Gate const core::mem::needs_drop behind const_needs_drop

src/libcore/mem.rs

@@ -483,6 +483,7 @@ pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
 /// ```
 #[inline]
 #[stable(feature = "needs_drop", since = "1.21.0")]
+#[rustc_const_unstable(feature = "const_needs_drop")]
 #[cfg(not(stage0))]
 pub const fn needs_drop<T>() -> bool {
     intrinsics::needs_drop::<T>()

src/test/run-pass/const-needs_drop.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(const_needs_drop)]
+
 use std::mem;
 
 struct Trivial(u8, f32);