Add test for closure drop.

tests/run-pass/closure-drop.rs

@@ -0,0 +1,30 @@
+struct Foo<'a>(&'a mut bool);
+
+impl<'a> Drop for Foo<'a> {
+    fn drop(&mut self) {
+        *self.0 = true;
+    }
+}
+
+fn f<T: FnOnce()>(t: T) {
+    t()
+}
+
+fn main() {
+    let mut ran_drop = false;
+    {
+        // FIXME: v is a temporary hack to force the below closure to be a FnOnce-only closure
+        // (with sig fn(self)). Without it, the closure sig would be fn(&self) which requires a
+        // shim to call via FnOnce::call_once, and Miri's current shim doesn't correctly call
+        // destructors.
+        let v = vec![1];
+        let x = Foo(&mut ran_drop);
+        let g = move || {
+            let _ = x;
+            drop(v); // Force the closure to be FnOnce-only by using a capture by-value.
+        };
+        f(g);
+    }
+    assert!(ran_drop);
+}
+