chore: remove mutable references to vecs

Author: TomAFrench

compiler/noirc_evaluator/src/ssa/opt/unrolling.rs

@@ -44,8 +44,7 @@ impl Ssa {
     pub(crate) fn unroll_loops_iteratively(mut ssa: Ssa) -> Result<Ssa, RuntimeError> {
         for (_, function) in ssa.functions.iter_mut() {
             // Try to unroll loops first:
-            let mut unroll_errors: Vec<RuntimeError> = Vec::new();
-            function.try_to_unroll_loops(&mut unroll_errors);
+            let unroll_errors = function.try_to_unroll_loops();
 
             // Keep unrolling until no more errors are found
             while !unroll_errors.is_empty() {
@@ -60,8 +59,7 @@ impl Ssa {
                 function.mem2reg();
 
                 // Unroll again
-                unroll_errors = Vec::new();
-                function.try_to_unroll_loops(&mut unroll_errors);
+                let mut unroll_errors = function.try_to_unroll_loops();
                 // If we didn't manage to unroll any more loops, exit
                 if unroll_errors.len() >= prev_unroll_err_count {
                     return Err(unroll_errors.swap_remove(0));
@@ -74,12 +72,14 @@ impl Ssa {
 }
 
 impl Function {
-    pub(crate) fn try_to_unroll_loops(&mut self, errors: &mut Vec<RuntimeError>) {
+    fn try_to_unroll_loops(&mut self) -> Vec<RuntimeError> {
         // Loop unrolling in brillig can lead to a code explosion currently. This can
         // also be true for ACIR, but we have no alternative to unrolling in ACIR.
         // Brillig also generally prefers smaller code rather than faster code.
         if !matches!(self.runtime(), RuntimeType::Brillig(_)) {
-            errors.extend(find_all_loops(self).unroll_each_loop(self));
+            find_all_loops(self).unroll_each_loop(self)
+        } else {
+            Vec::new()
         }
     }
 }