Make Pin::new_static const.

Author: m-ou-se

library/core/src/pin.rs

@@ -787,7 +787,8 @@ impl<T: ?Sized> Pin<&'static T> {
     /// This is safe, because the `'static` lifetime guarantees the data will
     /// never be moved.
     #[unstable(feature = "pin_static_ref", issue = "none")]
-    pub fn new_static(r: &'static T) -> Pin<&'static T> {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    pub const fn new_static(r: &'static T) -> Pin<&'static T> {
         // SAFETY: The 'static lifetime guarantees the data will not be
         // moved/invalidated until it gets dropped (which is never).
         unsafe { Pin::new_unchecked(r) }