Defer specialization by one level only

compiler/build/src/program.rs

@@ -227,8 +227,14 @@ pub fn build(
 
     // Populate Procs further and get the low-level Expr from the canonical Expr
     let main_body = Expr::new(&mut mono_env, loc_expr.value, &mut procs);
+    let mut headers = {
+        let num_headers = match &procs.pending_specializations {
+            Some(map) => map.len(),
+            None => 0,
+        };
 
-    let mut headers = Vec::with_capacity(procs.pending_specializations.len());
+        Vec::with_capacity(num_headers)
+    };
     let mut layout_cache = LayoutCache::default();
     let mut procs = roc_mono::expr::specialize_all(&mut mono_env, procs, &mut layout_cache);