From 1a2c8163e81a0464b04cad0c01e3740e258d3abb Mon Sep 17 00:00:00 2001
From: Ivan Babrou <ivan@cloudflare.com>
Date: Fri, 10 Oct 2025 19:09:55 -0700
Subject: [PATCH] Properly size the destination in flatten_segments

We've observed unexpected reallocations on some flamegraphs here.
---
 capnp/src/serialize.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/capnp/src/serialize.rs b/capnp/src/serialize.rs
index 803303e23..0290a7f06 100644
--- a/capnp/src/serialize.rs
+++ b/capnp/src/serialize.rs
@@ -536,7 +536,7 @@ where
 
 #[cfg(feature = "alloc")]
 fn flatten_segments<R: message::ReaderSegments + ?Sized>(segments: &R) -> alloc::vec::Vec<u8> {
-    let word_count = compute_serialized_size(segments);
+    let word_count = compute_serialized_size(segments) * BYTES_PER_WORD;
     let segment_count = segments.len();
     let table_size = segment_count / 2 + 1;
     let mut result = alloc::vec::Vec::with_capacity(word_count);
@@ -549,6 +549,10 @@ fn flatten_segments<R: message::ReaderSegments + ?Sized>(segments: &R) -> alloc:
         let segment = segments.get_segment(i as u32).unwrap();
         result.extend(segment);
     }
+    debug_assert!(
+        result.capacity() == word_count,
+        "unexpected result capacity growth"
+    );
     result
 }