rust: do not use MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy but is not available as a "const" function
until Rust 1.75.0.

Remove the default implementation of Zeroable::ZERO, and write by hand
the definitions for those types that need it.  It may be possible to
add automatic implementation of the trait, via a procedural macro and/or
a trick similar to offset_of!, but do it the easy way for now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/zeroable.rs

@@ -1,23 +1,86 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+use std::ptr;
+
 /// Encapsulates the requirement that
-/// `MaybeUninit::<Self>::zeroed().assume_init()` does not cause
-/// undefined behavior.
+/// `MaybeUninit::<Self>::zeroed().assume_init()` does not cause undefined
+/// behavior.  This trait in principle could be implemented as just:
+///
+/// ```
+///     const ZERO: Self = unsafe {
+///         ::core::mem::MaybeUninit::<$crate::bindings::Property>::zeroed().assume_init()
+///     },
+/// ```
+///
+/// The need for a manual implementation is only because `zeroed()` cannot
+/// be used as a `const fn` prior to Rust 1.75.0. Once we can assume a new
+/// enough version of the compiler, we could provide a `#[derive(Zeroable)]`
+/// macro to check at compile-time that all struct fields are Zeroable, and
+/// use the above blanket implementation of the `ZERO` constant.
 ///
 /// # 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<>`.
+/// Because the implementation of `ZERO` is manual, it does not make
+/// any assumption on the safety of `zeroed()`.  However, other users of the
+/// trait could use it that way.  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
 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() };
+    const ZERO: Self;
 }
 
-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 {}
+unsafe impl Zeroable for crate::bindings::Property__bindgen_ty_1 {
+    const ZERO: Self = Self { i: 0 };
+}
+
+unsafe impl Zeroable for crate::bindings::Property {
+    const ZERO: Self = Self {
+        name: ptr::null(),
+        info: ptr::null(),
+        offset: 0,
+        bitnr: 0,
+        bitmask: 0,
+        set_default: false,
+        defval: Zeroable::ZERO,
+        arrayoffset: 0,
+        arrayinfo: ptr::null(),
+        arrayfieldsize: 0,
+        link_type: ptr::null(),
+    };
+}
+
+unsafe impl Zeroable for crate::bindings::VMStateDescription {
+    const ZERO: Self = Self {
+        name: ptr::null(),
+        unmigratable: false,
+        early_setup: false,
+        version_id: 0,
+        minimum_version_id: 0,
+        priority: crate::bindings::MigrationPriority::MIG_PRI_DEFAULT,
+        pre_load: None,
+        post_load: None,
+        pre_save: None,
+        post_save: None,
+        needed: None,
+        dev_unplug_pending: None,
+        fields: ptr::null(),
+        subsections: ptr::null(),
+    };
+}
+
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_1 {
+    const ZERO: Self = Self {
+        min_access_size: 0,
+        max_access_size: 0,
+        unaligned: false,
+        accepts: None,
+    };
+}
+
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_2 {
+    const ZERO: Self = Self {
+        min_access_size: 0,
+        max_access_size: 0,
+        unaligned: false,
+    };
+}