attributes: Apply fake return edge only to async fns

The fake return edge was added in tokio-rs#2270 to improve type
errors in instrumented async functions. However, it meant that the user
block was being modified outside of `gen_block`. This created a negative
interaction with tokio-rs#1614, which suppressed a clippy lint
internally while explicitly enabling it on the user block. The installed
fake return edge generated this same lint, but the user had no way to
suppress it: lint directives above the instrumentation were ignored
because of the internal suppression, and lints inside the user block
could not influence the fake return edge's scope.

We now avoid modifying the user block outside of `gen_block`, and
restrict the fake return edge to async functions. We also apply the same
clippy lint suppression technique to the installed fake return edge.

Closes tokio-rs#2410.

tracing-attributes/src/expand.rs

@@ -14,6 +14,39 @@ use crate::{
     MaybeItemFn, MaybeItemFnRef,
 };
 
+#[derive(Debug, Default)]
+struct AsyncContext {
+    fake_return_edge: Option<proc_macro2::TokenStream>,
+}
+
+impl AsyncContext {
+    /// Adds a fake return statement that can be installed as the first thing in the
+    /// function body, so that we eagerly infer that the return type is what was declared
+    /// in the async fn signature.
+    fn with_fake_return_edge(mut self, ident: &Ident, output: &ReturnType) -> Self {
+        let (return_type, return_span) = if let ReturnType::Type(_, return_type) = &output {
+            (erase_impl_trait(return_type), return_type.span())
+        } else {
+            // Point at function name if we don't have an explicit return type
+            (syn::parse_quote! { () }, ident.span())
+        };
+        // 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)]
+            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;
+            }
+        };
+
+        self.fake_return_edge = Some(fake_return_edge);
+        self
+    }
+}
+
 /// Given an existing function, generate an instrumented version of that function
 pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
     input: MaybeItemFnRef<'a, B>,
@@ -51,37 +84,13 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
 
     let warnings = args.warnings();
 
-    let (return_type, return_span) = if let ReturnType::Type(_, return_type) = &output {
-        (erase_impl_trait(return_type), return_type.span())
-    } else {
-        // Point at function name if we don't have an explicit return type
-        (syn::parse_quote! { () }, ident.span())
-    };
-    // 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)]
-        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;
-        }
-    };
-    let block = quote! {
-        {
-            #fake_return_edge
-            #block
-        }
-    };
+    let async_context =
+        asyncness.map(|_| AsyncContext::default().with_fake_return_edge(ident, output));
 
     let body = gen_block(
-        &block,
+        block,
         params,
-        asyncness.is_some(),
+        async_context,
         args,
         instrumented_function_name,
         self_type,
@@ -103,7 +112,7 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
 fn gen_block<B: ToTokens>(
     block: &B,
     params: &Punctuated<FnArg, Token![,]>,
-    async_context: bool,
+    async_context: Option<AsyncContext>,
     mut args: InstrumentArgs,
     instrumented_function_name: &str,
     self_type: Option<&TypePath>,
@@ -257,7 +266,30 @@ fn gen_block<B: ToTokens>(
     // If `err` is in args, instrument any resulting `Err`s.
     // If `ret` is in args, instrument any resulting `Ok`s when the function
     // returns `Result`s, otherwise instrument any resulting values.
-    if async_context {
+    if let Some(context) = async_context {
+        let block = if let Some(fake_return_edge) = context.fake_return_edge {
+            // Install the fake return edge.
+            quote! {
+                {
+                    // Because `quote` produces a stream of tokens _without_ whitespace,
+                    // the `if` and the block will appear directly next to each other.
+                    // This generates a clippy lint about suspicious `if/else` formatting.
+                    // Therefore, suppress the lint inside the generated code...
+                    #[allow(clippy::suspicious_else_formatting)]
+                    {
+                        #fake_return_edge
+                        // ...but turn the lint back on inside the function body.
+                        #[warn(clippy::suspicious_else_formatting)]
+                        #block
+                    }
+                }
+            }
+        } else {
+            // Re-quote the block for type matching.
+            quote! {
+                #block
+            }
+        };
         let mk_fut = match (err_event, ret_event) {
             (Some(err_event), Some(ret_event)) => quote_spanned!(block.span()=>
                 async move {
@@ -697,7 +729,7 @@ impl<'block> AsyncInfo<'block> {
                     let instrumented_block = gen_block(
                         &async_expr.block,
                         &self.input.sig.inputs,
-                        true,
+                        Some(AsyncContext::default()),
                         args,
                         instrumented_function_name,
                         None,