Support impl Trait in return type

Cargo.toml

@@ -18,7 +18,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0.74"
 quote = "1.0.35"
-syn = { version = "2.0.46", default-features = false, features = ["full", "parsing", "printing", "proc-macro", "visit-mut"] }
+syn = { version = "2.0.46", default-features = false, features = ["clone-impls", "full", "parsing", "printing", "proc-macro", "visit-mut"] }
 
 [dev-dependencies]
 futures = "0.3.30"

src/expand.rs

@@ -12,7 +12,7 @@ use syn::visit_mut::{self, VisitMut};
 use syn::{
     parse_quote, parse_quote_spanned, Attribute, Block, FnArg, GenericArgument, GenericParam,
     Generics, Ident, ImplItem, Lifetime, LifetimeParam, Pat, PatIdent, PathArguments, Receiver,
-    ReturnType, Signature, Token, TraitItem, Type, TypePath, WhereClause,
+    ReturnType, Signature, Token, TraitItem, Type, TypeInfer, TypePath, WhereClause,
 };
 
 impl ToTokens for Item {
@@ -410,6 +410,8 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
                     quote!(#(#decls)* { #(#stmts)* })
                 }
             } else {
+                let mut ret = ret.clone();
+                replace_impl_trait_with_infer(&mut ret);
                 quote! {
                     if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<#ret> {
                         #[allow(unreachable_code)]
@@ -475,3 +477,20 @@ fn where_clause_or_default(clause: &mut Option<WhereClause>) -> &mut WhereClause
         predicates: Punctuated::new(),
     })
 }
+
+fn replace_impl_trait_with_infer(ty: &mut Type) {
+    struct ReplaceImplTraitWithInfer;
+
+    impl VisitMut for ReplaceImplTraitWithInfer {
+        fn visit_type_mut(&mut self, ty: &mut Type) {
+            if let Type::ImplTrait(impl_trait) = ty {
+                *ty = Type::Infer(TypeInfer {
+                    underscore_token: Token![_](impl_trait.impl_token.span),
+                });
+            }
+            visit_mut::visit_type_mut(self, ty);
+        }
+    }
+
+    ReplaceImplTraitWithInfer.visit_type_mut(ty);
+}