From d66b5c42a05fec352593a34d4f26a9799165cdc5 Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@electriccoin.co>
Date: Thu, 4 Nov 2021 13:38:38 +0000
Subject: [PATCH] `impl std::error::Error for Error`

---
 CHANGELOG.md       |  1 +
 src/plonk/error.rs | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 49459e62..ebd97dee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ and this project adheres to Rust's notion of
 ## [Unreleased]
 ### Changed
 - `halo2::plonk::Error` has been overhauled:
+  - `Error` now implements `std::fmt::Display` and `std::error::Error`.
   - `Error` no longer implements `PartialEq`. Tests can check for specific error
     cases with `assert!(matches!(..))`.
   - `Error::IncompatibleParams` is now `Error::InvalidInstances`.
diff --git a/src/plonk/error.rs b/src/plonk/error.rs
index 7058f79b..18f695e3 100644
--- a/src/plonk/error.rs
+++ b/src/plonk/error.rs
@@ -1,3 +1,5 @@
+use std::error;
+use std::fmt;
 use std::io;
 
 /// This is an error that could occur during proving or circuit synthesis.
@@ -32,3 +34,33 @@ impl From<io::Error> for Error {
         Error::Transcript(error)
     }
 }
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::Synthesis => write!(f, "General synthesis error"),
+            Error::InvalidInstances => write!(f, "Provided instances do not match the circuit"),
+            Error::ConstraintSystemFailure => write!(f, "The constraint system is not satisfied"),
+            Error::BoundsFailure => write!(f, "An out-of-bounds index was passed to the backend"),
+            Error::Opening => write!(f, "Multi-opening proof was invalid"),
+            Error::Transcript(e) => write!(f, "Transcript error: {}", e),
+            Error::NotEnoughRowsAvailable => write!(f, "`k` is too small for the given circuit"),
+            Error::InstanceTooLarge => write!(f, "Instance vectors are larger than the circuit"),
+            Error::NotEnoughColumnsForConstants => {
+                write!(
+                    f,
+                    "Too few fixed columns are enabled for global constants usage"
+                )
+            }
+        }
+    }
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self {
+            Error::Transcript(e) => Some(e),
+            _ => None,
+        }
+    }
+}