Apply the static level optimization to async fns and error printing

Author: oli-obk

tracing-attributes/src/lib.rs

@@ -516,7 +516,9 @@ fn gen_block(
         if err {
             quote_spanned!(block.span()=>
                 let __tracing_attr_span = #span;
-                tracing::Instrument::instrument(async move {
+                // See comment on the default case at the end of this function
+                // for why we do this a bit roundabout.
+                let fut = async move {
                     match async move { #block }.await {
                         #[allow(clippy::unit_arg)]
                         Ok(x) => Ok(x),
@@ -525,22 +527,46 @@ fn gen_block(
                             Err(e)
                         }
                     }
-                }, __tracing_attr_span).await
+                };
+                if tracing::level_enabled!(#level) {
+                    tracing::Instrument::instrument(
+                        fut,
+                        __tracing_attr_span
+                    )
+                    .await
+                } else {
+                    fut.await
+                }
             )
         } else {
             quote_spanned!(block.span()=>
                 let __tracing_attr_span = #span;
+                // See comment on the default case at the end of this function
+                // for why we do this a bit roundabout.
+                let fut = async move { #block };
+                if tracing::level_enabled!(#level) {
                     tracing::Instrument::instrument(
-                        async move { #block },
+                        fut,
                         __tracing_attr_span
                     )
                     .await
+                } else {
+                    fut.await
+                }
             )
         }
     } else if err {
         quote_spanned!(block.span()=>
-            let __tracing_attr_span = #span;
-            let __tracing_attr_guard = __tracing_attr_span.enter();
+            // See comment on the default case at the end of this function
+            // for why we do this a bit roundabout.
+            let __tracing_attr_span;
+            let __tracing_attr_guard;
+            if tracing::level_enabled!(#level) {
+                __tracing_attr_span = #span;
+                __tracing_attr_guard = __tracing_attr_span.enter();
+            }
+            // pacify clippy::suspicious_else_formatting
+            let _ = ();
             #[allow(clippy::redundant_closure_call)]
             match (move || #block)() {
                 #[allow(clippy::unit_arg)]