attributes: fix clippy::let_with_type_underscore in generated code (#…

…2609)

Co-authored-by: Hayden Stainsby <hds@caffeineconcepts.com>
Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing-attributes/src/expand.rs

@@ -64,7 +64,10 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
     // 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)]
+        #[allow(
+            unreachable_code, clippy::diverging_sub_expression, clippy::let_unit_value,
+            clippy::unreachable, clippy::let_with_type_underscore
+        )]
         if false {
             let __tracing_attr_fake_return: #return_type =
                 unreachable!("this is just for type inference, and is unreachable code");