rust: provide safe wrapper for MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy, but it introduces unsafe (and has a
pretty heavy syntax in general).  Introduce a trait that provides the
same functionality while staying within safe Rust.

In addition, MaybeUninit::zeroed() is not available as a "const"
function until Rust 1.75.0, so this also prepares for having handwritten
implementations of the trait until we can assume that version.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/zeroable.rs

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/// Encapsulates the requirement that
+/// `MaybeUninit::<Self>::zeroed().assume_init()` does not cause
+/// undefined behavior.
+///
+/// # Safety
+///
+/// Do not add this trait to a type unless all-zeroes is
+/// a valid value for the type.  In particular, remember that raw
+/// pointers can be zero, but references and `NonNull<T>` cannot
+/// unless wrapped with `Option<>`.
+pub unsafe trait Zeroable: Default {
+    /// SAFETY: If the trait was added to a type, then by definition
+    /// this is safe.
+    const ZERO: Self = unsafe { ::core::mem::MaybeUninit::<Self>::zeroed().assume_init() };
+}
+
+unsafe impl Zeroable for crate::bindings::Property__bindgen_ty_1 {}
+unsafe impl Zeroable for crate::bindings::Property {}
+unsafe impl Zeroable for crate::bindings::VMStateDescription {}
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_1 {}
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_2 {}