Merge #779

779: Don't call assume_init on Deferred's Data r=taiki-e a=saethlin

This situation can be observed by running `MIRIFLAGS="-Zmiri-check-number-validity" cargo miri test` in `crossbeam/crossbeam-deque`:
```
test is_empty ... error: Undefined Behavior: type validation failed at .value[0]: encountered uninitialized bytes
  --> /home/ben/crossbeam/crossbeam-epoch/src/deferred.rs:49:27a
   |
49 |                     data: data.assume_init(),
   |                           ^^^^^^^^^^^^^^^^^^ type validation failed at .value[0]: encountered uninitialized bytes
   |
```

In the crossbeam-deque test suite, a `Deferred` was created from a `FnOnce` which is smaller than the `Data`. This makes the call to `MaybeUninit::assume_init()` immediate UB (the reference to it created upon call is probably UB too). [The docs for `MaybeUninit::assume_init()`](https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#safety) say this:
> It is up to the caller to guarantee that the MaybeUninit<T> really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior. The [type-level documentation](https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#initialization-invariant) contains more information about this initialization invariant.

 Since `Data` doesn't have a `Drop` impl, we can just leave it in the `MaybeUninit` wrapper. This removes all issues about type validity.

Co-authored-by: Ben Kimock <kimockb@gmail.com>

crossbeam-epoch/src/deferred.rs

@@ -18,7 +18,7 @@ type Data = [usize; DATA_WORDS];
 /// This is a handy way of keeping an unsized `FnOnce()` within a sized structure.
 pub(crate) struct Deferred {
     call: unsafe fn(*mut u8),
-    data: Data,
+    data: MaybeUninit<Data>,
     _marker: PhantomData<*mut ()>, // !Send + !Sync
 }
 
@@ -46,7 +46,7 @@ impl Deferred {
 
                 Deferred {
                     call: call::<F>,
-                    data: data.assume_init(),
+                    data,
                     _marker: PhantomData,
                 }
             } else {
@@ -64,7 +64,7 @@ impl Deferred {
 
                 Deferred {
                     call: call::<F>,
-                    data: data.assume_init(),
+                    data,
                     _marker: PhantomData,
                 }
             }
@@ -75,7 +75,7 @@ impl Deferred {
     #[inline]
     pub(crate) fn call(mut self) {
         let call = self.call;
-        unsafe { call(&mut self.data as *mut Data as *mut u8) };
+        unsafe { call(self.data.as_mut_ptr() as *mut u8) };
     }
 }