From 6a9e39e4852c1781903a6bd0dd3f849b87a58ed4 Mon Sep 17 00:00:00 2001
From: Paul DeLucia <69597248+pauldelucia@users.noreply.github.com>
Date: Mon, 20 Apr 2026 17:16:35 +0700
Subject: [PATCH] fix(serdes): usize serializes as fixed-width u64 on 32-bit
 targets
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The `exp_serde_for_number!(usize, 8)` macro expands to a `usize::from_le_bytes([u8; 8])` call that fails on wasm32 (and any other 32-bit target) where `usize::from_le_bytes` takes `[u8; 4]`.

Proofs are produced on 64-bit hosts, so the wire format already stores usize as 8 bytes; this change just makes the verifier decode them correctly on 32-bit targets by reading a u64 and try_from-ing to usize. Values above `usize::MAX` on 32-bit (4 GiB) decode to `SerdeError::DeserializeError`.

Unblocks WASM verifier builds — the verifier needs to deserialize `Circuit<C>` and `Proof` on wasm32-unknown-unknown for browser/mobile full-GKR verification.
---
 serdes/src/serdes.rs | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/serdes/src/serdes.rs b/serdes/src/serdes.rs
index 0de383403..cbf903311 100644
--- a/serdes/src/serdes.rs
+++ b/serdes/src/serdes.rs
@@ -32,7 +32,20 @@ impl ExpSerde for () {
 }
 
 exp_serde_for_number!(u64, 8);
-exp_serde_for_number!(usize, 8);
+// `usize` serializes as a fixed-width u64 so proofs produced on 64-bit
+// hosts (prover) decode on 32-bit targets too (wasm32-unknown-unknown
+// consumers — mobile, browser, any WASM verifier). The generic
+// `exp_serde_for_number!(usize, 8)` macro hits an 8-vs-4 byte mismatch
+// on 32-bit targets where `usize::from_le_bytes` takes `[u8; 4]`.
+impl ExpSerde for usize {
+    fn serialize_into<W: Write>(&self, writer: W) -> SerdeResult<()> {
+        (*self as u64).serialize_into(writer)
+    }
+    fn deserialize_from<R: Read>(mut reader: R) -> SerdeResult<Self> {
+        let v = u64::deserialize_from(&mut reader)?;
+        usize::try_from(v).map_err(|_| SerdeError::DeserializeError)
+    }
+}
 exp_serde_for_number!(u8, 1);
 exp_serde_for_number!(f64, 8);
 exp_serde_for_number!(u128, 16);