attributes: fix compile error with instrumented async functions

tracing-attributes/src/lib.rs

@@ -533,15 +533,16 @@ fn gen_block(
         };
 
         return quote_spanned!(block.span()=>
-            if tracing::level_enabled!(#level) {
-                let __tracing_attr_span = #span;
+            let __tracing_attr_span = #span;
+            let __tracing_instrument_future = #mk_fut;
+            if !__tracing_attr_span.is_disabled() {
                 tracing::Instrument::instrument(
-                    #mk_fut,
+                    __tracing_instrument_future,
                     __tracing_attr_span
                 )
                 .await
             } else {
-                #mk_fut.await
+                __tracing_instrument_future.await
             }
         );
     }

tracing-attributes/tests/async_fn.rs

@@ -15,6 +15,21 @@ async fn test_async_fn(polls: usize) -> Result<(), ()> {
     future.await
 }
 
+// Reproduces a compile error when returning an `impl Trait` from an
+// instrumented async fn (see https://github.com/tokio-rs/tracing/issues/1615)
+#[instrument]
+async fn test_ret_impl_trait(n: i32) -> Result<impl Iterator<Item = i32>, ()> {
+    let n = n;
+    Ok((0..10).filter(move |x| *x < n))
+}
+
+// Reproduces a compile error when returning an `impl Trait` from an
+// instrumented async fn (see https://github.com/tokio-rs/tracing/issues/1615)
+#[instrument(err)]
+async fn test_ret_impl_trait_err(n: i32) -> Result<impl Iterator<Item = i32>, &'static str> {
+    Ok((0..10).filter(move |x| *x < n))
+}
+
 #[instrument]
 async fn test_async_fn_empty() {}