From b911157579a4259ca20a38e1b1b17514752b139e Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Fri, 4 Feb 2022 20:17:26 -0500
Subject: [PATCH] Don't call assume_init on Deferred's Data

In the crossbeam-deque test suite, a Deferred is created from a FnOnce
which is smaller than the Data. This makes the call to assume_init()
immediate UB (the reference to it created upon call is probably UB too).
Since Data doesn't have a Drop impl, we can just leave it in the MaybeUninit
wrapper.
---
 crossbeam-epoch/src/deferred.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/crossbeam-epoch/src/deferred.rs b/crossbeam-epoch/src/deferred.rs
index 741d8c82a..7ec8f6d2e 100644
--- a/crossbeam-epoch/src/deferred.rs
+++ b/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) };
     }
 }