From bf1a93f44422d6f048f185d36d30f5acb76d5d94 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Wed, 17 Apr 2019 14:00:00 +1000
Subject: [PATCH] Allocate correctly for tree hash

---
 eth2/utils/tree_hash_derive/src/lib.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/eth2/utils/tree_hash_derive/src/lib.rs b/eth2/utils/tree_hash_derive/src/lib.rs
index 4b7761f91..343287313 100644
--- a/eth2/utils/tree_hash_derive/src/lib.rs
+++ b/eth2/utils/tree_hash_derive/src/lib.rs
@@ -166,11 +166,12 @@ pub fn tree_hash_signed_root_derive(input: TokenStream) -> TokenStream {
     };
 
     let idents = get_signed_root_named_field_idents(&struct_data);
+    let num_elems = idents.len();
 
     let output = quote! {
         impl tree_hash::SignedRoot for #name {
             fn signed_root(&self) -> Vec<u8> {
-                let mut leaves = Vec::with_capacity(4 * tree_hash::HASHSIZE);
+                let mut leaves = Vec::with_capacity(#num_elems * tree_hash::HASHSIZE);
 
                 #(
                     leaves.append(&mut self.#idents.tree_hash_root());