Check each PolynomialValues len

I.e. that it is the length of some power-of-two subgroup.

field/src/fft.rs

@@ -61,7 +61,7 @@ pub fn fft_with_options<F: Field>(
 ) -> PolynomialValues<F> {
     let PolynomialCoeffs { coeffs: mut buffer } = poly;
     fft_dispatch(&mut buffer, zero_factor, root_table);
-    PolynomialValues { values: buffer }
+    PolynomialValues::new(buffer)
 }
 
 #[inline]

field/src/interpolation.rs

@@ -19,9 +19,7 @@ pub fn interpolant<F: Field>(points: &[(F, F)]) -> PolynomialCoeffs<F> {
         .map(|x| interpolate(points, x, &barycentric_weights))
         .collect();
 
-    let mut coeffs = ifft(PolynomialValues {
-        values: subgroup_evals,
-    });
+    let mut coeffs = ifft(PolynomialValues::new(subgroup_evals));
     coeffs.trim();
     coeffs
 }

field/src/polynomial/mod.rs

@@ -24,6 +24,8 @@ pub struct PolynomialValues<F: Field> {
 
 impl<F: Field> PolynomialValues<F> {
     pub fn new(values: Vec<F>) -> Self {
+        // Check that a subgroup exists of this size, which should be a power of two.
+        debug_assert!(log2_strict(values.len()) <= F::TWO_ADICITY);
         PolynomialValues { values }
     }