New lint affecting tracing::instrument macro when returning an impl

[damccull]

When I updated to the latest clippy via rustup update, I ran into a new lint that doesn't seem to like returning impl SomeTrait: https://rust-lang.github.io/rust-clippy/master/index.html#/let_with_type_underscore

So tracing, when used like this is causing this lint to trigger.

#[tracing::instrument(name = "Login form", skip(flashes))]
pub async fn login_form(flashes: IncomingFlashes) -> impl IntoResponse {
//...
}

[sandhose]

The code which triggers this lint is generated here:

tracing/tracing-attributes/src/expand.rs

Lines 60 to 73 in e603c2a

	// Install a fake return statement as the first thing in the function
	// body, so that we eagerly infer that the return type is what we
	// declared in the async fn signature.
	// The `#[allow(..)]` is given because the return statement is
	// unreachable, but does affect inference, so it needs to be written
	// exactly that way for it to do its magic.
	let fake_return_edge = quote_spanned! {return_span=>
	#[allow(unreachable_code, clippy::diverging_sub_expression, clippy::let_unit_value, clippy::unreachable)]
	if false {
	let __tracing_attr_fake_return: #return_type =
	unreachable!("this is just for type inference, and is unreachable code");
	return __tracing_attr_fake_return;
	}
	};

When the return type is an impl Trait, the #return_type here is _.

I think there are two solutions for this:

• either add the clippy::let_with_type_underscore to the allowed lints
• or completely skip this block when the return type is an impl Trait, because that block doesn't help with inference in those cases?

[mladedav]

This issue seems to have been fixed #2609.

However, what this did to us is clippy now fails for us because we're using an older rust version, where this lint is not present yet.

I don't think adding a warning to compilation is a breaking change, but it might be worthwhile to also add #![allow(unknown_lints)] for this and future similar cases where we do not want to break all compilations, right?

[mladedav]

This issue has been fixed and can be closed now.