Auto merge of #61983 - Centril:rollup-wnfo07y, r=Centril

Rollup of 4 pull requests

Successful merges:

 - #60454 (Add custom nth_back to Skip)
 - #60772 (Implement nth_back for slice::{Iter, IterMut})
 - #61782 (suggest tuple struct syntax)
 - #61968 (rustc: disallow cloning HIR nodes.)

Failed merges:

r? @ghost

2 files changed, 11 insertions, 2 deletions

diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 55db8da3276..337b8424736 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -12,6 +12,8 @@
 #![deny(unused_lifetimes)]
 
 #![feature(bind_by_move_pattern_guards)]
+#![feature(const_fn)]
+#![feature(const_transmute)]
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]
 #![feature(nll)]
diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index d577243fb3d..f0cfa5a84a8 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -133,8 +133,15 @@ impl<T: Encodable> Encodable for P<T> {
 }
 
 impl<T> P<[T]> {
-    pub fn new() -> P<[T]> {
-        P { ptr: Default::default() }
+    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]>)
+            },
+        }
     }
 
     #[inline(never)]