dev: Handle too-small `k` error in `CircuitLayout::render`

In most places within `CircuitLayout`, we are flexible on `k`. However,
for tracking selectors we need to know the number of rows. We could
change this, but given that we also want the number of rows to be
accurate for rendering unusable rows, we can instead use this as an
opportunity to inform the user that they will need to bump `k`.

CHANGELOG.md

@@ -18,6 +18,8 @@ and this project adheres to Rust's notion of
   - `Error::SynthesisError` is now `Error::Synthesis`.
   - `Error::TranscriptError` is now `Error::Transcript`, and stores the
     underlying `io::Error`.
+- `halo2::dev::CircuitLayout::render` now takes `k` as a `u32`, matching the
+  regular parameter APIs.
 
 ### Removed
 - `halo2::arithmetic::BatchInvert` (use `ff::BatchInvert` instead).

src/dev/graph/layout.rs

@@ -33,7 +33,8 @@ use crate::plonk::{
 ///     .unwrap();
 ///
 /// let circuit = MyCircuit::default();
-/// CircuitLayout::default().render(&circuit, &drawing_area).unwrap();
+/// let k = 5; // Suitable size for MyCircuit
+/// CircuitLayout::default().render(k, &circuit, &drawing_area).unwrap();
 /// ```
 #[derive(Debug, Default)]
 pub struct CircuitLayout {
@@ -84,7 +85,7 @@ impl CircuitLayout {
     /// Renders the given circuit on the given drawing area.
     pub fn render<F: Field, ConcreteCircuit: Circuit<F>, DB: DrawingBackend>(
         self,
-        k: usize,
+        k: u32,
         circuit: &ConcreteCircuit,
         drawing_area: &DrawingArea<DB, Shift>,
     ) -> Result<(), DrawingAreaErrorKind<DB::ErrorType>> {
@@ -95,7 +96,7 @@ impl CircuitLayout {
         // Collect the layout details.
         let mut cs = ConstraintSystem::default();
         let config = ConcreteCircuit::configure(&mut cs);
-        let mut layout = Layout::new(n, cs.num_selectors);
+        let mut layout = Layout::new(k, n, cs.num_selectors);
         ConcreteCircuit::FloorPlanner::synthesize(
             &mut layout,
             circuit,
@@ -336,6 +337,7 @@ struct Region {
 
 #[derive(Default)]
 struct Layout {
+    k: u32,
     regions: Vec<Region>,
     current_region: Option<usize>,
     total_rows: usize,
@@ -349,8 +351,9 @@ struct Layout {
 }
 
 impl Layout {
-    fn new(n: usize, num_selectors: usize) -> Self {
+    fn new(k: u32, n: usize, num_selectors: usize) -> Self {
         Layout {
+            k,
             regions: vec![],
             current_region: None,
             total_rows: 0,
@@ -417,7 +420,12 @@ impl<F: Field> Assignment<F> for Layout {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        self.selectors[selector.0][row] = true;
+        if let Some(cell) = self.selectors[selector.0].get_mut(row) {
+            *cell = true;
+        } else {
+            return Err(Error::not_enough_rows_available(self.k, row + 1));
+        }
+
         self.update((*selector).into(), row);
         Ok(())
     }