Replace Option<T> with MaybeUninit<T>

This avoids unneeded tag in `Option` and addresses FIXME comment.

Author: Kixunil

src/inline_lazy.rs

@@ -9,43 +9,30 @@ extern crate core;
 extern crate std;
 
 use self::std::cell::Cell;
-use self::std::hint::unreachable_unchecked;
+use self::std::mem::MaybeUninit;
 use self::std::prelude::v1::*;
 use self::std::sync::Once;
 #[allow(deprecated)]
 pub use self::std::sync::ONCE_INIT;
 
-// FIXME: Replace Option<T> with MaybeUninit<T> (stable since 1.36.0)
-pub struct Lazy<T: Sync>(Cell<Option<T>>, Once);
+pub struct Lazy<T: Sync>(Cell<MaybeUninit<T>>, Once);
 
 impl<T: Sync> Lazy<T> {
     #[allow(deprecated)]
-    pub const INIT: Self = Lazy(Cell::new(None), ONCE_INIT);
+    pub const INIT: Self = Lazy(Cell::new(MaybeUninit::uninit()), ONCE_INIT);
 
     #[inline(always)]
     pub fn get<F>(&'static self, f: F) -> &T
     where
         F: FnOnce() -> T,
     {
         self.1.call_once(|| {
-            self.0.set(Some(f()));
+            self.0.set(MaybeUninit::new(f()));
         });
 
-        // `self.0` is guaranteed to be `Some` by this point
+        // `self.0` is guaranteed to be initialized by this point
         // The `Once` will catch and propagate panics
-        unsafe {
-            match *self.0.as_ptr() {
-                Some(ref x) => x,
-                None => {
-                    debug_assert!(
-                        false,
-                        "attempted to dereference an uninitialized lazy static. This is a bug"
-                    );
-
-                    unreachable_unchecked()
-                }
-            }
-        }
+        unsafe { &*(*self.0.as_ptr()).as_ptr() }
     }
 }