circuit: Remove `Copy` impl from `poseidon::pow5::StateWord`

We will be making it a newtype around `halo2::circuit::AssignedCell`,
which does not impl `Copy`.

src/circuit/gadget/poseidon.rs

@@ -1,6 +1,7 @@
 //! Gadget and chips for the Poseidon algebraic hash function.
 
 use std::array;
+use std::convert::TryInto;
 use std::fmt;
 
 use halo2::{
@@ -20,7 +21,7 @@ pub trait PoseidonInstructions<F: FieldExt, S: Spec<F, T, RATE>, const T: usize,
     Chip<F>
 {
     /// Variable representing the word over which the Poseidon permutation operates.
-    type Word: Copy + fmt::Debug + From<CellValue<F>> + Into<CellValue<F>>;
+    type Word: Clone + fmt::Debug + From<CellValue<F>> + Into<CellValue<F>>;
 
     /// Applies the Poseidon permutation to the given state.
     fn permute(
@@ -82,7 +83,7 @@ impl<
 {
     /// The word contained in this gadget.
     pub fn inner(&self) -> PoseidonChip::Word {
-        self.inner
+        self.inner.clone()
     }
 
     /// Construct a [`Word`] gadget from the inner word.
@@ -144,7 +145,13 @@ impl<
         chip.initial_state(&mut layouter, &domain)
             .map(|state| Duplex {
                 chip,
-                sponge: Sponge::Absorbing([None; RATE]),
+                sponge: Sponge::Absorbing(
+                    (0..RATE)
+                        .map(|_| None)
+                        .collect::<Vec<_>>()
+                        .try_into()
+                        .unwrap(),
+                ),
                 state,
                 domain,
             })
@@ -205,7 +212,13 @@ impl<
                     }
 
                     // We've already squeezed out all available elements
-                    self.sponge = Sponge::Absorbing([None; RATE]);
+                    self.sponge = Sponge::Absorbing(
+                        (0..RATE)
+                            .map(|_| None)
+                            .collect::<Vec<_>>()
+                            .try_into()
+                            .unwrap(),
+                    );
                 }
             }
         }

src/circuit/gadget/poseidon/pow5.rs

@@ -341,7 +341,7 @@ impl<F: FieldExt, S: Spec<F, WIDTH, RATE>, const WIDTH: usize, const RATE: usize
 
                 // Load the input and padding into this region.
                 let load_input_word = |i: usize| {
-                    let (constraint_var, value) = match (input[i], padding_values[i]) {
+                    let (constraint_var, value) = match (input[i].clone(), padding_values[i]) {
                         (Some(word), None) => (word.var, word.value),
                         (None, Some(padding_value)) => {
                             let padding_var = region.assign_fixed(
@@ -395,7 +395,7 @@ impl<F: FieldExt, S: Spec<F, WIDTH, RATE>, const WIDTH: usize, const RATE: usize
     fn get_output(state: &State<Self::Word, WIDTH>) -> SpongeState<Self::Word, RATE> {
         state[..RATE]
             .iter()
-            .map(|word| Some(*word))
+            .map(|word| Some(word.clone()))
             .collect::<Vec<_>>()
             .try_into()
             .unwrap()
@@ -403,7 +403,7 @@ impl<F: FieldExt, S: Spec<F, WIDTH, RATE>, const WIDTH: usize, const RATE: usize
 }
 
 /// A word in the Poseidon state.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Debug)]
 pub struct StateWord<F: FieldExt> {
     var: Cell,
     value: Option<F>,

src/primitives/poseidon.rs

@@ -1,6 +1,7 @@
 //! The Poseidon algebraic hash function.
 
 use std::array;
+use std::convert::TryInto;
 use std::fmt;
 use std::iter;
 use std::marker::PhantomData;
@@ -147,9 +148,13 @@ pub(crate) enum Sponge<F, const RATE: usize> {
     Squeezing(SpongeState<F, RATE>),
 }
 
-impl<F: Copy, const RATE: usize> Sponge<F, RATE> {
+impl<F: fmt::Debug, const RATE: usize> Sponge<F, RATE> {
     pub(crate) fn absorb(val: F) -> Self {
-        let mut input = [None; RATE];
+        let mut input: [Option<F>; RATE] = (0..RATE)
+            .map(|_| None)
+            .collect::<Vec<_>>()
+            .try_into()
+            .unwrap();
         input[0] = Some(val);
         Sponge::Absorbing(input)
     }