From a687464ea1f241f69611ea9603d70bc2943208fc Mon Sep 17 00:00:00 2001
From: Michael Sproul <michael@sigmaprime.io>
Date: Wed, 3 Aug 2022 13:08:03 +1000
Subject: [PATCH 1/2] Pre-allocate vectors in SSZ decoding

---
 consensus/ssz/src/decode/try_from_iter.rs | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/consensus/ssz/src/decode/try_from_iter.rs b/consensus/ssz/src/decode/try_from_iter.rs
index 22db02d4fc8..dc33b2538be 100644
--- a/consensus/ssz/src/decode/try_from_iter.rs
+++ b/consensus/ssz/src/decode/try_from_iter.rs
@@ -29,11 +29,18 @@ pub trait TryFromIter<T>: Sized {
 impl<T> TryFromIter<T> for Vec<T> {
     type Error = Infallible;
 
-    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    fn try_from_iter<I>(values: I) -> Result<Self, Self::Error>
     where
         I: IntoIterator<Item = T>,
     {
-        Ok(Self::from_iter(iter))
+        // Pre-allocate the max size of the Vec. We expect to receive vastly more valid
+        // messages to decode, and the max length has already been vetted by
+        // `decode_list_of_variable_length_items`.
+        let iter = values.into_iter();
+        let (_, opt_max_len) = iter.size_hint();
+        let mut vec = Vec::with_capacity(opt_max_len.unwrap_or(0));
+        vec.extend(iter);
+        Ok(vec)
     }
 }
 

From ce5ecf9ca67013b1bb9245de54d4ebcf55b85161 Mon Sep 17 00:00:00 2001
From: Michael Sproul <michael@sigmaprime.io>
Date: Wed, 14 Sep 2022 11:23:50 +1000
Subject: [PATCH 2/2] Clarify comment (thanks Sean)

---
 consensus/ssz/src/decode/try_from_iter.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/consensus/ssz/src/decode/try_from_iter.rs b/consensus/ssz/src/decode/try_from_iter.rs
index dc33b2538be..1ff89a107f4 100644
--- a/consensus/ssz/src/decode/try_from_iter.rs
+++ b/consensus/ssz/src/decode/try_from_iter.rs
@@ -33,9 +33,9 @@ impl<T> TryFromIter<T> for Vec<T> {
     where
         I: IntoIterator<Item = T>,
     {
-        // Pre-allocate the max size of the Vec. We expect to receive vastly more valid
-        // messages to decode, and the max length has already been vetted by
-        // `decode_list_of_variable_length_items`.
+        // Pre-allocate the expected size of the Vec, which is parsed from the SSZ input bytes as
+        // `num_items`. This length has already been checked to be less than or equal to the type's
+        // maximum length in `decode_list_of_variable_length_items`.
         let iter = values.into_iter();
         let (_, opt_max_len) = iter.size_hint();
         let mut vec = Vec::with_capacity(opt_max_len.unwrap_or(0));