not insta-stable

src/liballoc/lib.rs

@@ -123,6 +123,7 @@
 #![feature(pointer_methods)]
 #![feature(inclusive_range_fields)]
 #![cfg_attr(stage0, feature(generic_param_attrs))]
+#![feature(rustc_const_unstable)]
 
 #![cfg_attr(not(test), feature(fn_traits, i128))]
 #![cfg_attr(test, feature(test))]

src/liballoc/vec.rs

@@ -322,6 +322,7 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_vec_new")]
     pub const fn new() -> Vec<T> {
         Vec {
             buf: RawVec::empty(),