dev: Move halo2::dev::circuit_layout to halo2::dev::CircuitLayout::render

This makes it easier to add configuration to the renderer.

examples/circuit-layout.rs

@@ -1,6 +1,6 @@
 use halo2::{
     arithmetic::FieldExt,
-    dev::circuit_layout,
+    dev::CircuitLayout,
     pasta::Fp,
     plonk::{Advice, Assignment, Circuit, Column, ConstraintSystem, Error, Fixed, Permutation},
     poly::Rotation,
@@ -396,5 +396,5 @@ fn main() {
         .titled("Example Circuit Layout", ("sans-serif", 60))
         .unwrap();
 
-    circuit_layout(&circuit, &root).unwrap();
+    CircuitLayout::default().render(&circuit, &root).unwrap();
 }

src/dev.rs

@@ -20,7 +20,7 @@ mod graph;
 
 #[cfg(feature = "dev-graph")]
 #[cfg_attr(docsrs, doc(cfg(feature = "dev-graph")))]
-pub use graph::{circuit_dot_graph, layout::circuit_layout};
+pub use graph::{circuit_dot_graph, layout::CircuitLayout};
 
 /// Cells that haven't been explicitly assigned to, default to zero.
 fn cell_value<F: Field>(cell: Option<F>) -> F {

src/dev/graph/layout.rs

@@ -10,7 +10,7 @@ use crate::plonk::{
     Advice, Any, Assignment, Circuit, Column, ConstraintSystem, Error, Fixed, Permutation, Selector,
 };
 
-/// Renders the circuit layout on the given drawing area.
+/// Graphical renderer for circuit layouts.
 ///
 /// Cells that have been assigned to by the circuit will be shaded. If any cells are
 /// assigned to more than once (which is usually a mistake), they will be shaded darker
@@ -19,7 +19,7 @@ use crate::plonk::{
 /// # Examples
 ///
 /// ```ignore
-/// use halo2::dev::circuit_layout;
+/// use halo2::dev::CircuitLayout;
 /// use plotters::prelude::*;
 ///
 /// let drawing_area = BitMapBackend::new("example-circuit-layout.png", (1024, 768))
@@ -30,12 +30,18 @@ use crate::plonk::{
 ///     .unwrap();
 ///
 /// let circuit = MyCircuit::default();
-/// circuit_layout(&circuit, &drawing_area).unwrap();
+/// CircuitLayout::default().render(&circuit, &drawing_area).unwrap();
 /// ```
-pub fn circuit_layout<F: Field, ConcreteCircuit: Circuit<F>, DB: DrawingBackend>(
+#[derive(Debug, Default)]
+pub struct CircuitLayout {}
+
+impl CircuitLayout {
+    /// Renders the given circuit on the given drawing area.
+    pub fn render<F: Field, ConcreteCircuit: Circuit<F>, DB: DrawingBackend>(
+        self,
         circuit: &ConcreteCircuit,
         drawing_area: &DrawingArea<DB, Shift>,
-) -> Result<(), DrawingAreaErrorKind<DB::ErrorType>> {
+    ) -> Result<(), DrawingAreaErrorKind<DB::ErrorType>> {
         use plotters::coord::types::RangedCoordusize;
         use plotters::prelude::*;
 
@@ -153,6 +159,7 @@ pub fn circuit_layout<F: Field, ConcreteCircuit: Circuit<F>, DB: DrawingBackend>
         }
 
         Ok(())
+    }
 }
 
 #[derive(Debug)]