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

[Paolo Bonzini <pbonzini@redhat.com>]

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