Add test with by value type containing mut ref.

stainless_frontend/tests/pass/mut_ref_borrow_11.rs

@@ -1,51 +1,16 @@
 extern crate stainless;
 
-/// This benchmark stems from [RustVis Specs](http://erik.vestera.as/rustvis/specs)
-/// and was adapted to work with stainless.
-
-struct Container<K, V> {
-  pair: Option<(K, V)>,
-}
-
-impl<K, V> Container<K, V> {
-  pub fn new() -> Self {
-    Container { pair: None }
-  }
-
-  pub fn insert(&mut self, k: K, v: V) {
-    self.pair = Some((k, v))
-  }
-}
-
-impl<V> Container<String, V> {
-  pub fn get_mut(&mut self, key: &String) -> Option<&mut V> {
-    match &mut self.pair {
-      Some((k, v)) if *k == *key => Some(v),
-      _ => None,
-    }
+#[allow(unused_mut, unused_assignments)]
+fn change(mut opt: Option<&mut i32>) {
+  if let Some(x) = opt {
+    *x = 456;
   }
+  opt = None;
 }
 
 pub fn main() {
-  let mut target = Container::new();
-  let key = "foo".to_string();
-
-  let ref1 = &mut target;
-  match ref1.get_mut(&key) {
-    Some(value) => *value = 123,
-    _ => {
-      target.insert(key.clone(), 5);
-      match target.get_mut(&key) {
-        Some(v) => *v = 123,
-        _ => panic!("no value"),
-      }
-    }
-  }
-
-  assert!(matches!(
-    target,
-    Container {
-      pair: Some((key, 123))
-    } if key == "foo"
-  ))
+  let mut x = 123;
+  change(None);
+  change(Some(&mut x));
+  assert!(x == 456)
 }