Explain possible better future for async fn that need to be Send

rust/src/storage/db/mod.rs

@@ -33,8 +33,37 @@ pub trait DatabasePool: Send + Sync {
     /// Starts a transaction on the database and runs the given function,
     /// returning its result.
     ///
+    /// `name` should be a descriptive identifier for logging/metrics
+    ///
     /// `func` may be called multiple times under certain failure modes (like
     /// serialization and deadlock errors), so it is `Fn` rather than `FnOnce`.
+    ///
+    /// Usage:
+    /// ```rust
+    /// db_pool
+    /// .run_interaction(|txn| {
+    ///     async move {
+    ///         /* do stuff with txn */
+    ///     }
+    ///     .boxed()
+    /// })
+    /// ```
+    //
+    // Ideally, this method signature would be slightly different to allow downstream
+    // usage to look like the following (simpler) but because allow the work to happen
+    // on other threads, the `Future` needs to be `Send`; As of 2026-06-22, the
+    // `AsyncFn` trait doesn't have a clean way to express that "the future this async
+    // closure produces is `Send`."
+    // ```
+    // db_pool.run_interaction("description", async move |txn| {
+    //     /* do stuff with txn */
+    // })
+    // ```
+    //
+    // Refs:
+    //  - [RFC 3668: Async closures](https://github.com/rust-lang/rfcs/pull/3668)
+    //  - [RFC 3654: Return Type Notation](https://github.com/rust-lang/rfcs/pull/3654)
+    //  - [Tracking Issue for return type notation](https://github.com/rust-lang/rust/issues/109417)
     fn run_interaction<R, F>(
         &self,
         name: &'static str,

rust/src/storage/store.rs

@@ -63,7 +63,7 @@ impl<P: DatabasePool> Store<P> {
         //
         // Owned copies so the callback can be `'static` (it may be moved to
         // another thread and called multiple times under retries).
-        let user_id = user_id.to_string();
+        let user_id = user_id.to_owned();
         let feature = feature.to_string();
 
         let is_feature_enabled_for_user = self