unit_arg lint for Rocket handlers returning unit causes a warning

[KamilaBorowska]

Lint name:

unit_arg

I tried this code:

#[rocket::get("/")]
fn handler() {}

fn main() {
    println!("Hello, world!");
}

with the following Cargo.toml:

[package]
name = "rocket-example"
version = "0.1.0"
edition = "2018"

[dependencies.rocket]
git = "https://github.com/SergioBenitez/Rocket.git"
rev = "9671115796e42865eaebd020ac27542802027b02"

I expected to see this happen: no warning

Instead, this happened:

warning: passing a unit value to a function
 --> src/main.rs:2:4
  |
2 | fn handler() {}
  |    ^^^^^^^
  |
  = 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 and replace it with the unit literal `()`
  |
2 | fn handler;
3 | ()() {}
  |

warning: 1 warning emitted

See also rwf2/Rocket#1495.

Meta

• cargo clippy -V: clippy 0.0.212 (2987785 2020-12-28)
• rustc -Vv:

  rustc 1.51.0-nightly (2987785df 2020-12-28)
  binary: rustc
  commit-hash: 2987785df3d46d5ff144a5c67fbb8f5cca798d78
  commit-date: 2020-12-28
  host: x86_64-unknown-linux-gnu
  release: 1.51.0-nightly
  

[phansch]

Alright, I managed to extract a proc_macro_attribute function from Rocket's codegen code that reproduces the issue:

#[proc_macro_attribute]
pub fn rocket_get(_args: TokenStream, input: TokenStream) -> TokenStream {
    let input_fn: syn::ItemFn = syn::parse(input).unwrap();

    // This needs to be quote_spanned! with the span of the input signature otherwise the issue couldn't be reproduced
    let spanned = quote_spanned! { input_fn.sig.ident.span() =>
        fn unitfn(_: ()) {}
        #[allow(clippy::no_effect)]
        unitfn({ 1; });
    };

    let output = quote! {
        #[allow(unused, clippy::no_effect)]
        #input_fn

        fn foo() {
            #spanned
        }
    };
    output.into()
}

Unfortunately a simple external macro check didn't fix the issue, so I will continue working on this tomorrow =)

[phansch]

Unfortunately, neither Clippy nor rustc currently have a way to tell apart a quote_spanned span from a non-macro span, so this is currently impossible to fix on our side (Also see #6249 (comment)).

[camsteffen]

I don't have much experience with procedural macros. Is this something that can/should be fixed in the library? Maybe using Span::mixed_site or something like that?

[phansch]

Personally, I would consider that only a workaround (I don't know if mixed_site would fix it). Given that the library is using stable proc_macro APIs that are causing these problems, I would say it's something that needs to be fixed in the compiler.

There has been some more discussion on Zulip: https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/Telling.20apart.20.60quote_spanned.60.20from.20non-macro.20.20spans.3F

Essentially, we would need some extra 'this span was produced by macro expansion' state that is not modified by any of the proc_macro APIs.