ff: Remove PrimeFieldRepr trait

The ff::PrimeField::Repr associated type now has the minimal necessary
bounds, which can be satisfied by a newtype around a byte array.

zcash_client_backend/src/proto/mod.rs

@@ -1,6 +1,6 @@
 //! Generated code for handling light client protobuf structs.
 
-use ff::{PrimeField, PrimeFieldRepr};
+use ff::PrimeField;
 use pairing::bls12_381::{Bls12, Fr, FrRepr};
 use zcash_primitives::{
     block::{BlockHash, BlockHeader},
@@ -67,8 +67,8 @@ impl compact_formats::CompactOutput {
     /// [`CompactOutput.cmu`]: #structfield.cmu
     pub fn cmu(&self) -> Result<Fr, ()> {
         let mut repr = FrRepr::default();
-        repr.read_le(&self.cmu[..]).map_err(|_| ())?;
-        Fr::from_repr(repr).map_err(|_| ())
+        repr.as_mut().copy_from_slice(&self.cmu[..]);
+        Fr::from_repr(repr).ok_or(())
     }
 
     /// Returns the ephemeral public key for this output.

zcash_client_backend/src/welding_rig.rs

@@ -183,7 +183,7 @@ pub fn scan_block(
 
 #[cfg(test)]
 mod tests {
-    use ff::{Field, PrimeField, PrimeFieldRepr};
+    use ff::{Field, PrimeField};
     use pairing::bls12_381::{Bls12, Fr};
     use rand_core::{OsRng, RngCore};
     use zcash_primitives::{
@@ -207,9 +207,7 @@ mod tests {
         };
         let fake_cmu = {
             let fake_cmu = Fr::random(rng);
-            let mut bytes = vec![];
-            fake_cmu.into_repr().write_le(&mut bytes).unwrap();
-            bytes
+            fake_cmu.into_repr().as_ref().to_owned()
         };
         let fake_epk = {
             let mut buffer = vec![0; 64];
@@ -264,8 +262,7 @@ mod tests {
             Memo::default(),
             &mut rng,
         );
-        let mut cmu = vec![];
-        note.cm(&JUBJUB).into_repr().write_le(&mut cmu).unwrap();
+        let cmu = note.cm(&JUBJUB).into_repr().as_ref().to_owned();
         let mut epk = vec![];
         encryptor.epk().write(&mut epk).unwrap();
         let enc_ciphertext = encryptor.encrypt_note_plaintext();