clippy: "warning: passing a unit value to a function"

[NyxCode]

version: latest master

os: arch/x86_64

description:

When a route returns a unit (()), clippy emits this warning:

warning: passing a unit value to a function
  --> src/rest/api/auth/mod.rs:26:8
   |
26 | pub fn logout(ctx: &App, jwt: &JWT) {
   |        ^^^^^^
   |
   = note: `#[warn(clippy::unit_arg)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
help: move the expression in front of the call...
   |
26 | pub fn logout;
   |        ^^^^^^^
help: ...and use a unit literal instead
   |
26 | pub fn ()(ctx: &App, jwt: &JWT) {
   |    

adding a return type resolves this warning.
The snipped which provoked this warning is this:

#[post("/logout")]
pub fn logout(ctx: &App, jwt: &JWT) {
    ctx.jwt.invalidated_users.invalidate(jwt.user_id);
}

[jebrosen]

The generated code for such a route includes a pattern similar to this:

{
    let ___responder = hello2();
    rocket::handler::Outcome::from(__req, ___responder)
})

I think clippy is too eager to lint this; there's a similar case at rust-lang/rust-clippy#6447. If there is a reasonable way to generate code differently to avoid the lint I'd be interested, but it looks like the code is generated this way in order to provide better error messages.

[NyxCode]

couldn't we just put a #[allow(clippy::unit_arg)] in there?

[jebrosen]

In principle we can (and I have opened a PR doing that). I'm not super happy about adding arbitrary clippy exemptions in generated code, since the exemptions may hide errors introduced in the future and this one really does look like a bug in clippy.

[SergioBenitez]

This appears to have either been resolved upstream or no longer occur with the latest codegen:

❯ cat src/main.rs                                                                                                                                                                                                                
use rocket::*;

#[get("/")]
fn index() { }

#[launch]
fn rocket() -> _ {
    rocket::build().mount("/", routes![index])
}

❯ cargo clippy                                                                                                                                                                                                                   master ●
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s

Closing as "resolved".

[tobymurray]

Using 0.5.0-dec (rocket v0.5.0-dev (https://github.com/SergioBenitez/Rocket.git?branch=master#801e04bd))and clippy 0.1.51 (2fd73fa 2021-03-23) I see this warning on routes that accept data. Maybe it came back?

Is there anything an end user had to do to silence these?

[jebrosen]

rust-lang/rust-clippy#6447 was merged which probably fixed this, but that fix might not have made it to the stable releases yet. It ought to hit stable the week of May 6 (or June 17, if I miscounted).