Implement Debug for MaybeUninit

Precedent: UnsafeCell implements Debug even though it can't actually
display the value.

src/libcore/mem/maybe_uninit.rs

@@ -1,3 +1,5 @@
+use crate::any::type_name;
+use crate::fmt;
 use crate::intrinsics;
 use crate::mem::ManuallyDrop;
 
@@ -230,6 +232,13 @@ impl<T: Copy> Clone for MaybeUninit<T> {
     }
 }
 
+#[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
+impl<T> fmt::Debug for MaybeUninit<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad(type_name::<Self>())
+    }
+}
+
 impl<T> MaybeUninit<T> {
     /// Creates a new `MaybeUninit<T>` initialized with the given value.
     /// It is safe to call [`assume_init`] on the return value of this function.