attributes: generate less dead code for async block return type hint (#…

…2709)

## Motivation

`#[tracing::instrument]` uses `unreachable!()` macro which needlessly
expands to panicking and formatting code. It only needs any `!` type.

## Solution

`loop {}` works just as well for a `!` type, and it crates less work for
the compiler. The code is truly unreachable, so the message would never
be useful. Rust used to be concerned about semantics of empty loops in
LLVM, but this [has been solved](https://reviews.llvm.org/D85393).

tracing-attributes/src/expand.rs

@@ -66,11 +66,11 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
     let fake_return_edge = quote_spanned! {return_span=>
         #[allow(
             unknown_lints, unreachable_code, clippy::diverging_sub_expression,
-            clippy::let_unit_value, clippy::unreachable, clippy::let_with_type_underscore
+            clippy::let_unit_value, clippy::unreachable, clippy::let_with_type_underscore,
+            clippy::empty_loop
         )]
         if false {
-            let __tracing_attr_fake_return: #return_type =
-                unreachable!("this is just for type inference, and is unreachable code");
+            let __tracing_attr_fake_return: #return_type = loop {};
             return __tracing_attr_fake_return;
         }
     };