test new From instances for Rc

tests/run-pass/rc.rs

@@ -1,11 +1,11 @@
 use std::cell::RefCell;
 use std::rc::Rc;
 
-fn rc_refcell() -> i32 {
+fn rc_refcell() {
     let r = Rc::new(RefCell::new(42));
     *r.borrow_mut() += 10;
     let x = *r.borrow();
-    x
+    assert_eq!(x, 52);
 }
 
 fn rc_raw() {
@@ -17,7 +17,23 @@ fn rc_raw() {
     assert!(Rc::try_unwrap(r2).is_ok());
 }
 
+// Make sure this Rc doesn't fall apart when touched
+fn check_unique_rc<T: ?Sized>(mut r: Rc<T>) {
+    let r2 = r.clone();
+    assert!(Rc::get_mut(&mut r).is_none());
+    drop(r2);
+    assert!(Rc::get_mut(&mut r).is_some());
+}
+
+fn rc_from() {
+    check_unique_rc::<[_]>(Rc::from(&[1,2,3] as &[_]));
+    check_unique_rc::<[_]>(Rc::from(vec![1,2,3]));
+    check_unique_rc::<[_]>(Rc::from(Box::new([1,2,3]) as Box<[_]>));
+    check_unique_rc::<str>(Rc::from("Hello, World!"));
+}
+
 fn main() {
     rc_refcell();
     rc_raw();
+    rc_from();
 }