make rustc compilable

compiler/rustc_ast/src/lib.rs

@@ -10,7 +10,6 @@
 )]
 #![feature(associated_type_bounds)]
 #![feature(box_patterns)]
-#![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]

compiler/rustc_ast/src/ptr.rs

@@ -126,7 +126,8 @@ impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<T> {
 }
 
 impl<T> P<[T]> {
-    pub const fn new() -> P<[T]> {
+    // FIXME(const-hack) make this const again
+    pub fn new() -> P<[T]> {
         P { ptr: Box::default() }
     }