Lift the Default bound for the Circuit trait

The `Compiler::compile` function keeps this bound since it actually
requires the circuit to have a `Default` implementation, but all other
functionality now works without said bound.

Resolves #715

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Lift `Default` bound on the `Circuit` trait [#715]
+
 ## [0.13.1] - 2022-10-26
 
 ### Fixed
@@ -444,6 +448,7 @@ is necessary since `rkyv/validation` was required as a bound.
 - Proof system module.
 
 <!-- ISSUES -->
+[#715]: https://github.com/dusk-network/plonk/issues/715
 [#709]: https://github.com/dusk-network/plonk/issues/709
 [#697]: https://github.com/dusk-network/plonk/issues/697
 [#688]: https://github.com/dusk-network/plonk/issues/688

src/composer/circuit.rs

@@ -11,7 +11,7 @@ use super::Composer;
 /// Circuit implementation that can be proved by a Composer
 ///
 /// The default implementation will be used to generate the proving arguments.
-pub trait Circuit: Default {
+pub trait Circuit {
     /// Circuit definition
     fn circuit<C>(&self, composer: &mut C) -> Result<(), Error>
     where

src/composer/compiler.rs

@@ -26,7 +26,7 @@ impl Compiler {
         label: &[u8],
     ) -> Result<(Prover<C>, Verifier<C>), Error>
     where
-        C: Circuit,
+        C: Default + Circuit,
     {
         Self::compile_with_circuit(pp, label, &Default::default())
     }