From 5114aee5cf08e48137ebfe7da85c9960a4e7bc37 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Tue, 17 Nov 2020 04:28:40 +0000
Subject: [PATCH] Avoid allocations on VariableList (#1921)

## Issue Addressed

NA

## Proposed Changes

Avoids lots of grow allocations when decoding a `VariableList` of fixed-length items. This is the function used for decoding the `state.validators` list.

## Additional Info

NA
---
 consensus/ssz_types/src/variable_list.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/consensus/ssz_types/src/variable_list.rs b/consensus/ssz_types/src/variable_list.rs
index 774d4a358..599e4f5b6 100644
--- a/consensus/ssz_types/src/variable_list.rs
+++ b/consensus/ssz_types/src/variable_list.rs
@@ -247,8 +247,10 @@ where
 
             bytes
                 .chunks(T::ssz_fixed_len())
-                .map(|chunk| T::from_ssz_bytes(chunk))
-                .collect::<Result<Vec<_>, _>>()
+                .try_fold(Vec::with_capacity(num_items), |mut vec, chunk| {
+                    vec.push(T::from_ssz_bytes(chunk)?);
+                    Ok(vec)
+                })
                 .map(Into::into)
         } else {
             ssz::decode_list_of_variable_length_items(bytes, Some(max_len)).map(|vec| vec.into())