diff --git a/.gitignore b/.gitignore
index 1d76c98a1ac..334901860d9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -56,3 +56,6 @@ codegen
 
 **/cspell.json
 !./cspell.json
+
+mutants.out
+mutants.out.old
diff --git a/acvm-repo/acvm/src/pwg/arithmetic.rs b/acvm-repo/acvm/src/pwg/arithmetic.rs
index 78da63ddc0f..a2921bcbc9b 100644
--- a/acvm-repo/acvm/src/pwg/arithmetic.rs
+++ b/acvm-repo/acvm/src/pwg/arithmetic.rs
@@ -254,7 +254,52 @@ mod tests {
     use acir::FieldElement;
 
     #[test]
-    fn expression_solver_smoke_test() {
+    fn solves_simple_assignment() {
+        let a = Witness(0);
+
+        // a - 1 == 0;
+        let opcode_a = Expression {
+            mul_terms: vec![],
+            linear_combinations: vec![(FieldElement::one(), a)],
+            q_c: -FieldElement::one(),
+        };
+
+        let mut values = WitnessMap::new();
+        assert_eq!(ExpressionSolver::solve(&mut values, &opcode_a), Ok(()));
+
+        assert_eq!(values.get(&a).unwrap(), &FieldElement::from(1_i128));
+    }
+
+    #[test]
+    fn solves_unknown_in_mul_term() {
+        let a = Witness(0);
+        let b = Witness(1);
+        let c = Witness(2);
+        let d = Witness(3);
+
+        // a * b - b - c - d == 0;
+        let opcode_a = Expression {
+            mul_terms: vec![(FieldElement::one(), a, b)],
+            linear_combinations: vec![
+                (-FieldElement::one(), b),
+                (-FieldElement::one(), c),
+                (-FieldElement::one(), d),
+            ],
+            q_c: FieldElement::zero(),
+        };
+
+        let mut values = WitnessMap::new();
+        values.insert(b, FieldElement::from(2_i128));
+        values.insert(c, FieldElement::from(1_i128));
+        values.insert(d, FieldElement::from(1_i128));
+
+        assert_eq!(ExpressionSolver::solve(&mut values, &opcode_a), Ok(()));
+
+        assert_eq!(values.get(&a).unwrap(), &FieldElement::from(2_i128));
+    }
+
+    #[test]
+    fn solves_unknown_in_linear_term() {
         let a = Witness(0);
         let b = Witness(1);
         let c = Witness(2);
diff --git a/acvm-repo/acvm/src/pwg/blackbox/hash.rs b/acvm-repo/acvm/src/pwg/blackbox/hash.rs
index 93f9ea410bc..a4af9de55cf 100644
--- a/acvm-repo/acvm/src/pwg/blackbox/hash.rs
+++ b/acvm-repo/acvm/src/pwg/blackbox/hash.rs
@@ -136,11 +136,10 @@ pub(crate) fn solve_poseidon2_permutation_opcode<F: AcirField>(
     }
 
     // Read witness assignments
-    let mut state = Vec::new();
-    for input in inputs.iter() {
-        let witness_assignment = input_to_value(initial_witness, *input, false)?;
-        state.push(witness_assignment);
-    }
+    let state: Vec<F> = inputs
+        .iter()
+        .map(|input| input_to_value(initial_witness, *input, false))
+        .collect::<Result<_, _>>()?;
 
     let state = backend.poseidon2_permutation(&state, len)?;
 
diff --git a/acvm-repo/acvm/src/pwg/blackbox/range.rs b/acvm-repo/acvm/src/pwg/blackbox/range.rs
index 927706f4671..039a04b9063 100644
--- a/acvm-repo/acvm/src/pwg/blackbox/range.rs
+++ b/acvm-repo/acvm/src/pwg/blackbox/range.rs
@@ -21,3 +21,36 @@ pub(crate) fn solve_range_opcode<F: AcirField>(
     }
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeMap;
+
+    use acir::{
+        FieldElement,
+        circuit::opcodes::FunctionInput,
+        native_types::{Witness, WitnessMap},
+    };
+
+    use crate::pwg::blackbox::solve_range_opcode;
+
+    #[test]
+    fn rejects_too_large_inputs() {
+        let witness_map =
+            WitnessMap::from(BTreeMap::from([(Witness(0), FieldElement::from(256u32))]));
+        let input: FunctionInput<FieldElement> = FunctionInput::witness(Witness(0), 8);
+        assert!(solve_range_opcode(&witness_map, &input, false).is_err());
+    }
+
+    #[test]
+    fn accepts_valid_inputs() {
+        let values: [u32; 4] = [0, 1, 8, 255];
+
+        for value in values {
+            let witness_map =
+                WitnessMap::from(BTreeMap::from([(Witness(0), FieldElement::from(value))]));
+            let input: FunctionInput<FieldElement> = FunctionInput::witness(Witness(0), 8);
+            assert!(solve_range_opcode(&witness_map, &input, false).is_ok());
+        }
+    }
+}
diff --git a/acvm-repo/blackbox_solver/src/curve_specific_solver.rs b/acvm-repo/blackbox_solver/src/curve_specific_solver.rs
index 37fe5d05363..af0104b54f0 100644
--- a/acvm-repo/blackbox_solver/src/curve_specific_solver.rs
+++ b/acvm-repo/blackbox_solver/src/curve_specific_solver.rs
@@ -25,8 +25,8 @@ pub trait BlackBoxFunctionSolver<F> {
     ) -> Result<(F, F, F), BlackBoxResolutionError>;
     fn poseidon2_permutation(
         &self,
-        _inputs: &[F],
-        _len: u32,
+        inputs: &[F],
+        len: u32,
     ) -> Result<Vec<F>, BlackBoxResolutionError>;
 }