) {
         (None, n) => n,
         (Some(Vcs), _) | (_, Vcs) => Vcs,
+        // FSEvents reports a deleted file's history as create, remove, then modify: a
+        // modify can't bring back a path that went away, only a create can.
+        (Some(Transient), Modified) => Transient,
         (Some(Created | Transient), Modified | Created) => Created,
         (Some(Created | Transient), Removed) => Transient,
         (Some(Removed), Created | Modified) => Modified,
@@ -630,7 +633,7 @@ impl EventFilter {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use notify::event::{CreateKind, DataChange, RemoveKind};
+    use notify::event::{CreateKind, DataChange, MetadataKind, RemoveKind};
 
     fn ev(kind: EventKind, paths: Vec<PathBuf>) -> Event {
         Event { kind, paths, attrs: Default::default() }
@@ -788,6 +791,24 @@ mod tests {
         assert_eq!(batch.ignored, vec![root.join("debug.log")], "ignored files are reported apart");
     }
 
+    /// What FSEvents delivers when a file is deleted: its coalesced history flags as
+    /// create, remove, then modify events, all for a path that no longer exists.
+    #[test]
+    fn fsevents_deletion_with_history_flags_is_a_removal() {
+        let tmp = tempfile::tempdir().unwrap();
+        let root = dunce::canonicalize(tmp.path()).unwrap();
+        let filter = EventFilter::new(root.clone(), &[]);
+        let gone = root.join("src/Fresh.java");
+        let batch = filter.batch(&[
+            ev(EventKind::Create(CreateKind::File), vec![gone.clone()]),
+            ev(EventKind::Remove(RemoveKind::File), vec![gone.clone()]),
+            ev(EventKind::Modify(ModifyKind::Metadata(MetadataKind::Extended)), vec![gone.clone()]),
+            ev(EventKind::Modify(ModifyKind::Data(DataChange::Content)), vec![gone.clone()]),
+        ]);
+        assert_eq!(batch.removed, vec![gone]);
+        assert!(batch.created.is_empty() && batch.modified.is_empty());
+    }
+
     #[test]
     fn atomic_save_temp_files_are_not_reported() {
         let tmp = tempfile::tempdir().unwrap();