feat(transformer/statement-injector): add an assertion to check if it…

… still contains statements that don't inject (oxc-project#9063)

When there are still statements that aren't inserted after the visitor ends, it means statements inserted with an incorrect address or outdated address (e.g., the original statement has mutated to a new statement).

This PR adds an assertion to expose this potential bug as soon as possible on the development side.

crates/oxc_transformer/src/common/mod.rs

@@ -46,6 +46,7 @@ impl<'a> Traverse<'a> for Common<'a, '_> {
         self.var_declarations.exit_program(program, ctx);
         self.top_level_statements.exit_program(program, ctx);
         self.arrow_function_converter.exit_program(program, ctx);
+        self.statement_injector.exit_program(program, ctx);
     }
 
     fn enter_statements(

crates/oxc_transformer/src/common/statement_injector.rs

@@ -41,6 +41,11 @@ impl<'a> Traverse<'a> for StatementInjector<'a, '_> {
     ) {
         self.ctx.statement_injector.insert_into_statements(statements, ctx);
     }
+
+    #[inline]
+    fn exit_program(&mut self, _program: &mut Program<'a>, _ctx: &mut TraverseCtx<'a>) {
+        self.ctx.statement_injector.assert_no_insertions_remaining();
+    }
 }
 
 #[derive(Debug)]
@@ -195,4 +200,12 @@ impl<'a> StatementInjectorStore<'a> {
 
         *statements = new_statements;
     }
+
+    // Assertion for checking if no remaining insertions are left.
+    // `#[inline(always)]` because this is a no-op in release mode
+    #[expect(clippy::inline_always)]
+    #[inline(always)]
+    fn assert_no_insertions_remaining(&self) {
+        debug_assert!(self.insertions.borrow().is_empty());
+    }
 }