instrument macro sometimes triggers clippy::let_with_type_underscore

[jplatte]

Bug Report

Version

v0.1.37

Platform

Linux 6.2.2-arch1-1 #1 SMP PREEMPT_DYNAMIC Fri, 03 Mar 2023 15:58:31 +0000 x86_64 GNU/Linux

Description

Clippy introduced a new lint let_with_type_underscore that triggers on let pattern: _ = expression; in rust-lang/rust-clippy#10463. It seems to trigger on #[instrument]ed function that return impl Trait + 'lt, for instance one function I saw it was the function

#[instrument(name = "sync_stream", skip_all, parent = &self.inner.client.inner.root_span)]
pub fn stream(&self) -> impl Stream<Item = Result<UpdateSummary, crate::Error>> + '_ {
    // ...
}

(and naming the lifetime didn't help)

[BBaoVanC]

I got this issue with a function in my axum app:

#[instrument(skip(headers), fields(if_none_match = ?headers.get(header::IF_NONE_MATCH)))]
pub async fn handler(uri: Uri, headers: HeaderMap) -> impl IntoResponse {

In the macro expansion, a bit down I see:

            let __tracing_attr_fake_return: _ = ::core::panicking::panic_fmt(format_args!(
                "internal error: entered unreachable code: {0}",
                format_args!("this is just for type inference, and is unreachable code")
            ));
            return __tracing_attr_fake_return;

[mrnossiom]

This only happens with impl Trait because impl in return types removed as it is used later in a let binding. This happens because impl Trait in let bindings RFC is accepted but not implemented nor stable. It means that let foo: impl Trait = ... is not valid syntax yet. (See PR)

tracing/tracing-attributes/src/expand.rs

Lines 54 to 55 in a0126b2

	let (return_type, return_span) = if let ReturnType::Type(_, return_type) = &output {
	(erase_impl_trait(return_type), return_type.span())

Here, the return type is completely empty, so it equals _

tracing/tracing-attributes/src/expand.rs

Lines 69 to 70 in a0126b2

	let __tracing_attr_fake_return: #return_type =
	unreachable!("this is just for type inference, and is unreachable code");

I would love to make a PR, but I don't know if we should silence the warning with a allow(clippy::let_with_type_underscore) or in the expansion itself, not emitting to an underscore but rather nothing.

Thanks

[ranile]

Min repro on playground. This only happens if there's an impl Trait return type from my testing

This lint bug is triggered on stable now and breaks CI

[mrnossiom]

I made a PR two months ago. And yes, the lint is now warn by default on rust v1.70.0.