Merge pull request #743 from zcash/740-expose-instance-value

halo2_proofs: Add `Region::instance_value`

halo2_proofs/CHANGELOG.md

@@ -16,9 +16,11 @@ and this project adheres to Rust's notion of
     - `metadata::Region`
   - `halo2_proofs::poly::Rotation`
 - `halo2_proofs::arithmetic::FftGroup`
-- `halo2_proofs::circuit::layouter`:
-  - `RegionLayouter::instance_value` method added to provide access to
-    instance values within a region.
+- `halo2_proofs::circuit`:
+  - `Region::instance_value`, to provide access to instance values within a
+    region. This method is only provided for convenience; it does not create any
+    constraints. Callers still need to use `Region::assign_advice_from_instance`
+    to constrain the values in their circuit.
 
 ### Changed
 - Migrated to `ff 0.13`, `group 0.13`, `pasta_curves 0.5`.
@@ -27,7 +29,10 @@ and this project adheres to Rust's notion of
 - `halo2_proofs::arithmetic`:
   - `best_fft, recursive_butterfly_arithmetic` now use the `FftGroup` trait
     instead of the (now-removed) `pasta_curves::arithmetic::Group` trait.
-- `halo2::proofs::circuit`
+- `halo2_proofs::circuit::layouter`:
+  - The `RegionLayouter` trait now requires implementing an `instance_value`
+    method, to back `Region::instance_value`.
+- `halo2_proofs::plonk`
   - `VirtualCells`
     - `query_any` now panics if a non-`cur` `Rotation` is used with the
       `Column<Fixed>` variant.

halo2_proofs/src/circuit.rs

@@ -297,6 +297,19 @@ impl<'r, F: Field> Region<'r, F> {
         })
     }
 
+    /// Returns the value of the instance column's cell at absolute location `row`.
+    ///
+    /// This method is only provided for convenience; it does not create any constraints.
+    /// Callers still need to use [`Self::assign_advice_from_instance`] to constrain the
+    /// instance values in their circuit.
+    pub fn instance_value(
+        &mut self,
+        instance: Column<Instance>,
+        row: usize,
+    ) -> Result<Value<F>, Error> {
+        self.region.instance_value(instance, row)
+    }
+
     /// Assign a fixed value.
     ///
     /// Even though `to` has `FnMut` bounds, it is guaranteed to be called at most once.