Work around lifetime that does not appear in bounds

Author: dtolnay

src/expand.rs

@@ -1,4 +1,4 @@
-use crate::lifetime::CollectLifetimes;
+use crate::lifetime::{has_async_lifetime, CollectLifetimes};
 use crate::parse::Item;
 use crate::receiver::{has_self_in_block, has_self_in_sig, ReplaceReceiver};
 use proc_macro2::{Span, TokenStream};
@@ -249,11 +249,13 @@ fn transform_block(context: Context, sig: &mut MethodSig, block: &mut Block, has
             .extend(where_clause.predicates);
     }
 
-    standalone
-        .decl
-        .generics
-        .params
-        .push(parse_quote!('async_trait));
+    if has_async_lifetime(&mut standalone, block) {
+        standalone
+            .decl
+            .generics
+            .params
+            .push(parse_quote!('async_trait));
+    }
 
     let mut types = standalone
         .decl

src/lifetime.rs

@@ -1,6 +1,25 @@
 use proc_macro2::Span;
 use syn::visit_mut::{self, VisitMut};
-use syn::{ArgSelfRef, GenericArgument, Lifetime, TypeReference};
+use syn::{ArgSelfRef, Block, GenericArgument, Item, Lifetime, MethodSig, TypeReference};
+
+pub fn has_async_lifetime(sig: &mut MethodSig, block: &mut Block) -> bool {
+    let mut visitor = HasAsyncLifetime(false);
+    visitor.visit_method_sig_mut(sig);
+    visitor.visit_block_mut(block);
+    visitor.0
+}
+
+struct HasAsyncLifetime(bool);
+
+impl VisitMut for HasAsyncLifetime {
+    fn visit_lifetime_mut(&mut self, life: &mut Lifetime) {
+        self.0 |= life.to_string() == "'async_trait";
+    }
+
+    fn visit_item_mut(&mut self, _: &mut Item) {
+        // Do not recurse into nested items.
+    }
+}
 
 pub struct CollectLifetimes {
     pub lifetimes: Vec<Lifetime>,