Do you have to clone it twice? I think the latter one can be just moved.

I would rather have #[allow(dead_code)] here than doing the "prefix with _" paradigm for structs.

Or come up with some other paradigm, but I would like to avoid intermediate diffs to bend over backwards for linting rules which won't apply in later diffs.

The #[allow(dead_code)] is good until you forget to remove it after the first usage

One option is to simply remove all occurrences in the last diff of the stack

I've tested it initially and only cloning twice worked for me, but I'll check once again

In this case I think that @jon is right, as this struct is deserialized and prefixing might've broken something.

I'm not 100% what's happening here, but if we remove the second copy we receive an error

error[E0507]: cannot move out of `db_clone`, a captured variable in an `Fn` closure
  --> src/service.rs:21:34
   |
18 |     let db_clone = self.db.clone();
   |         -------- captured outer variable
19 |     HttpServer::new(move || {
   |                     ------- captured by this `Fn` closure
20 |       App::new()
21 |         .app_data(web::Data::new(db_clone))
   |                                  ^^^^^^^^ move occurs because `db_clone` has type `DatabaseClient`, which does not implement the `Copy` trait

It might be caused by a fact that app_data expects <U: 'static>.

We can modify the code to be more explicit about moving here by using to_owned which usually uses clone inside.