let_with_type_underscore false positive with '_ lifetime and tracing::instrument

[sciguy16]

Summary

The new let_with_type_underscore lint is reported when an anonymous lifetime is required in a function return type when the function is instrumented with tracing::instrument, as in the below example. Without the #[tracing::instrument] clippy correctly produces no warnings. Following clippy's suggestion to remove the '_ results in a compilation error.

Lint Name

let_with_type_underscore

Reproducer

I tried this code (playground):

#[derive(Debug)]
struct Wrapper(Vec<i32>);

impl Wrapper {
    #[tracing::instrument]
    fn items(&self) -> impl Iterator<Item = i32> + '_ {
        self.0.iter().cloned()
    }
}

fn main() {
    let a = Wrapper(vec![1, 2, 3]);
    let _ = a.items();
}

I saw this happen:

warning: variable declared with type underscore
 --> src/main.rs:6:24
  |
6 |     fn items(&self) -> impl Iterator<Item = i32> + '_ {
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: remove the explicit type `_` declaration
 --> src/main.rs:6:54
  |
6 |     fn items(&self) -> impl Iterator<Item = i32> + '_ {
  |                                                      ^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_with_type_underscore
  = note: `#[warn(clippy::let_with_type_underscore)]` on by default

I expected to see this happen:
[no clippy warning]

Version

rustc 1.70.0-nightly (7b4f48927 2023-03-12)
binary: rustc
commit-hash: 7b4f48927dce585f747a58083b45ab62b9d73a53
commit-date: 2023-03-12
host: x86_64-unknown-linux-gnu
release: 1.70.0-nightly
LLVM version: 15.0.7

Additional Labels

No response

[AdamGS]

I have a similiar issue, but the code looks a bit different (inspired by @sciguy16's example). No lifetimes but just an impl Iterator return type.

#[derive(Debug)]
struct Wrapper(Vec<String>);

impl Wrapper {
    #[tracing::instrument]
    fn make_iter(self) -> impl Iterator<Item = String> {
        self.0.into_iter()
    }   
}

[brainstorm]

I'm seemingly hitting the same issue in umccr/htsget-rs#171, offending code:

/// Gets the JSON to return for a service-info endpoint
#[instrument(skip(app_state))]
pub fn get_service_info_json<H: HtsGet + Send + Sync + 'static>(
  app_state: &AppState<H>,
  endpoint: Endpoint,
) -> impl Responder {
  info!(endpoint = ?endpoint, "service info request");
  PrettyJson(get_base_service_info_json(
    endpoint,
    app_state.htsget.clone(),
    &app_state.config_service_info,
  ))
}

And clippy error:

warning: variable declared with type underscore
  --> htsget-actix/src/handlers/service_info.rs:18:6
   |
18 | ) -> impl Responder {
   |      ^^^^^^^^^^^^^^
   |
help: remove the explicit type `_` declaration
  --> htsget-actix/src/handlers/service_info.rs:18:20
   |
18 | ) -> impl Responder {
   |                    ^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_with_type_underscore

warning: `htsget-actix` (lib) generated 5 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 4.79s

Version

% rustc --version
rustc 1.71.0-nightly (473f916d8 2023-05-03)

/cc @mmalenic

[denschub]

Jup, this seemly affects a lot of usage of tracing. This Axum handler function now results in a rather confusing error warning from clippy:

warning: variable declared with type underscore
  --> src/server.rs:57:6
   |
57 | ) -> impl IntoResponse {
   |      ^^^^
   |
help: remove the explicit type `_` declaration
  --> src/server.rs:57:10
   |
57 | ) -> impl IntoResponse {
   |          ^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_with_type_underscore
   = note: `#[warn(clippy::let_with_type_underscore)]` on by default