feat: add halo2_debug package (#346)

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "halo2_frontend",
     "halo2_middleware",
     "halo2_backend",
+    "halo2_debug",
     "p3_frontend",
 ]
 resolver = "2"

halo2_backend/src/plonk/circuit.rs

@@ -25,6 +25,12 @@ pub enum VarBack {
     Challenge(ChallengeMid),
 }
 
+impl std::fmt::Display for VarBack {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
 impl Variable for VarBack {
     fn degree(&self) -> usize {
         match self {
@@ -40,8 +46,8 @@ impl Variable for VarBack {
         }
     }
 
-    fn write_identifier<W: std::io::Write>(&self, _writer: &mut W) -> std::io::Result<()> {
-        unimplemented!("unused method")
+    fn write_identifier<W: std::io::Write>(&self, writer: &mut W) -> std::io::Result<()> {
+        write!(writer, "{}", self)
     }
 }
 

halo2_debug/Cargo.toml

@@ -0,0 +1,27 @@
+[package]
+name = "halo2_debug"
+version = "0.3.0"
+authors = [
+    "Privacy Scaling Explorations team",
+]
+edition = "2021"
+rust-version = "1.66.0"
+description = """
+Halo2 Debug.  This package contains utilities for debugging and testing within
+the halo2 ecosystem.
+"""
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/privacy-scaling-explorations/halo2"
+documentation = "https://privacy-scaling-explorations.github.io/halo2/"
+categories = ["cryptography"]
+keywords = ["halo", "proofs", "zkp", "zkSNARKs"]
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs", "--html-in-header", "katex-header.html"]
+
+[dependencies]
+ff = "0.13"
+halo2curves = { version = "0.6.1", default-features = false }
+num-bigint = "0.4.5"
+halo2_middleware = { path = "../halo2_middleware" }