fix(ssa_interpreter): push_back and pop_back to slices with padding

compiler/noirc_evaluator/src/ssa/interpreter/intrinsics.rs

@@ -510,6 +510,9 @@ impl<W: Write> Interpreter<'_, W> {
         let mut new_elements = slice.elements.borrow().to_vec();
         let element_types = slice.element_types.clone();
 
+        // The slice might contain more elements than its length.
+        // We need to either insert before the extras, overwrite, or remove them.
+        new_elements.truncate(element_types.len() * length as usize);
         for arg in args.iter().skip(2) {
             new_elements.push(self.lookup(*arg)?);
         }
@@ -548,6 +551,10 @@ impl<W: Write> Interpreter<'_, W> {
         }
         check_slice_can_pop_all_element_types(args[1], &slice)?;
 
+        // The slice might contain more elements than its length.
+        // We want the last valid element, ignoring any extras following it.
+        // We don't ever access the extras, so we might as well remove any.
+        slice_elements.truncate(element_types.len() * length as usize);
         let mut popped_elements = vecmap(0..element_types.len(), |_| slice_elements.pop().unwrap());
         popped_elements.reverse();
 

test_programs/execution_success/regression_9312/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "regression_9312"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/regression_9312/Prover.toml

@@ -0,0 +1,2 @@
+a = true
+return = [[1, 20], [3, 60]]

test_programs/execution_success/regression_9312/src/main.nr

@@ -0,0 +1,6 @@
+fn main(a: bool) -> pub ((u32, Field), (u32, Field)) {
+    let s0: [Field] = if a { &[10, 20] } else { &[30, 40, 50] };
+    let (s1, f) = s0.pop_back();
+    let s2 = s0.push_back(60);
+    ((s1.len(), f), (s2.len(), s2[2]))
+}