fix: expect [] on a non-empty list now fails.

crates/aiken-lang/src/gen_uplc.rs

@@ -1824,7 +1824,7 @@ impl<'a> CodeGenerator<'a> {
                         value_stack,
                     );
                 } else {
-                    pattern_stack.let_assignment("_", value_stack);
+                    pattern_stack.list_empty(value_stack);
                 }
 
                 pattern_stack.merge_child(elements_stack);
@@ -4696,6 +4696,17 @@ impl<'a> CodeGenerator<'a> {
 
                 arg_stack.push(term);
             }
+            Air::ListEmpty { .. } => {
+                let value = arg_stack.pop().unwrap();
+                let mut term = arg_stack.pop().unwrap();
+
+                term = value.delayed_choose_list(
+                    term,
+                    Term::Error.trace(Term::string("Expected no items for List")),
+                );
+
+                arg_stack.push(term);
+            }
             Air::Tuple { tipo, count, .. } => {
                 let mut args = vec![];
 

crates/aiken-lang/src/gen_uplc/air.rs

@@ -219,6 +219,9 @@ pub enum Air {
     FieldsEmpty {
         scope: Scope,
     },
+    ListEmpty {
+        scope: Scope,
+    },
 }
 
 impl Air {
@@ -257,6 +260,7 @@ impl Air {
             | Air::RecordAccess { scope, .. }
             | Air::FieldsExpose { scope, .. }
             | Air::FieldsEmpty { scope }
+            | Air::ListEmpty { scope }
             | Air::ListAccessor { scope, .. }
             | Air::ListExpose { scope, .. }
             | Air::TupleAccessor { scope, .. }
@@ -301,6 +305,7 @@ impl Air {
             | Air::RecordAccess { scope, .. }
             | Air::FieldsExpose { scope, .. }
             | Air::FieldsEmpty { scope }
+            | Air::ListEmpty { scope }
             | Air::ListAccessor { scope, .. }
             | Air::ListExpose { scope, .. }
             | Air::TupleAccessor { scope, .. }
@@ -398,6 +403,7 @@ impl Air {
             | Air::Finally { .. }
             | Air::FieldsExpose { .. }
             | Air::FieldsEmpty { .. }
+            | Air::ListEmpty { .. }
             | Air::NoOp { .. } => None,
             Air::UnOp { op, .. } => match op {
                 UnOp::Not => Some(

crates/aiken-lang/src/gen_uplc/stack.rs

@@ -781,6 +781,16 @@ impl AirStack {
 
         self.call(void(), expect_stack, vec![tail_stack, arg_stack2])
     }
+
+    pub fn list_empty(&mut self, value_stack: AirStack) {
+        self.new_scope();
+
+        self.air.push(Air::ListEmpty {
+            scope: self.scope.clone(),
+        });
+
+        self.merge_child(value_stack);
+    }
 }
 
 #[cfg(test)]