Fix nightly warnings 2024 june (#8151)

## Problem

new clippy warnings on nightly.

## Summary of changes

broken up each commit by warning type.
1. Remove some unnecessary refs.
2. In edition 2024, inference will default to `!` and not `()`.
3. Clippy complains about doc comment indentation
4. Fix `Trait + ?Sized` where `Trait: Sized`.
5. diesel_derives triggering `non_local_defintions`

proxy/src/waiters.rs

@@ -111,7 +111,7 @@ mod tests {
 
         let waiters = Arc::clone(&waiters);
         let notifier = tokio::spawn(async move {
-            waiters.notify(key, Default::default())?;
+            waiters.notify(key, ())?;
             Ok(())
         });