refactor: remove usage of unstable generic_const_exprs in starky

This PR mostly lifts 0xPolygonZero#1246
down into the lib level into the starky crate, getting rid of the
unstable `generic_const_exprs` feature from the crate entirely. This
should work towards the 0xPolygonZero#417
as well.

This would probably warrant a minor version bump in the starky crate to
0.2.0, since this would be an API change for anyone relying on starky.

starky/src/evaluation_frame.rs

@@ -0,0 +1,66 @@
+/// A trait for viewing an evaluation frame of a STARK table.
+///
+/// It allows to access the current and next rows at a given step
+/// and can be used to implement constraint evaluation both natively
+/// and recursively.
+pub trait StarkEvaluationFrame<T: Copy + Clone + Default, U: Copy + Clone + Default>:
+    Sized
+{
+    /// The number of columns for the STARK table this evaluation frame views.
+    const COLUMNS: usize;
+    const PUBLIC_INPUTS: usize;
+
+    /// Returns the local values (i.e. current row) for this evaluation frame.
+    fn get_local_values(&self) -> &[T];
+    /// Returns the next values (i.e. next row) for this evaluation frame.
+    fn get_next_values(&self) -> &[T];
+
+    fn get_public_inputs(&self) -> &[U];
+    /// Outputs a new evaluation frame from the provided local and next values.
+    ///
+    /// **NOTE**: Concrete implementations of this method SHOULD ensure that
+    /// the provided slices lengths match the `Self::COLUMNS` value.
+    fn from_values(lv: &[T], nv: &[T], pis: &[U]) -> Self;
+}
+
+pub struct StarkFrame<
+    T: Copy + Clone + Default,
+    U: Copy + Clone + Default,
+    const N: usize,
+    const N2: usize,
+> {
+    local_values: [T; N],
+    next_values: [T; N],
+    public_inputs: [U; N2],
+}
+
+impl<T: Copy + Clone + Default, U: Copy + Clone + Default, const N: usize, const N2: usize>
+    StarkEvaluationFrame<T, U> for StarkFrame<T, U, N, N2>
+{
+    const COLUMNS: usize = N;
+    const PUBLIC_INPUTS: usize = N2;
+
+    fn get_local_values(&self) -> &[T] {
+        &self.local_values
+    }
+
+    fn get_next_values(&self) -> &[T] {
+        &self.next_values
+    }
+
+    fn get_public_inputs(&self) -> &[U] {
+        &self.public_inputs
+    }
+
+    fn from_values(lv: &[T], nv: &[T], pis: &[U]) -> Self {
+        assert_eq!(lv.len(), Self::COLUMNS);
+        assert_eq!(nv.len(), Self::COLUMNS);
+        assert_eq!(pis.len(), Self::PUBLIC_INPUTS);
+
+        Self {
+            local_values: lv.try_into().unwrap(),
+            next_values: nv.try_into().unwrap(),
+            public_inputs: pis.try_into().unwrap(),
+        }
+    }
+}

starky/src/fibonacci_stark.rs

@@ -6,13 +6,14 @@ use plonky2::field::extension::{Extendable, FieldExtension};
 use plonky2::field::packed::PackedField;
 use plonky2::field::polynomial::PolynomialValues;
 use plonky2::hash::hash_types::RichField;
+use plonky2::iop::ext_target::ExtensionTarget;
 use plonky2::plonk::circuit_builder::CircuitBuilder;
 
 use crate::constraint_consumer::{ConstraintConsumer, RecursiveConstraintConsumer};
+use crate::evaluation_frame::{StarkEvaluationFrame, StarkFrame};
 use crate::permutation::PermutationPair;
 use crate::stark::Stark;
 use crate::util::trace_rows_to_poly_values;
-use crate::vars::{StarkEvaluationTargets, StarkEvaluationVars};
 
 /// Toy STARK system used for testing.
 /// Computes a Fibonacci sequence with state `[x0, x1, i, j]` using the state transition
@@ -57,57 +58,67 @@ impl<F: RichField + Extendable<D>, const D: usize> FibonacciStark<F, D> {
     }
 }
 
+const COLUMNS: usize = 4;
+const PUBLIC_INPUTS: usize = 3;
+
 impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for FibonacciStark<F, D> {
-    const COLUMNS: usize = 4;
-    const PUBLIC_INPUTS: usize = 3;
+    type EvaluationFrame<FE, P, const D2: usize> = StarkFrame<P, P::Scalar, COLUMNS, PUBLIC_INPUTS>
+    where
+        FE: FieldExtension<D2, BaseField = F>,
+        P: PackedField<Scalar = FE>;
+
+    type EvaluationFrameTarget =
+        StarkFrame<ExtensionTarget<D>, ExtensionTarget<D>, COLUMNS, PUBLIC_INPUTS>;
 
     fn eval_packed_generic<FE, P, const D2: usize>(
         &self,
-        vars: StarkEvaluationVars<FE, P, { Self::COLUMNS }, { Self::PUBLIC_INPUTS }>,
+        vars: &Self::EvaluationFrame<FE, P, D2>,
         yield_constr: &mut ConstraintConsumer<P>,
     ) where
         FE: FieldExtension<D2, BaseField = F>,
         P: PackedField<Scalar = FE>,
     {
+        let local_values = vars.get_local_values();
+        let next_values = vars.get_next_values();
+        let public_inputs = vars.get_public_inputs();
+
         // Check public inputs.
-        yield_constr
-            .constraint_first_row(vars.local_values[0] - vars.public_inputs[Self::PI_INDEX_X0]);
-        yield_constr
-            .constraint_first_row(vars.local_values[1] - vars.public_inputs[Self::PI_INDEX_X1]);
-        yield_constr
-            .constraint_last_row(vars.local_values[1] - vars.public_inputs[Self::PI_INDEX_RES]);
+        yield_constr.constraint_first_row(local_values[0] - public_inputs[Self::PI_INDEX_X0]);
+        yield_constr.constraint_first_row(local_values[1] - public_inputs[Self::PI_INDEX_X1]);
+        yield_constr.constraint_last_row(local_values[1] - public_inputs[Self::PI_INDEX_RES]);
 
         // x0' <- x1
-        yield_constr.constraint_transition(vars.next_values[0] - vars.local_values[1]);
+        yield_constr.constraint_transition(next_values[0] - local_values[1]);
         // x1' <- x0 + x1
-        yield_constr.constraint_transition(
-            vars.next_values[1] - vars.local_values[0] - vars.local_values[1],
-        );
+        yield_constr.constraint_transition(next_values[1] - local_values[0] - local_values[1]);
     }
 
     fn eval_ext_circuit(
         &self,
         builder: &mut CircuitBuilder<F, D>,
-        vars: StarkEvaluationTargets<D, { Self::COLUMNS }, { Self::PUBLIC_INPUTS }>,
+        vars: &Self::EvaluationFrameTarget,
         yield_constr: &mut RecursiveConstraintConsumer<F, D>,
     ) {
+        let local_values = vars.get_local_values();
+        let next_values = vars.get_next_values();
+        let public_inputs = vars.get_public_inputs();
         // Check public inputs.
         let pis_constraints = [
-            builder.sub_extension(vars.local_values[0], vars.public_inputs[Self::PI_INDEX_X0]),
-            builder.sub_extension(vars.local_values[1], vars.public_inputs[Self::PI_INDEX_X1]),
-            builder.sub_extension(vars.local_values[1], vars.public_inputs[Self::PI_INDEX_RES]),
+            builder.sub_extension(local_values[0], public_inputs[Self::PI_INDEX_X0]),
+            builder.sub_extension(local_values[1], public_inputs[Self::PI_INDEX_X1]),
+            builder.sub_extension(local_values[1], public_inputs[Self::PI_INDEX_RES]),
         ];
         yield_constr.constraint_first_row(builder, pis_constraints[0]);
         yield_constr.constraint_first_row(builder, pis_constraints[1]);
         yield_constr.constraint_last_row(builder, pis_constraints[2]);
 
         // x0' <- x1
-        let first_col_constraint = builder.sub_extension(vars.next_values[0], vars.local_values[1]);
+        let first_col_constraint = builder.sub_extension(next_values[0], local_values[1]);
         yield_constr.constraint_transition(builder, first_col_constraint);
         // x1' <- x0 + x1
         let second_col_constraint = {
-            let tmp = builder.sub_extension(vars.next_values[1], vars.local_values[0]);
-            builder.sub_extension(tmp, vars.local_values[1])
+            let tmp = builder.sub_extension(next_values[1], local_values[0]);
+            builder.sub_extension(tmp, local_values[1])
         };
         yield_constr.constraint_transition(builder, second_col_constraint);
     }
@@ -165,7 +176,7 @@ mod tests {
             stark,
             &config,
             trace,
-            public_inputs,
+            &public_inputs,
             &mut TimingTree::default(),
         )?;
 
@@ -213,7 +224,7 @@ mod tests {
             stark,
             &config,
             trace,
-            public_inputs,
+            &public_inputs,
             &mut TimingTree::default(),
         )?;
         verify_stark_proof(stark, proof.clone(), &config)?;
@@ -235,8 +246,6 @@ mod tests {
     ) -> Result<()>
     where
         InnerC::Hasher: AlgebraicHasher<F>,
-        [(); S::COLUMNS]:,
-        [(); S::PUBLIC_INPUTS]:,
     {
         let circuit_config = CircuitConfig::standard_recursion_config();
         let mut builder = CircuitBuilder::<F, D>::new(circuit_config);

starky/src/lib.rs

@@ -1,7 +1,6 @@
 #![allow(incomplete_features)]
 #![allow(clippy::too_many_arguments)]
 #![allow(clippy::type_complexity)]
-#![feature(generic_const_exprs)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
 extern crate alloc;
@@ -10,6 +9,7 @@ mod get_challenges;
 
 pub mod config;
 pub mod constraint_consumer;
+pub mod evaluation_frame;
 pub mod permutation;
 pub mod proof;
 pub mod prover;
@@ -18,7 +18,6 @@ pub mod stark;
 pub mod stark_testing;
 pub mod util;
 pub mod vanishing_poly;
-pub mod vars;
 pub mod verifier;
 
 #[cfg(test)]

starky/src/permutation.rs

@@ -20,8 +20,8 @@ use plonky2_maybe_rayon::*;
 
 use crate::config::StarkConfig;
 use crate::constraint_consumer::{ConstraintConsumer, RecursiveConstraintConsumer};
+use crate::evaluation_frame::StarkEvaluationFrame;
 use crate::stark::Stark;
-use crate::vars::{StarkEvaluationTargets, StarkEvaluationVars};
 
 /// A pair of lists of columns, `lhs` and `rhs`, that should be permutations of one another.
 /// In particular, there should exist some permutation `pi` such that for any `i`,
@@ -262,17 +262,17 @@ where
 pub(crate) fn eval_permutation_checks<F, FE, P, S, const D: usize, const D2: usize>(
     stark: &S,
     config: &StarkConfig,
-    vars: StarkEvaluationVars<FE, P, { S::COLUMNS }, { S::PUBLIC_INPUTS }>,
+    vars: &S::EvaluationFrame<FE, P, D2>,
     permutation_data: PermutationCheckVars<F, FE, P, D2>,
     consumer: &mut ConstraintConsumer<P>,
 ) where
     F: RichField + Extendable<D>,
     FE: FieldExtension<D2, BaseField = F>,
     P: PackedField<Scalar = FE>,
     S: Stark<F, D>,
-    [(); S::COLUMNS]:,
-    [(); S::PUBLIC_INPUTS]:,
 {
+    let local_values = vars.get_local_values();
+
     let PermutationCheckVars {
         local_zs,
         next_zs,
@@ -306,7 +306,7 @@ pub(crate) fn eval_permutation_checks<F, FE, P, S, const D: usize, const D2: usi
                 let mut factor = ReducingFactor::new(*beta);
                 let (lhs, rhs): (Vec<_>, Vec<_>) = column_pairs
                     .iter()
-                    .map(|&(i, j)| (vars.local_values[i], vars.local_values[j]))
+                    .map(|&(i, j)| (local_values[i], local_values[j]))
                     .unzip();
                 (
                     factor.reduce_ext(lhs.into_iter()) + FE::from_basefield(*gamma),
@@ -330,15 +330,15 @@ pub(crate) fn eval_permutation_checks_circuit<F, S, const D: usize>(
     builder: &mut CircuitBuilder<F, D>,
     stark: &S,
     config: &StarkConfig,
-    vars: StarkEvaluationTargets<D, { S::COLUMNS }, { S::PUBLIC_INPUTS }>,
+    vars: &S::EvaluationFrameTarget,
     permutation_data: PermutationCheckDataTarget<D>,
     consumer: &mut RecursiveConstraintConsumer<F, D>,
 ) where
     F: RichField + Extendable<D>,
     S: Stark<F, D>,
-    [(); S::COLUMNS]:,
-    [(); S::PUBLIC_INPUTS]:,
 {
+    let local_values = vars.get_local_values();
+
     let PermutationCheckDataTarget {
         local_zs,
         next_zs,
@@ -376,7 +376,7 @@ pub(crate) fn eval_permutation_checks_circuit<F, S, const D: usize>(
                     let mut factor = ReducingFactorTarget::new(beta_ext);
                     let (lhs, rhs): (Vec<_>, Vec<_>) = column_pairs
                         .iter()
-                        .map(|&(i, j)| (vars.local_values[i], vars.local_values[j]))
+                        .map(|&(i, j)| (local_values[i], local_values[j]))
                         .unzip();
                     let reduced_lhs = factor.reduce(&lhs, builder);
                     let reduced_rhs = factor.reduce(&rhs, builder);

starky/src/prover.rs

@@ -20,28 +20,26 @@ use plonky2_maybe_rayon::*;
 
 use crate::config::StarkConfig;
 use crate::constraint_consumer::ConstraintConsumer;
+use crate::evaluation_frame::StarkEvaluationFrame;
 use crate::permutation::{
     compute_permutation_z_polys, get_n_permutation_challenge_sets, PermutationChallengeSet,
     PermutationCheckVars,
 };
 use crate::proof::{StarkOpeningSet, StarkProof, StarkProofWithPublicInputs};
 use crate::stark::Stark;
 use crate::vanishing_poly::eval_vanishing_poly;
-use crate::vars::StarkEvaluationVars;
 
 pub fn prove<F, C, S, const D: usize>(
     stark: S,
     config: &StarkConfig,
     trace_poly_values: Vec<PolynomialValues<F>>,
-    public_inputs: [F; S::PUBLIC_INPUTS],
+    public_inputs: &[F],
     timing: &mut TimingTree,
 ) -> Result<StarkProofWithPublicInputs<F, C, D>>
 where
     F: RichField + Extendable<D>,
     C: GenericConfig<D, F = F>,
     S: Stark<F, D>,
-    [(); S::COLUMNS]:,
-    [(); S::PUBLIC_INPUTS]:,
 {
     let degree = trace_poly_values[0].len();
     let degree_bits = log2_strict(degree);
@@ -202,7 +200,7 @@ fn compute_quotient_polys<'a, F, P, C, S, const D: usize>(
         PolynomialBatch<F, C, D>,
         Vec<PermutationChallengeSet<F>>,
     )>,
-    public_inputs: [F; S::PUBLIC_INPUTS],
+    public_inputs: &[F],
     alphas: Vec<F>,
     degree_bits: usize,
     config: &StarkConfig,
@@ -212,8 +210,6 @@ where
     P: PackedField<Scalar = F>,
     C: GenericConfig<D, F = F>,
     S: Stark<F, D>,
-    [(); S::COLUMNS]:,
-    [(); S::PUBLIC_INPUTS]:,
 {
     let degree = 1 << degree_bits;
     let rate_bits = config.fri_config.rate_bits;
@@ -236,12 +232,8 @@ where
     let z_h_on_coset = ZeroPolyOnCoset::<F>::new(degree_bits, quotient_degree_bits);
 
     // Retrieve the LDE values at index `i`.
-    let get_trace_values_packed = |i_start| -> [P; S::COLUMNS] {
-        trace_commitment
-            .get_lde_values_packed(i_start, step)
-            .try_into()
-            .unwrap()
-    };
+    let get_trace_values_packed =
+        |i_start| -> Vec<P> { trace_commitment.get_lde_values_packed(i_start, step) };
 
     // Last element of the subgroup.
     let last = F::primitive_root_of_unity(degree_bits).inverse();
@@ -272,11 +264,11 @@ where
                 lagrange_basis_first,
                 lagrange_basis_last,
             );
-            let vars = StarkEvaluationVars {
-                local_values: &get_trace_values_packed(i_start),
-                next_values: &get_trace_values_packed(i_next_start),
-                public_inputs: &public_inputs,
-            };
+            let vars = S::EvaluationFrame::from_values(
+                &get_trace_values_packed(i_start),
+                &get_trace_values_packed(i_next_start),
+                public_inputs,
+            );
             let permutation_check_data = permutation_zs_commitment_challenges.as_ref().map(
                 |(permutation_zs_commitment, permutation_challenge_sets)| PermutationCheckVars {
                     local_zs: permutation_zs_commitment.get_lde_values_packed(i_start, step),
@@ -287,7 +279,7 @@ where
             eval_vanishing_poly::<F, F, P, S, D, 1>(
                 stark,
                 config,
-                vars,
+                &vars,
                 permutation_check_data,
                 &mut consumer,
             );