From c023ba67e07283bad3a43ef9bcc0336c94aa42ae Mon Sep 17 00:00:00 2001
From: overlookmotel <557937+overlookmotel@users.noreply.github.com>
Date: Mon, 17 Nov 2025 08:57:54 +0000
Subject: [PATCH] fix(semantic): do not duplicate statements in temp `Vec` when
 binding `TSModuleDeclaration`s (#15724)

I *think* this was a bug, but I don't really understand the logic, so not 100% sure.

When binding a `TSModuleDeclaration`, references to all its statements are collected into a `Vec` called `module_declaration_stmts`:

https://github.com/oxc-project/oxc/blob/4608549fc7715ee76c4a6d7a78069ff339f52267/crates/oxc_semantic/src/binder.rs#L434-L454

But `module_declaration_stmts.extend(block.body.iter())` is *inside* the loop, meaning that *all* the statements are inserted over and over until a statement is found which instantiates the namespace.

e.g. this would push 25 `&Statement`s into the `Vec`:

```ts
namespace X {
  type A = string;
  type B = number;
  type C = string;
  type D = number;
  interface E {}
}
```

The number of entries pushed to the `Vec` rises exponentially with the number of statements in the namespace (100 statements -> 10,000 entries pushed).

I assume this is unintentional.

Move `module_declaration_stmts.extend(block.body.iter());` to before the loop.

---

Do the `&Statement`s need to be pushed before the loop? Could it be delayed until after? That would be more performant. Though, as I said, I don't understand the logic, so I'm not sure if that'd break it.
---
 crates/oxc_semantic/src/binder.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/crates/oxc_semantic/src/binder.rs b/crates/oxc_semantic/src/binder.rs
index 9378da62c094b..29f57a235c0ac 100644
--- a/crates/oxc_semantic/src/binder.rs
+++ b/crates/oxc_semantic/src/binder.rs
@@ -433,9 +433,10 @@ fn get_module_instance_state_impl<'a, 'b>(
     // A module is uninstantiated if it contains only specific declarations
     let state = match body {
         TSModuleDeclarationBody::TSModuleBlock(block) => {
+            module_declaration_stmts.extend(block.body.iter());
+
             let mut child_state = ModuleInstanceState::NonInstantiated;
             for stmt in &block.body {
-                module_declaration_stmts.extend(block.body.iter());
                 child_state = get_module_instance_state_for_statement(
                     builder,
                     stmt,