Retain Rust code items from mod decorated with subxt attribute (#721)

* refactor: Simplify collecting type substitutes

* Simplify ItemMod::from

* Preserve inner Rust items when expanding the subxt::subxt macro

* No named lifetimes

* Update codegen/src/api/mod.rs

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Move passing UI tests under `ui_tests`

Co-authored-by: Andrew Jones <ascjones@gmail.com>

codegen/src/api/mod.rs

@@ -204,9 +204,7 @@ impl RuntimeGenerator {
         })
         .collect::<HashMap<_, _>>();
 
-        for (path, substitute) in item_mod_ir.type_substitutes().iter() {
-            type_substitutes.insert(path.to_string(), substitute.clone());
-        }
+        type_substitutes.extend(item_mod_ir.type_substitutes().into_iter());
 
         let type_gen = TypeGenerator::new(
             &self.metadata.types,
@@ -302,7 +300,7 @@ impl RuntimeGenerator {
             }
         };
 
-        let mod_ident = item_mod_ir.ident;
+        let mod_ident = &item_mod_ir.ident;
         let pallets_with_constants: Vec<_> = pallets_with_mod_names
             .iter()
             .filter_map(|(pallet, pallet_mod_name)| {
@@ -324,9 +322,14 @@ impl RuntimeGenerator {
             })
             .collect();
 
+        let rust_items = item_mod_ir.rust_items();
+
         quote! {
             #[allow(dead_code, unused_imports, non_camel_case_types)]
             pub mod #mod_ident {
+                // Preserve any Rust items that were previously defined in the adorned module
+                #( #rust_items ) *
+
                 // Make it easy to access the root via `root_mod` at different levels:
                 use super::#mod_ident as root_mod;
                 // Identify the pallets composing the static metadata by name.