Extend Circuit trait to take parameters in config

The Circuit trait is extended with the following:
```
pub trait Circuit<F: Field> {
    /// [...]
    type Params: Default;

    fn params(&self) -> Self::Params {
        Self::Params::default()
    }

    fn configure_with_params(meta: &mut ConstraintSystem<F>, params: &Self::Params) -> Self::Config {
        Self::configure(meta)
    }

    fn configure(meta: &mut ConstraintSystem<F>) -> Self::Config;
}
```

This allows runtime parametrization of the circuit configuration.  The extension to the Circuit trait has been designed to minimize the breaking change: existing circuits only need to define the associated `type Params`.

Unfortunately "Associated type defaults" are unstable in Rust, otherwise this would be a non-breaking change.  See rust-lang/rust#29661

halo2_gadgets/benches/poseidon.rs

@@ -53,6 +53,7 @@ where
 {
     type Config = MyConfig<WIDTH, RATE, L>;
     type FloorPlanner = SimpleFloorPlanner;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self {

halo2_gadgets/benches/sha256.rs

@@ -37,6 +37,7 @@ fn bench(name: &str, k: u32, c: &mut Criterion) {
     impl Circuit<pallas::Base> for MyCircuit {
         type Config = Table16Config;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             Self::default()

halo2_gadgets/src/ecc.rs

@@ -731,6 +731,7 @@ pub(crate) mod tests {
     impl Circuit<pallas::Base> for MyCircuit {
         type Config = EccConfig<TestFixedBases>;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             MyCircuit { test_errors: false }

halo2_gadgets/src/ecc/chip/mul_fixed/short.rs

@@ -434,6 +434,7 @@ pub mod tests {
         impl Circuit<pallas::Base> for MyCircuit {
             type Config = EccConfig<TestFixedBases>;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 Self::default()

halo2_gadgets/src/poseidon/pow5.rs

@@ -620,6 +620,7 @@ mod tests {
     {
         type Config = Pow5Config<Fp, WIDTH, RATE>;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             PermuteCircuit::<S, WIDTH, RATE>(PhantomData)
@@ -735,6 +736,7 @@ mod tests {
     {
         type Config = Pow5Config<Fp, WIDTH, RATE>;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             Self {

halo2_gadgets/src/sha256/table16.rs

@@ -468,6 +468,7 @@ mod tests {
         impl Circuit<pallas::Base> for MyCircuit {
             type Config = Table16Config;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_gadgets/src/sha256/table16/compression.rs

@@ -954,6 +954,7 @@ mod tests {
         impl Circuit<pallas::Base> for MyCircuit {
             type Config = Table16Config;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_gadgets/src/sha256/table16/message_schedule.rs

@@ -411,6 +411,7 @@ mod tests {
         impl Circuit<pallas::Base> for MyCircuit {
             type Config = Table16Config;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_gadgets/src/sha256/table16/spread_table.rs

@@ -304,6 +304,7 @@ mod tests {
         impl<F: PrimeField> Circuit<F> for MyCircuit {
             type Config = SpreadTableConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_gadgets/src/sinsemilla.rs

@@ -525,6 +525,7 @@ pub(crate) mod tests {
             SinsemillaConfig<TestHashDomain, TestCommitDomain, TestFixedBases>,
         );
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             MyCircuit {}

halo2_gadgets/src/sinsemilla/merkle.rs

@@ -213,6 +213,7 @@ pub mod tests {
             MerkleConfig<TestHashDomain, TestCommitDomain, TestFixedBases>,
         );
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             Self::default()

halo2_gadgets/src/utilities.rs

@@ -271,6 +271,7 @@ mod tests {
         impl<const RANGE: usize> Circuit<pallas::Base> for MyCircuit<RANGE> {
             type Config = Config;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit(self.0)

halo2_gadgets/src/utilities/cond_swap.rs

@@ -217,6 +217,7 @@ mod tests {
         impl<F: PrimeField> Circuit<F> for MyCircuit<F> {
             type Config = CondSwapConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 Self::default()

halo2_gadgets/src/utilities/decompose_running_sum.rs

@@ -243,6 +243,7 @@ mod tests {
         {
             type Config = RunningSumConfig<F, WINDOW_NUM_BITS>;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 Self {

halo2_gadgets/src/utilities/lookup_range_check.rs

@@ -410,6 +410,7 @@ mod tests {
         impl<F: PrimeFieldBits> Circuit<F> for MyCircuit<F> {
             type Config = LookupRangeCheckConfig<F, K>;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 *self
@@ -506,6 +507,7 @@ mod tests {
         impl<F: PrimeFieldBits> Circuit<F> for MyCircuit<F> {
             type Config = LookupRangeCheckConfig<F, K>;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {

halo2_proofs/benches/dev_lookup.rs

@@ -28,6 +28,7 @@ fn criterion_benchmark(c: &mut Criterion) {
     impl<F: PrimeField> Circuit<F> for MyCircuit<F> {
         type Config = MyConfig;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             Self::default()

halo2_proofs/benches/plonk.rs

@@ -183,6 +183,7 @@ fn criterion_benchmark(c: &mut Criterion) {
     impl<F: Field> Circuit<F> for MyCircuit<F> {
         type Config = PlonkConfig;
         type FloorPlanner = SimpleFloorPlanner;
+        type Params = ();
 
         fn without_witnesses(&self) -> Self {
             Self {

halo2_proofs/examples/circuit-layout.rs

@@ -161,6 +161,7 @@ impl<FF: Field> StandardCs<FF> for StandardPlonk<FF> {
 impl<F: Field> Circuit<F> for MyCircuit<F> {
     type Config = PlonkConfig;
     type FloorPlanner = SimpleFloorPlanner;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self {

halo2_proofs/examples/serialization.rs

@@ -86,6 +86,7 @@ struct StandardPlonk(Fr);
 impl Circuit<Fr> for StandardPlonk {
     type Config = StandardPlonkConfig;
     type FloorPlanner = SimpleFloorPlanner;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self::default()
@@ -140,8 +141,12 @@ fn main() {
 
     let f = File::open("serialization-test.pk").unwrap();
     let mut reader = BufReader::new(f);
-    let pk = ProvingKey::<G1Affine>::read::<_, StandardPlonk>(&mut reader, SerdeFormat::RawBytes)
-        .unwrap();
+    let pk = ProvingKey::<G1Affine>::read::<_, StandardPlonk>(
+        &mut reader,
+        SerdeFormat::RawBytes,
+        &circuit.params(),
+    )
+    .unwrap();
 
     std::fs::remove_file("serialization-test.pk").unwrap();
 

halo2_proofs/examples/shuffle.rs

@@ -136,6 +136,7 @@ impl<F: Field, const W: usize, const H: usize> MyCircuit<F, W, H> {
 impl<F: Field, const W: usize, const H: usize> Circuit<F> for MyCircuit<F, W, H> {
     type Config = MyConfig<W>;
     type FloorPlanner = V1;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self::default()

halo2_proofs/examples/simple-example.rs

@@ -248,6 +248,7 @@ impl<F: Field> Circuit<F> for MyCircuit<F> {
     // Since we are using a single chip for everything, we can just reuse its config.
     type Config = FieldConfig;
     type FloorPlanner = SimpleFloorPlanner;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self::default()

halo2_proofs/examples/two-chip.rs

@@ -458,6 +458,7 @@ impl<F: Field> Circuit<F> for MyCircuit<F> {
     // Since we are using a single chip for everything, we can just reuse its config.
     type Config = FieldConfig;
     type FloorPlanner = SimpleFloorPlanner;
+    type Params = ();
 
     fn without_witnesses(&self) -> Self {
         Self::default()

halo2_proofs/src/circuit/floor_planner/single_pass.rs

@@ -479,6 +479,7 @@ mod tests {
         impl Circuit<vesta::Scalar> for MyCircuit {
             type Config = Column<Advice>;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_proofs/src/circuit/floor_planner/v1.rs

@@ -517,6 +517,7 @@ mod tests {
         impl Circuit<vesta::Scalar> for MyCircuit {
             type Config = Column<Advice>;
             type FloorPlanner = super::V1;
+            type Params = ();
 
             fn without_witnesses(&self) -> Self {
                 MyCircuit {}

halo2_proofs/src/dev.rs

@@ -208,6 +208,7 @@ impl<F: Field> Mul<F> for Value<F> {
 /// impl<F: PrimeField> Circuit<F> for MyCircuit {
 ///     type Config = MyConfig;
 ///     type FloorPlanner = SimpleFloorPlanner;
+///     type Params = ();
 ///
 ///     fn without_witnesses(&self) -> Self {
 ///         Self::default()
@@ -601,7 +602,7 @@ impl<F: FromUniformBytes<64> + Ord> MockProver<F> {
         let n = 1 << k;
 
         let mut cs = ConstraintSystem::default();
-        let config = ConcreteCircuit::configure(&mut cs);
+        let config = ConcreteCircuit::configure_with_params(&mut cs, &circuit.params());
         let cs = cs;
 
         assert!(
@@ -1536,6 +1537,7 @@ mod tests {
         impl Circuit<Fp> for FaultyCircuit {
             type Config = FaultyCircuitConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn configure(meta: &mut ConstraintSystem<Fp>) -> Self::Config {
                 let a = meta.advice_column();
@@ -1622,6 +1624,7 @@ mod tests {
         impl Circuit<Fp> for FaultyCircuit {
             type Config = FaultyCircuitConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn configure(meta: &mut ConstraintSystem<Fp>) -> Self::Config {
                 let a = meta.advice_column();
@@ -1791,6 +1794,7 @@ mod tests {
         impl Circuit<Fp> for FaultyCircuit {
             type Config = FaultyCircuitConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn configure(meta: &mut ConstraintSystem<Fp>) -> Self::Config {
                 let a = meta.advice_column();
@@ -1925,6 +1929,7 @@ mod tests {
         impl Circuit<Fp> for FaultyCircuit {
             type Config = FaultyCircuitConfig;
             type FloorPlanner = SimpleFloorPlanner;
+            type Params = ();
 
             fn configure(meta: &mut ConstraintSystem<Fp>) -> Self::Config {
                 let a = meta.advice_column();

halo2_proofs/src/dev/cost.rs

@@ -150,7 +150,7 @@ impl<G: PrimeGroup, ConcreteCircuit: Circuit<G::Scalar>> CircuitCost<G, Concrete
     pub fn measure(k: usize, circuit: &ConcreteCircuit) -> Self {
         // Collect the layout details.
         let mut cs = ConstraintSystem::default();
-        let config = ConcreteCircuit::configure(&mut cs);
+        let config = ConcreteCircuit::configure_with_params(&mut cs, &circuit.params());
         let mut assembly = Assembly {
             selectors: vec![vec![false; 1 << k]; cs.num_selectors],
         };

halo2_proofs/src/dev/gates.rs

@@ -49,6 +49,7 @@ struct Gate {
 /// impl<F: Field> Circuit<F> for MyCircuit {
 ///     type Config = MyConfig;
 ///     type FloorPlanner = SimpleFloorPlanner;
+///     type Params = ();
 ///
 ///     fn without_witnesses(&self) -> Self {
 ///         Self::default()
@@ -79,7 +80,7 @@ struct Gate {
 ///     }
 /// }
 ///
-/// let gates = CircuitGates::collect::<pallas::Base, MyCircuit>();
+/// let gates = CircuitGates::collect::<pallas::Base, MyCircuit>(&());
 /// assert_eq!(
 ///     format!("{}", gates),
 ///     r#####"R1CS constraint:
@@ -103,10 +104,10 @@ pub struct CircuitGates {
 
 impl CircuitGates {
     /// Collects the gates from within the circuit.
-    pub fn collect<F: PrimeField, C: Circuit<F>>() -> Self {
+    pub fn collect<F: PrimeField, C: Circuit<F>>(params: &C::Params) -> Self {
         // Collect the graph details.
         let mut cs = ConstraintSystem::default();
-        let _ = C::configure(&mut cs);
+        let _ = C::configure_with_params(&mut cs, params);
 
         let gates = cs
             .gates

halo2_proofs/src/plonk.rs

@@ -101,11 +101,12 @@ where
     pub fn read<R: io::Read, ConcreteCircuit: Circuit<C::Scalar>>(
         reader: &mut R,
         format: SerdeFormat,
+        params: &ConcreteCircuit::Params,
     ) -> io::Result<Self> {
         let mut k = [0u8; 4];
         reader.read_exact(&mut k)?;
         let k = u32::from_be_bytes(k);
-        let (domain, cs, _) = keygen::create_domain::<C, ConcreteCircuit>(k);
+        let (domain, cs, _) = keygen::create_domain::<C, ConcreteCircuit>(k, params);
         let mut num_fixed_columns = [0u8; 4];
         reader.read_exact(&mut num_fixed_columns)?;
         let num_fixed_columns = u32::from_be_bytes(num_fixed_columns);
@@ -150,8 +151,9 @@ where
     pub fn from_bytes<ConcreteCircuit: Circuit<C::Scalar>>(
         mut bytes: &[u8],
         format: SerdeFormat,
+        params: &ConcreteCircuit::Params,
     ) -> io::Result<Self> {
-        Self::read::<_, ConcreteCircuit>(&mut bytes, format)
+        Self::read::<_, ConcreteCircuit>(&mut bytes, format, params)
     }
 }
 
@@ -335,8 +337,9 @@ where
     pub fn read<R: io::Read, ConcreteCircuit: Circuit<C::Scalar>>(
         reader: &mut R,
         format: SerdeFormat,
+        params: &ConcreteCircuit::Params,
     ) -> io::Result<Self> {
-        let vk = VerifyingKey::<C>::read::<R, ConcreteCircuit>(reader, format)?;
+        let vk = VerifyingKey::<C>::read::<R, ConcreteCircuit>(reader, format, params)?;
         let l0 = Polynomial::read(reader, format)?;
         let l_last = Polynomial::read(reader, format)?;
         let l_active_row = Polynomial::read(reader, format)?;
@@ -369,8 +372,9 @@ where
     pub fn from_bytes<ConcreteCircuit: Circuit<C::Scalar>>(
         mut bytes: &[u8],
         format: SerdeFormat,
+        params: &ConcreteCircuit::Params,
     ) -> io::Result<Self> {
-        Self::read::<_, ConcreteCircuit>(&mut bytes, format)
+        Self::read::<_, ConcreteCircuit>(&mut bytes, format, params)
     }
 }
 

halo2_proofs/src/plonk/circuit.rs

@@ -664,13 +664,29 @@ pub trait Circuit<F: Field> {
     /// The floor planner used for this circuit. This is an associated type of the
     /// `Circuit` trait because its behaviour is circuit-critical.
     type FloorPlanner: FloorPlanner;
+    /// Optional circuit configuration parameters.
+    type Params: Default;
 
     /// Returns a copy of this circuit with no witness values (i.e. all witnesses set to
     /// `None`). For most circuits, this will be equal to `Self::default()`.
     fn without_witnesses(&self) -> Self;
 
+    /// Returns a reference to the parameters that should be used to configure the circuit.
+    fn params(&self) -> Self::Params {
+        Self::Params::default()
+    }
+
     /// The circuit is given an opportunity to describe the exact gate
     /// arrangement, column arrangement, etc.
+    fn configure_with_params(
+        meta: &mut ConstraintSystem<F>,
+        _params: &Self::Params,
+    ) -> Self::Config {
+        Self::configure(meta)
+    }
+
+    /// Configuration function without parameters.  This method is usually only called via the default
+    /// `configure_with_params` implementation for backwards-compatibility purposes.
     fn configure(meta: &mut ConstraintSystem<F>) -> Self::Config;
 
     /// Given the provided `cs`, synthesize the circuit. The concrete type of