Significant drop tightening raises at an effective last statement

Summary

I was using an asynchronous lock. The last usage of that lock is inside a match branch and an if. There is no more statements after the match, so it is a tail call and a manual drop doesn't help.

https://github.com/jdh8/natsuki/blob/d4d22b3eee530b47d2d24a98ac394647daa593fd/src/topgg.rs#L58

Lint Name

significant-drop-tightening

Reproducer

I tried this code:

async fn ready(&self, _: serenity::Context, ready: serenity::Ready) {
    let Some(token) = self.token.as_deref() else { return };
    match ready.shard {
        None => post(token, Stats { guilds: ready.guilds.len() as u64, shards: 0 }).await,
        Some([_, shards]) => {
            let mut stats = self.stats.lock().await;
            stats.guilds += ready.guilds.len() as u64;
            stats.shards += 1;
            
            if stats.shards == shards {
                post(token, *stats).await;
            }
        },
    };
}

I saw this happen:

warning: temporary with significant `Drop` can be early dropped
  --> src/topgg.rs:53:25
   |
52 |               Some([_, shards]) => {
   |  __________________________________-
53 | |                 let mut stats = self.stats.lock().await;
   | |                         ^^^^^
54 | |                 stats.guilds += ready.guilds.len() as u64;
55 | |                 stats.shards += 1;
...  |
59 | |                 }
60 | |             },
   | |_____________- temporary `stats` is currently being dropped at the end of its contained scope
   |
   = note: this might lead to unnecessary resource contention
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
   = note: `-W clippy::significant-drop-tightening` implied by `-W clippy::nursery`
help: drop the temporary after the end of its last usage
   |
58 ~                     post(token, *stats).await;
59 +                     drop(stats);
   |

I expected to see this happen:

No warning

Version

rustc 1.72.0 (5680fa18f 2023-08-23) (Fedora 1.72.0-1.fc38)
binary: rustc
commit-hash: 5680fa18feaa87f3ff04063800aec256c3d4b4be
commit-date: 2023-08-23
host: x86_64-unknown-linux-gnu
release: 1.72.0
LLVM version: 16.0.6

Additional Labels

No response