alloc: restrict impl ZeroableOption for Box to T: Sized

examples/pthread_mutex.rs

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-// inspired by https://github.com/nbdd0121/pin-init/blob/trunk/examples/pthread_mutex.rs
+// inspired by <https://github.com/nbdd0121/pin-init/blob/trunk/examples/pthread_mutex.rs>
 #![allow(clippy::undocumented_unsafe_blocks)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
 #[cfg(not(windows))]

src/alloc.rs

@@ -17,11 +17,9 @@ use crate::{
 
 pub extern crate alloc;
 
-// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
-//
-// In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant and there
-// is no problem with a VTABLE pointer being null.
-unsafe impl<T: ?Sized> ZeroableOption for Box<T> {}
+// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee:
+// <https://doc.rust-lang.org/stable/std/option/index.html#representation>).
+unsafe impl<T> ZeroableOption for Box<T> {}
 
 /// Smart pointer that can initialize memory in-place.
 pub trait InPlaceInit<T>: Sized {

src/lib.rs

@@ -1447,7 +1447,7 @@ impl_zeroable! {
     {<T: ?Sized + Zeroable>} UnsafeCell<T>,
 
     // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee:
-    // https://doc.rust-lang.org/stable/std/option/index.html#representation).
+    // <https://doc.rust-lang.org/stable/std/option/index.html#representation>).
     Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
     Option<NonZeroU128>, Option<NonZeroUsize>,
     Option<NonZeroI8>, Option<NonZeroI16>, Option<NonZeroI32>, Option<NonZeroI64>,