diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index 538bfc2129017..a7c23dbb79ca5 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -9,6 +9,8 @@
     test(attr(deny(warnings)))
 )]
 #![feature(box_patterns)]
+#![feature(const_default_impls)]
+#![feature(const_trait_impl)]
 #![feature(crate_visibility_modifier)]
 #![feature(if_let_guard)]
 #![feature(label_break_value)]
diff --git a/compiler/rustc_ast/src/ptr.rs b/compiler/rustc_ast/src/ptr.rs
index 70dbda8222445..89a0857992e49 100644
--- a/compiler/rustc_ast/src/ptr.rs
+++ b/compiler/rustc_ast/src/ptr.rs
@@ -128,14 +128,7 @@ impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<T> {
 
 impl<T> P<[T]> {
     pub const fn new() -> P<[T]> {
-        // HACK(eddyb) bypass the lack of a `const fn` to create an empty `Box<[T]>`
-        // (as trait methods, `default` in this case, can't be `const fn` yet).
-        P {
-            ptr: unsafe {
-                use std::ptr::NonNull;
-                std::mem::transmute(NonNull::<[T; 0]>::dangling() as NonNull<[T]>)
-            },
-        }
+        P { ptr: Box::default() }
     }
 
     #[inline(never)]