Clippy lint: temporary with significant Drop can be early dropped

[madser123]

Rocket Version

0.5.0

Operating System

Ubuntu 22.04

Rust Toolchain Version

rustc 1.77.2 (25ef9e3d8 2024-04-09)

What happened?

Whenever i use my Database-pool in a route, clippy warns me of a temporary with significant 'Drop' can be early dropped.

Test Case

/// Library "hipster":
use rocket_sync_db_pools::{database, diesel};

#[database("hipster")]
pub struct DBPool(diesel::PgConnection);

/// Rocket application:
#[get("/some/route")]
pub fn route(conn: hipster::DBPool) {
    conn.run(//do stuff...);
}

Log Output

warning: temporary with significant `Drop` can be early dropped
  --> bin/hipster-backend/src/web/api/mod.rs:69:35
   |
68 | #[get("/validate_credentials/<credentials>")]
   | --------------------------------------------- temporary `__rocket_conn` is currently being dropped at the end of its contained scope
69 | pub async fn validate_credentials(conn: hipster::DBPool, credentials: &str) -> Json<Validation> {
   |                                   ^^^^
   |
   = 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: to override `-W clippy::nursery` add `#[allow(clippy::significant_drop_tightening)]`
help: merge the temporary construction with its single usage
   |
69 ~ pub async fn validate_credentials(conn: 
70 ~ let Json = hipster.;::DBPool, credentials: &str) -> Json<Validation> {
   |
help: remove separated single usage
   |
69 - pub async fn validate_credentials(conn: hipster::DBPool, credentials: &str) -> Json<Validation> {
69 + pub async fn validate_credentials(conn: hipster::DBPool, credentials: &str) -> <Validation> {
   |

Additional Context

The warning is one out of many - It warns with every route i use my pool on.

System Checks

• My bug report relates to functionality.
• I have tested against the latest Rocket release or a recent git commit.
• I have tested against the latest stable rustc toolchain.
• I was unable to find this issue previously reported.

[SergioBenitez]

I don't believe the lint is correct in this case. This is likely why it's a "nursery" lint.

The generated code looks like:

                let __rocket__db: DBPool = match <DBPool as ::rocket::request::FromRequest>::from_request( __req,) .await
                {
                    ::rocket::outcome::Outcome::Success(__v) => __v,
                    ::rocket::outcome::Outcome::Forward(__e) => {
                        return ::rocket::outcome::Outcome::Forward((__data, __e));
                    }
                    ::rocket::outcome::Outcome::Error((__c, __e)) => {
                        return ::rocket::outcome::Outcome::Error(__c);
                    }
                };

                let ___responder = route(__rocket__db);

So the value is not being dropped at the end of the scope.

There's nothing we can do here. This is an issue with clippy.