fix: generate better code.

tasks/ast_codegen/src/generators/derive_clone_in.rs

@@ -103,18 +103,30 @@ where
     F: FnOnce() -> (/* allocator name */ Ident, TokenStream),
 {
     let (alloc, body) = body();
-    let (old_lifetime, new_lifetime) = if has_lifetime {
-        (quote!(<'old_alloc>), quote!(<'new_alloc>))
+    let (impl_lifetimes, trait_lifetime, old_lifetime, new_lifetime, alloc_ref) = if has_lifetime {
+        (
+            quote!(<'old_alloc, 'new_alloc>),
+            quote!(<'new_alloc>),
+            quote!(<'old_alloc>),
+            quote!(<'new_alloc>),
+            quote!(&'new_alloc),
+        )
     } else {
-        Default::default()
+        (
+            quote!(<'alloc>),
+            quote!(<'alloc>),
+            TokenStream::default(),
+            TokenStream::default(),
+            quote!(&'alloc),
+        )
     };
 
     quote! {
         endl!();
-        impl<'old_alloc, 'new_alloc> CloneIn<'new_alloc> for #ty_ident #old_lifetime {
+        impl #impl_lifetimes CloneIn #trait_lifetime for #ty_ident #old_lifetime {
             type Cloned = #ty_ident #new_lifetime;
 
-            fn clone_in(&self, #alloc: &'new_alloc Allocator) -> Self::Cloned {
+            fn clone_in(&self, #alloc: #alloc_ref Allocator) -> Self::Cloned {
                 #body
             }
         }