EccInstructions: introduce witness_scalar_var() instruction.

halo2_gadgets/src/ecc.rs

@@ -74,6 +74,13 @@ pub trait EccInstructions<C: CurveAffine>:
         value: Option<C>,
     ) -> Result<Self::NonIdentityPoint, Error>;
 
+    /// Witnesses a full-width scalar to be used in variable-base multiplication.
+    fn witness_scalar_var(
+        &self,
+        layouter: &mut impl Layouter<C::Base>,
+        value: Option<C::Scalar>,
+    ) -> Result<Self::ScalarVar, Error>;
+
     /// Extracts the x-coordinate of a point.
     fn extract_p<Point: Into<Self::Point> + Clone>(point: &Point) -> Self::X;
 

halo2_gadgets/src/ecc/chip.rs

@@ -435,6 +435,15 @@ where
         )
     }
 
+    fn witness_scalar_var(
+        &self,
+        _layouter: &mut impl Layouter<pallas::Base>,
+        _value: Option<pallas::Scalar>,
+    ) -> Result<Self::ScalarVar, Error> {
+        // This is unimplemented for halo2_gadgets v0.1.0.
+        todo!()
+    }
+
     fn extract_p<Point: Into<Self::Point> + Clone>(point: &Point) -> Self::X {
         let point: EccPoint = (point.clone()).into();
         point.x()