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/hw/char/pl011/src/device_class.rs

@@ -4,7 +4,7 @@
 
 use core::ptr::NonNull;
 
-use qemu_api::{bindings::*, definitions::ObjectImpl};
+use qemu_api::{bindings::*, definitions::ObjectImpl, zeroable::Zeroable};
 
 use crate::device::PL011State;
 
@@ -12,7 +12,7 @@
 pub static VMSTATE_PL011: VMStateDescription = VMStateDescription {
     name: PL011State::TYPE_INFO.name,
     unmigratable: true,
-    ..unsafe { ::core::mem::MaybeUninit::<VMStateDescription>::zeroed().assume_init() }
+    ..Zeroable::ZERO
 };
 
 qemu_api::declare_properties! {

rust/hw/char/pl011/src/memory_ops.rs

@@ -2,9 +2,9 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use core::{mem::MaybeUninit, ptr::NonNull};
+use core::ptr::NonNull;
 
-use qemu_api::bindings::*;
+use qemu_api::{bindings::*, zeroable::Zeroable};
 
 use crate::device::PL011State;
 
@@ -14,11 +14,11 @@
     read_with_attrs: None,
     write_with_attrs: None,
     endianness: device_endian::DEVICE_NATIVE_ENDIAN,
-    valid: unsafe { MaybeUninit::<MemoryRegionOps__bindgen_ty_1>::zeroed().assume_init() },
+    valid: Zeroable::ZERO,
     impl_: MemoryRegionOps__bindgen_ty_2 {
         min_access_size: 4,
         max_access_size: 4,
-        ..unsafe { MaybeUninit::<MemoryRegionOps__bindgen_ty_2>::zeroed().assume_init() }
+        ..Zeroable::ZERO
     },
 };
 

rust/qemu-api/meson.build

@@ -5,6 +5,7 @@ _qemu_api_rs = static_library(
       'src/lib.rs',
       'src/definitions.rs',
       'src/device_class.rs',
+      'src/zeroable.rs',
     ],
     {'.' : bindings_rs},
   ),

rust/qemu-api/src/device_class.rs

@@ -31,7 +31,7 @@ macro_rules! define_property {
             offset: ::core::mem::offset_of!($state, $field) as isize,
             set_default: true,
             defval: $crate::bindings::Property__bindgen_ty_1 { u: $defval as u64 },
-            ..unsafe { ::core::mem::MaybeUninit::<$crate::bindings::Property>::zeroed().assume_init() }
+            ..$crate::zeroable::Zeroable::ZERO
         }
     };
     ($name:expr, $state:ty, $field:expr, $prop:expr, $type:expr$(,)*) => {
@@ -41,7 +41,7 @@ macro_rules! define_property {
             info: $prop,
             offset: ::core::mem::offset_of!($state, $field) as isize,
             set_default: false,
-            ..unsafe { ::core::mem::MaybeUninit::<$crate::bindings::Property>::zeroed().assume_init() }
+            ..$crate::zeroable::Zeroable::ZERO
         }
     };
 }
@@ -58,7 +58,7 @@ macro_rules! declare_properties {
             len
         }] = [
             $($prop),*,
-            unsafe { ::core::mem::MaybeUninit::<$crate::bindings::Property>::zeroed().assume_init() },
+            $crate::zeroable::Zeroable::ZERO,
         ];
     };
 }
@@ -79,7 +79,7 @@ macro_rules! vm_state_description {
                 $vname.as_ptr()
             },)*
             unmigratable: true,
-            ..unsafe { ::core::mem::MaybeUninit::<$crate::bindings::VMStateDescription>::zeroed().assume_init() }
+            ..$crate::zeroable::Zeroable::ZERO
         };
     }
 }

rust/qemu-api/src/lib.rs

@@ -29,6 +29,7 @@ unsafe impl Sync for bindings::VMStateDescription {}
 
 pub mod definitions;
 pub mod device_class;
+pub mod zeroable;
 
 use std::alloc::{GlobalAlloc, Layout};
 

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 {}