/auto (#16696)

Add `/auto` behind a feature flag that's disabled for now, even for
staff.

We've decided on a different design for context inference, but there are
parts of /auto that will be useful for that, so we want them in the code
base even if they're unused for now.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/semantic_index/src/indexing.rs

@@ -0,0 +1,49 @@
+use collections::HashSet;
+use parking_lot::Mutex;
+use project::ProjectEntryId;
+use smol::channel;
+use std::sync::{Arc, Weak};
+
+/// The set of entries that are currently being indexed.
+pub struct IndexingEntrySet {
+    entry_ids: Mutex<HashSet<ProjectEntryId>>,
+    tx: channel::Sender<()>,
+}
+
+/// When dropped, removes the entry from the set of entries that are being indexed.
+#[derive(Clone)]
+pub(crate) struct IndexingEntryHandle {
+    entry_id: ProjectEntryId,
+    set: Weak<IndexingEntrySet>,
+}
+
+impl IndexingEntrySet {
+    pub fn new(tx: channel::Sender<()>) -> Self {
+        Self {
+            entry_ids: Default::default(),
+            tx,
+        }
+    }
+
+    pub fn insert(self: &Arc<Self>, entry_id: ProjectEntryId) -> IndexingEntryHandle {
+        self.entry_ids.lock().insert(entry_id);
+        self.tx.send_blocking(()).ok();
+        IndexingEntryHandle {
+            entry_id,
+            set: Arc::downgrade(self),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.entry_ids.lock().len()
+    }
+}
+
+impl Drop for IndexingEntryHandle {
+    fn drop(&mut self) {
+        if let Some(set) = self.set.upgrade() {
+            set.tx.send_blocking(()).ok();
+            set.entry_ids.lock().remove(&self.entry_id);
+        }
+    }
+}