use a const to hack around promotion limitations

Author: RalfJung

src/liballoc/lib.rs

@@ -85,6 +85,7 @@
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(fundamental)]
+#![feature(internal_uninit_const)]
 #![feature(lang_items)]
 #![feature(libc)]
 #![feature(nll)]

src/libcore/macros.rs

@@ -653,7 +653,7 @@ macro_rules! uninit_array {
 #[cfg(not(bootstrap))]
 macro_rules! uninit_array {
     ($t:ty; $size:expr) => (
-        [MaybeUninit::<$t>::uninit(); $size]
+        [MaybeUninit::<$t>::UNINIT; $size]
     );
 }
 

src/libcore/mem/maybe_uninit.rs

@@ -248,11 +248,18 @@ impl<T> MaybeUninit<T> {
     /// [type]: union.MaybeUninit.html
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[inline(always)]
-    #[rustc_promotable]
     pub const fn uninit() -> MaybeUninit<T> {
         MaybeUninit { uninit: () }
     }
 
+    /// A promotable constant, equivalent to `uninit()`.
+    #[unstable(
+        feature = "internal_uninit_const",
+        issue = "0",
+        reason = "hack to work around promotability",
+    )]
+    pub const UNINIT: Self = Self::uninit();
+
     /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
     /// filled with `0` bytes. It depends on `T` whether that already makes for
     /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,