Add Default impl for types with Self::new

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to Rust's notion of
   `bellman` will use. The default, which has not changed, is to use the same
   number of threads as logical CPUs.
 - `bellman::multicore::Waiter`
+- `Default` bound for `bellman::multiexp::DensityTracker`.
+- `Default` bound for `bellman::gadgets::test::TestConstraintSystem`.
 
 ### Changed
 - `bellman::multicore` has migrated from `crossbeam` to `rayon`:

src/gadgets/test/mod.rs

@@ -135,6 +135,12 @@ fn eval_lc<Scalar: PrimeField>(
     acc
 }
 
+impl<Scalar: PrimeField> Default for TestConstraintSystem<Scalar> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<Scalar: PrimeField> TestConstraintSystem<Scalar> {
     pub fn new() -> TestConstraintSystem<Scalar> {
         let mut map = HashMap::new();

src/multiexp.rs

@@ -130,6 +130,12 @@ impl<'a> QueryDensity for &'a DensityTracker {
     }
 }
 
+impl Default for DensityTracker {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl DensityTracker {
     pub fn new() -> DensityTracker {
         DensityTracker { bv: BitVec::new() }