Decrease peak mem usage when generating mod tree and add breaking note to changelog

Author: YuhanLiin

CHANGELOG.md

@@ -11,15 +11,15 @@
 
 ### Changed
 
-- Top-level APIs now return `micropb_gen::Error` instead of `io::Error`
-- Split `container-heapless` into `container-heapless-0-8` and `container-heapless-0-9`
-- Change `container-arrayvec` to `container-arrayvec-0-7`
+- **BREAKING:** Top-level APIs now return `micropb_gen::Error` instead of `io::Error`
+- **BREAKING:** Split `container-heapless` into `container-heapless-0-8` and `container-heapless-0-9`
+- **BREAKING:** Change `container-arrayvec` to `container-arrayvec-0-7`
 - `use_container_heapless` now sets `map_type` to `heapless::index_map::FnvIndexMap`, which is the correct path in v0.9
 - Bump MSRV to 1.88
 
 ### Removed
 
-- Remove `micropb` re-exports of `heapless` and `arrayvec`
+- **BREAKING:** Remove `micropb` re-exports of `heapless` and `arrayvec`
 
 ## 0.5.1
 
@@ -31,12 +31,12 @@
 
 ### Removed
 
-- Remove `field_lifetime` config
-- Remove `recursive_field` config
+- **BREAKING:** Remove `field_lifetime` config
+- **BREAKING:** Remove `recursive_field` config
 
 ### Changed
 
-- `MAX_SIZE` changed from `Option<usize>` to `Result<usize, &str>` for reporting why the max size wasn't generated
+- **BREAKING:** `MAX_SIZE` changed from `Option<usize>` to `Result<usize, &str>` for reporting why the max size wasn't generated
 - Lifetime params are now generated for parent messages if their child messages have lifetimes
 - Applying `no_debug_impl`, `no_clone_impl`, and `no_partial_eq_impl` on a message will also apply to all its ancestors
 - Recursively nested messages are automatically detected and prevented by boxing the field

micropb-gen/src/generator.rs

@@ -293,17 +293,13 @@ impl<'proto> Context<'proto> {
     }
 
     fn generate_mod_tree(&self, mod_node: &mut Node<TokenStream>) -> TokenStream {
-        let code = mod_node.value_mut().take().unwrap_or_default();
-        let submods = mod_node.children_mut().map(|(submod_name, inner_node)| {
+        let mut code = mod_node.value_mut().take().unwrap_or_default();
+        for (submod_name, inner_node) in mod_node.children_mut() {
             let submod_name = resolve_path_elem(submod_name, self.params.suffixed_package_names);
             let inner = self.generate_mod_tree(inner_node);
-            quote! { pub mod #submod_name { #inner } }
-        });
-
-        quote! {
-            #code
-            #(#submods)*
+            code.extend(quote! { pub mod #submod_name { #inner } });
         }
+        code
     }
 
     fn generate_enum(&self, e: Option<&Enum>) -> TokenStream {