Use pointer::is_aligned in ThinBox debug assert

Author: thomcc

library/alloc/src/boxed/thin.rs

@@ -224,7 +224,7 @@ impl<H> WithHeader<H> {
         //    will always result in an aligned header pointer, it just may not point to the
         //    beginning of the allocation.
         let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
-        debug_assert!((hp.addr() & (core::mem::align_of::<H>() - 1)) == 0);
+        debug_assert!(hp.is_aligned());
         hp
     }
 

library/alloc/src/lib.rs

@@ -168,6 +168,7 @@
 #![feature(nll)] // Not necessary, but here to test the `nll` feature.
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
+#![feature(pointer_is_aligned)]
 #![feature(slice_internals)]
 #![feature(staged_api)]
 #![cfg_attr(test, feature(test))]