FromZeroes for Option<T> where T is subject to NPO

For thin pointer types, `P` (`Box<T>`, `&T`, `&mut T`, and `NonNull<T>`
where `T: Sized`), implement `FromZeroes` for `Option<P>`.

Release 0.7.25.

Closes #293

Cargo.toml

@@ -15,7 +15,7 @@
 [package]
 edition = "2018"
 name = "zerocopy"
-version = "0.7.24"
+version = "0.7.25"
 authors = ["Joshua Liebow-Feeser <joshlf@google.com>"]
 description = "Utilities for zero-copy parsing and serialization"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"
@@ -49,7 +49,7 @@ simd-nightly = ["simd"]
 __internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd"]
 
 [dependencies]
-zerocopy-derive = { version = "=0.7.24", path = "zerocopy-derive", optional = true }
+zerocopy-derive = { version = "=0.7.25", path = "zerocopy-derive", optional = true }
 
 [dependencies.byteorder]
 version = "1.3"
@@ -60,7 +60,7 @@ optional = true
 # zerocopy-derive remain equal, even if the 'derive' feature isn't used.
 # See: https://github.com/matklad/macro-dep-test
 [target.'cfg(any())'.dependencies]
-zerocopy-derive = { version = "=0.7.24", path = "zerocopy-derive" }
+zerocopy-derive = { version = "=0.7.25", path = "zerocopy-derive" }
 
 [dev-dependencies]
 assert_matches = "1.5"
@@ -75,6 +75,6 @@ testutil = { path = "testutil" }
 # CI test failures.
 trybuild = { version = "=1.0.85", features = ["diff"] }
 # In tests, unlike in production, zerocopy-derive is not optional
-zerocopy-derive = { version = "=0.7.24", path = "zerocopy-derive" }
+zerocopy-derive = { version = "=0.7.25", path = "zerocopy-derive" }
 # TODO(#381) Remove this dependency once we have our own layout gadgets.
 elain = "0.3.0"

src/lib.rs

@@ -1880,6 +1880,37 @@ safety_comment! {
     unsafe_impl!(Option<NonZeroIsize>: FromZeroes, FromBytes, AsBytes);
 }
 
+safety_comment! {
+    /// SAFETY:
+    /// The following types can be transmuted from `[0u8; size_of::<T>()]`. [1]
+    /// None of them contain `UnsafeCell`s, and so they all soundly implement
+    /// `FromZeroes`.
+    ///
+    /// [1] Per
+    /// https://doc.rust-lang.org/nightly/core/option/index.html#representation:
+    ///
+    ///   Rust guarantees to optimize the following types `T` such that
+    ///   [`Option<T>`] has the same size and alignment as `T`. In some of these
+    ///   cases, Rust further guarantees that `transmute::<_, Option<T>>([0u8;
+    ///   size_of::<T>()])` is sound and produces `Option::<T>::None`. These
+    ///   cases are identified by the second column:
+    ///
+    ///   | `T`                 | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
+    ///   |---------------------|-----------------------------------------------------------|
+    ///   | [`Box<U>`]          | when `U: Sized`                                           |
+    ///   | `&U`                | when `U: Sized`                                           |
+    ///   | `&mut U`            | when `U: Sized`                                           |
+    ///   | [`ptr::NonNull<U>`] | when `U: Sized`                                           |
+    ///
+    /// TODO(#429), TODO(https://github.com/rust-lang/rust/pull/115333): Cite
+    /// the Stable docs once they're available.
+    #[cfg(feature = "alloc")]
+    unsafe_impl!(T => FromZeroes for Option<Box<T>>);
+    unsafe_impl!(T => FromZeroes for Option<&'_ T>);
+    unsafe_impl!(T => FromZeroes for Option<&'_ mut T>);
+    unsafe_impl!(T => FromZeroes for Option<NonNull<T>>);
+}
+
 safety_comment! {
     /// SAFETY:
     /// For all `T`, `PhantomData<T>` has size 0 and alignment 1. [1]
@@ -3820,7 +3851,7 @@ pub use alloc_support::*;
 mod tests {
     #![allow(clippy::unreadable_literal)]
 
-    use core::{convert::TryInto as _, ops::Deref};
+    use core::{cell::UnsafeCell, convert::TryInto as _, ops::Deref};
 
     use static_assertions::assert_impl_all;
 
@@ -5515,6 +5546,16 @@ mod tests {
         // Implements none of the ZC traits.
         struct NotZerocopy;
 
+        #[cfg(feature = "alloc")]
+        assert_impls!(Option<Box<UnsafeCell<NotZerocopy>>>: KnownLayout, FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<Box<[UnsafeCell<NotZerocopy>]>>: KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<&'static UnsafeCell<NotZerocopy>>: KnownLayout, FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<&'static [UnsafeCell<NotZerocopy>]>: KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<&'static mut UnsafeCell<NotZerocopy>>: KnownLayout, FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<&'static mut [UnsafeCell<NotZerocopy>]>: KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<NonNull<UnsafeCell<NotZerocopy>>>: KnownLayout, FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(Option<NonNull<[UnsafeCell<NotZerocopy>]>>: KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
+
         assert_impls!(PhantomData<NotZerocopy>: KnownLayout, FromZeroes, FromBytes, AsBytes, Unaligned);
         assert_impls!(PhantomData<[u8]>: KnownLayout, FromZeroes, FromBytes, AsBytes, Unaligned);
 

src/macros.rs

@@ -26,9 +26,9 @@
 /// The macro invocations are emitted, each decorated with the following
 /// attribute: `#[allow(clippy::undocumented_unsafe_blocks)]`.
 macro_rules! safety_comment {
-    (#[doc = r" SAFETY:"] $($(#[doc = $_doc:literal])* $macro:ident!$args:tt;)*) => {
-        #[allow(clippy::undocumented_unsafe_blocks)]
-        const _: () = { $($macro!$args;)* };
+    (#[doc = r" SAFETY:"] $($(#[$attr:meta])* $macro:ident!$args:tt;)*) => {
+        #[allow(clippy::undocumented_unsafe_blocks, unused_attributes)]
+        const _: () = { $($(#[$attr])* $macro!$args;)* };
     }
 }
 

zerocopy-derive/Cargo.toml

@@ -9,7 +9,7 @@
 [package]
 edition = "2018"
 name = "zerocopy-derive"
-version = "0.7.24"
+version = "0.7.25"
 authors = ["Joshua Liebow-Feeser <joshlf@google.com>"]
 description = "Custom derive for traits from the zerocopy crate"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"