From 7aa100978604d51dbff9b96d6ea921fa0962c3a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carlos=20P=C3=A9rez?=
 <37264926+CPerezz@users.noreply.github.com>
Date: Wed, 11 Jan 2023 13:11:59 +0100
Subject: [PATCH] fix: FailureLocation::find empty-region handling (#121)

After working on fixing
privacy-scaling-explorations/zkevm-circuits#1024, a bug was found in the
verification fn of the MockProver which implies that while finding a
FailureLocation, if a Region doesn't contain any rows.

This is fixed by introducing a 2-line solution suggested by @lispc.

Resolves: #117
---
 halo2_proofs/src/dev/failure.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/halo2_proofs/src/dev/failure.rs b/halo2_proofs/src/dev/failure.rs
index 2fcb2a1fc8..55505ae04c 100644
--- a/halo2_proofs/src/dev/failure.rs
+++ b/halo2_proofs/src/dev/failure.rs
@@ -90,6 +90,9 @@ impl FailureLocation {
             .iter()
             .enumerate()
             .find(|(_, r)| {
+                if r.rows.is_none() {
+                    return false;
+                }
                 let (start, end) = r.rows.unwrap();
                 // We match the region if any input columns overlap, rather than all of
                 // them, because matching complex selector columns is hard. As long as