Add #[must_use] to core and std constructors

library/std/src/sync/condvar.rs

@@ -121,6 +121,7 @@ impl Condvar {
     /// let condvar = Condvar::new();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn new() -> Condvar {
         Condvar { inner: sys::Condvar::new() }
     }