Replace `UtilitiesInstructions` usage with a dedicated helper

The new helper enables returning typed `AssignedCell`s, rather than only
`AssignedCell<F, F>`.

src/circuit.rs

@@ -4,7 +4,7 @@ use core::fmt;
 
 use group::{Curve, GroupEncoding};
 use halo2_proofs::{
-    circuit::{floor_planner, AssignedCell, Layouter},
+    circuit::{floor_planner, Layouter},
     plonk::{
         self, Advice, Column, Constraints, Expression, Instance as InstanceColumn, Selector,
         SingleVerifier,
@@ -18,7 +18,10 @@ use rand::RngCore;
 
 use self::{
     commit_ivk::{CommitIvkChip, CommitIvkConfig},
-    gadget::add_chip::{AddChip, AddConfig},
+    gadget::{
+        add_chip::{AddChip, AddConfig},
+        assign_free_advice,
+    },
     note_commit::NoteCommitConfig,
 };
 use crate::{
@@ -53,7 +56,7 @@ use halo2_gadgets::{
             MerklePath,
         },
     },
-    utilities::{lookup_range_check::LookupRangeCheckConfig, UtilitiesInstructions},
+    utilities::lookup_range_check::LookupRangeCheckConfig,
 };
 
 mod commit_ivk;
@@ -118,10 +121,6 @@ pub struct Circuit {
     pub(crate) rcv: Option<ValueCommitTrapdoor>,
 }
 
-impl UtilitiesInstructions<pallas::Base> for Circuit {
-    type Var = AssignedCell<pallas::Base, pallas::Base>;
-}
-
 impl plonk::Circuit<pallas::Base> for Circuit {
     type Config = Config;
     type FloorPlanner = floor_planner::V1;
@@ -332,14 +331,14 @@ impl plonk::Circuit<pallas::Base> for Circuit {
         // Witness private inputs that are used across multiple checks.
         let (psi_old, rho_old, cm_old, g_d_old, ak_P, nk, v_old, v_new) = {
             // Witness psi_old
-            let psi_old = self.load_private(
+            let psi_old = assign_free_advice(
                 layouter.namespace(|| "witness psi_old"),
                 config.advices[0],
                 self.psi_old,
             )?;
 
             // Witness rho_old
-            let rho_old = self.load_private(
+            let rho_old = assign_free_advice(
                 layouter.namespace(|| "witness rho_old"),
                 config.advices[0],
                 self.rho_old.map(|rho| rho.0),
@@ -368,21 +367,21 @@ impl plonk::Circuit<pallas::Base> for Circuit {
             )?;
 
             // Witness nk.
-            let nk = self.load_private(
+            let nk = assign_free_advice(
                 layouter.namespace(|| "witness nk"),
                 config.advices[0],
                 self.nk.map(|nk| nk.inner()),
             )?;
 
             // Witness v_old.
-            let v_old = self.load_private(
+            let v_old = assign_free_advice(
                 layouter.namespace(|| "witness v_old"),
                 config.advices[0],
                 self.v_old.map(|v_old| pallas::Base::from(v_old.inner())),
             )?;
 
             // Witness v_new.
-            let v_new = self.load_private(
+            let v_new = assign_free_advice(
                 layouter.namespace(|| "witness v_new"),
                 config.advices[0],
                 self.v_new.map(|v_new| pallas::Base::from(v_new.inner())),
@@ -426,12 +425,12 @@ impl plonk::Circuit<pallas::Base> for Circuit {
                     )
                 });
 
-                let magnitude = self.load_private(
+                let magnitude = assign_free_advice(
                     layouter.namespace(|| "v_net magnitude"),
                     config.advices[9],
                     magnitude_sign.map(|m_s| m_s.0),
                 )?;
-                let sign = self.load_private(
+                let sign = assign_free_advice(
                     layouter.namespace(|| "v_net sign"),
                     config.advices[9],
                     magnitude_sign.map(|m_s| m_s.1),
@@ -582,7 +581,7 @@ impl plonk::Circuit<pallas::Base> for Circuit {
             let rho_new = nf_old.inner().clone();
 
             // Witness psi_new
-            let psi_new = self.load_private(
+            let psi_new = assign_free_advice(
                 layouter.namespace(|| "witness psi_new"),
                 config.advices[0],
                 self.psi_new,

src/circuit/gadget.rs

@@ -1,5 +1,6 @@
 //! Gadgets used in the Orchard circuit.
 
+use ff::Field;
 use pasta_curves::pallas;
 
 use super::commit_ivk::CommitIvkChip;
@@ -18,7 +19,7 @@ use halo2_gadgets::{
 use halo2_proofs::{
     arithmetic::FieldExt,
     circuit::{AssignedCell, Chip, Layouter},
-    plonk,
+    plonk::{self, Advice, Assigned, Column},
 };
 
 pub(in crate::circuit) mod add_chip;
@@ -76,6 +77,32 @@ pub(in crate::circuit) trait AddInstruction<F: FieldExt>: Chip<F> {
     ) -> Result<AssignedCell<F, F>, plonk::Error>;
 }
 
+/// Witnesses the given value in a standalone region.
+///
+/// Usages of this helper are technically superfluous, as the single-cell region is only
+/// ever used in equality constraints. We could eliminate them with a write-on-copy
+/// abstraction (https://github.com/zcash/halo2/issues/334).
+pub(in crate::circuit) fn assign_free_advice<F: Field, V: Copy>(
+    mut layouter: impl Layouter<F>,
+    column: Column<Advice>,
+    value: Option<V>,
+) -> Result<AssignedCell<V, F>, plonk::Error>
+where
+    for<'v> Assigned<F>: From<&'v V>,
+{
+    layouter.assign_region(
+        || "load private",
+        |mut region| {
+            region.assign_advice(
+                || "load private",
+                column,
+                0,
+                || value.ok_or(plonk::Error::Synthesis),
+            )
+        },
+    )
+}
+
 /// `ValueCommit^Orchard` from [Section 5.4.8.3 Homomorphic Pedersen commitments (Sapling and Orchard)].
 ///
 /// [Section 5.4.8.3 Homomorphic Pedersen commitments (Sapling and Orchard)]: https://zips.z.cash/protocol/protocol.pdf#concretehomomorphiccommit

src/circuit/note_commit.rs

@@ -1488,9 +1488,12 @@ mod tests {
     use core::iter;
 
     use super::NoteCommitConfig;
-    use crate::constants::{
+    use crate::{
-        fixed_bases::NOTE_COMMITMENT_PERSONALIZATION, OrchardCommitDomains, OrchardFixedBases,
+        circuit::gadget::assign_free_advice,
-        OrchardHashDomains, L_ORCHARD_BASE, L_VALUE, T_Q,
+        constants::{
+            fixed_bases::NOTE_COMMITMENT_PERSONALIZATION, OrchardCommitDomains, OrchardFixedBases,
+            OrchardHashDomains, L_ORCHARD_BASE, L_VALUE, T_Q,
+        },
     };
     use halo2_gadgets::{
         ecc::{
@@ -1499,13 +1502,13 @@ mod tests {
         },
         primitives::sinsemilla::CommitDomain,
         sinsemilla::chip::SinsemillaChip,
-        utilities::{lookup_range_check::LookupRangeCheckConfig, UtilitiesInstructions},
+        utilities::lookup_range_check::LookupRangeCheckConfig,
     };
 
     use ff::{Field, PrimeField, PrimeFieldBits};
     use group::Curve;
     use halo2_proofs::{
-        circuit::{AssignedCell, Layouter, SimpleFloorPlanner},
+        circuit::{Layouter, SimpleFloorPlanner},
         dev::MockProver,
         plonk::{Circuit, ConstraintSystem, Error},
     };
@@ -1528,10 +1531,6 @@ mod tests {
             psi: Option<pallas::Base>,
         }
 
-        impl UtilitiesInstructions<pallas::Base> for MyCircuit {
-            type Var = AssignedCell<pallas::Base, pallas::Base>;
-        }
-
         impl Circuit<pallas::Base> for MyCircuit {
             type Config = (NoteCommitConfig, EccConfig<OrchardFixedBases>);
             type FloorPlanner = SimpleFloorPlanner;
@@ -1669,7 +1668,7 @@ mod tests {
                     pallas::Base::from(rng.next_u64())
                 };
                 let value_var = {
-                    self.load_private(
+                    assign_free_advice(
                         layouter.namespace(|| "witness value"),
                         note_commit_config.advices[0],
                         Some(value),
@@ -1677,14 +1676,14 @@ mod tests {
                 };
 
                 // Witness rho
-                let rho = self.load_private(
+                let rho = assign_free_advice(
                     layouter.namespace(|| "witness rho"),
                     note_commit_config.advices[0],
                     self.rho,
                 )?;
 
                 // Witness psi
-                let psi = self.load_private(
+                let psi = assign_free_advice(
                     layouter.namespace(|| "witness psi"),
                     note_commit_config.advices[0],
                     self.psi,