Add test with mutable pattern match.

stainless_frontend/tests/extraction_tests.rs

@@ -130,6 +130,7 @@ define_tests!(
   pass: mut_ref_borrow_6,
   pass: mut_ref_borrow_7,
   pass: mut_ref_borrow_8,
+  pass: mut_ref_borrow_9,
   pass: mut_ref_immut_borrow,
   pass: mut_return,
   pass: nested_spec,

stainless_frontend/tests/pass/mut_ref_borrow_9.rs

@@ -0,0 +1,43 @@
+extern crate stainless;
+
+struct S<T>(T);
+struct Container<T> {
+  s: S<T>,
+}
+
+pub fn main() {
+  let mut c = Container { s: S("hello") };
+
+  match &mut c.s {
+    S(v) if *v == "hello" => *v = "world",
+    _ => {}
+  };
+
+  assert!(c.s.0 == "world")
+}
+
+/*
+
+Desired Scala translation:
+
+import stainless.annotation._
+
+object Fields {
+  case class S[@mutable T](_0: MutCell[T])
+  case class MutCell[@mutable T](var value: T)
+  case class Container[@mutable T](s: MutCell[S[T]])
+
+  def main: Unit = {
+    var c: Container[String] = Container(MutCell(S(MutCell("hello"))))
+
+    (c.s match {
+      case MutCell(S(v)) if v.value == "hello" =>
+        v.value = "world"
+      case _ =>
+        ()
+    })
+    assert(c.s.value._0.value == "world")
+  }
+}
+
+ */