[PATCH 1/2] rust: Zeroable: allow struct update syntax outside init macros

Paolo Bonzini
2024-11-28T14:13:31Z

The Zeroable trait is a marker trait, even though the various init macros
use a "fake" struct update syntax. Sometimes, such a struct update
syntax can be useful even outside the init macros. Add an associated
const that returns an all-zero instance of a Zeroable type.

The exact syntax used by the init macros cannot be reproduced without
forgoing the ability to use Zeroable::ZERO in const context. However,
it might not be a good idea to add a fn zeroed() inside the
Zeroable trait, to avoid confusion with the init::zeroed() function
and because Zeroable::ZERO is unrelated to the Init and PinInit
traits. In other words, let's treat this difference as a
feature rather than a bug.

The definition of the ZERO constant requires adding a Sized boundary, but
this is not a problem either because neither slices nor trait objects
are zeroable.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
rust/kernel/init.rs | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletion(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index a17ac8762d8f..a00e7ff6a513 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1392,7 +1392,12 @@ pub unsafe trait PinnedDrop: __internal::HasPinData {
/// ```rust,ignore
/// let val: Self = unsafe { core::mem::zeroed() };
/// ```
-pub unsafe trait Zeroable {}
+pub unsafe trait Zeroable: Sized {
+ /// Return a value of Self whose memory representation consists of all zeroes.
+ // SAFETY: the Zeroable trait itself is unsafe, and declaring it (whether
+ // manually or via derivation) implies that this is not undefined behavior.
+ const ZERO: Self = unsafe { core::mem::zeroed() };
+}

/// Create a new zeroed T.
///
@@ -1444,7 +1444,7 @@ macro_rules! impl_zeroable {
{<T>} Opaque<T>,

// SAFETY: `T: Zeroable` and `UnsafeCell` is `repr(transparent)`.
- {<T: ?Sized + Zeroable>} UnsafeCell<T>,
+ {<T: Zeroable>} UnsafeCell<T>,

// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
--
2.47.0