rust: use MaybeUninit::zeroed() in const context

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/rust.rst

@@ -83,10 +83,6 @@ are missing:
 * "Return position ``impl Trait`` in Traits" (1.75.0, blocker for including
   the pinned-init create).
 
-* ``MaybeUninit::zeroed()`` as a ``const`` function (1.75.0).  QEMU's
-  ``Zeroable`` trait can be implemented without ``MaybeUninit::zeroed()``,
-  so this would be just a cleanup.
-
 * ``c"" literals`` (stable in 1.77.0).  QEMU provides a ``c_str!()`` macro
   to define ``CStr`` constants easily
 

rust/hw/timer/hpet/src/fw_cfg.rs

@@ -4,7 +4,7 @@
 
 use std::ptr::addr_of_mut;
 
-use qemu_api::{cell::bql_locked, impl_zeroable, zeroable::Zeroable};
+use qemu_api::{cell::bql_locked, zeroable::Zeroable};
 
 /// Each `HPETState` represents a Event Timer Block. The v1 spec supports
 /// up to 8 blocks. QEMU only uses 1 block (in PC machine).
@@ -18,7 +18,7 @@ pub struct HPETFwEntry {
     pub min_tick: u16,
     pub page_prot: u8,
 }
-impl_zeroable!(HPETFwEntry);
+unsafe impl Zeroable for HPETFwEntry {}
 
 #[repr(C, packed)]
 #[derive(Copy, Clone, Default)]
@@ -26,7 +26,7 @@ pub struct HPETFwConfig {
     pub count: u8,
     pub hpet: [HPETFwEntry; HPET_MAX_NUM_EVENT_TIMER_BLOCK],
 }
-impl_zeroable!(HPETFwConfig);
+unsafe impl Zeroable for HPETFwConfig {}
 
 #[allow(non_upper_case_globals)]
 #[no_mangle]

rust/qemu-api/src/zeroable.rs

@@ -4,89 +4,17 @@
 
 /// Encapsulates the requirement that
 /// `MaybeUninit::<Self>::zeroed().assume_init()` does not cause undefined
-/// behavior.  This trait in principle could be implemented as just:
+/// behavior.
-///
-/// ```
-/// pub unsafe trait Zeroable: Default {
-///     const ZERO: Self = unsafe { ::core::mem::MaybeUninit::<Self>::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
 ///
-/// Because the implementation of `ZERO` is manual, it does not make
+/// Do not add this trait to a type unless all-zeroes is a valid value for the
-/// any assumption on the safety of `zeroed()`.  However, other users of the
+/// type.  In particular, raw pointers can be zero, but references and
-/// trait could use it that way.  Do not add this trait to a type unless
+/// `NonNull<T>` cannot.
-/// 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 {
-    const ZERO: Self;
+    /// Return a value of Self whose memory representation consists of all
-}
+    /// zeroes, with the possible exclusion of padding bytes.
-
+    const ZERO: Self = unsafe { ::core::mem::MaybeUninit::<Self>::zeroed().assume_init() };
-/// A macro that acts similarly to [`core::mem::zeroed()`], only is const
-///
-/// ## Safety
-///
-/// Similar to `core::mem::zeroed()`, except this zeroes padding bits. Zeroed
-/// padding usually isn't relevant to safety, but might be if a C union is used.
-///
-/// Just like for `core::mem::zeroed()`, an all zero byte pattern might not
-/// be a valid value for a type, as is the case for references `&T` and `&mut
-/// T`. Reference types trigger a (denied by default) lint and cause immediate
-/// undefined behavior if the lint is ignored
-///
-/// ```rust compile_fail
-/// use const_zero::const_zero;
-/// // error: any use of this value will cause an error
-/// // note: `#[deny(const_err)]` on by default
-/// const STR: &str = unsafe{const_zero!(&'static str)};
-/// ```
-///
-/// `const_zero` does not work on unsized types:
-///
-/// ```rust compile_fail
-/// use const_zero::const_zero;
-/// // error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
-/// const BYTES: [u8] = unsafe{const_zero!([u8])};
-/// ```
-/// ## Differences with `core::mem::zeroed`
-///
-/// `const_zero` zeroes padding bits, while `core::mem::zeroed` doesn't
-#[macro_export]
-macro_rules! const_zero {
-    // This macro to produce a type-generic zero constant is taken from the
-    // const_zero crate (v0.1.1):
-    //
-    //     https://docs.rs/const-zero/latest/src/const_zero/lib.rs.html
-    //
-    // and used under MIT license
-    ($type_:ty) => {{
-        const TYPE_SIZE: ::core::primitive::usize = ::core::mem::size_of::<$type_>();
-        union TypeAsBytes {
-            bytes: [::core::primitive::u8; TYPE_SIZE],
-            inner: ::core::mem::ManuallyDrop<$type_>,
-        }
-        const ZERO_BYTES: TypeAsBytes = TypeAsBytes {
-            bytes: [0; TYPE_SIZE],
-        };
-        ::core::mem::ManuallyDrop::<$type_>::into_inner(ZERO_BYTES.inner)
-    }};
-}
-
-/// A wrapper to implement the `Zeroable` trait through the `const_zero` macro.
-#[macro_export]
-macro_rules! impl_zeroable {
-    ($type:ty) => {
-        unsafe impl $crate::zeroable::Zeroable for $type {
-            const ZERO: Self = unsafe { $crate::const_zero!($type) };
-        }
-    };
 }
 
 // bindgen does not derive Default here
@@ -97,13 +25,13 @@ impl Default for crate::bindings::VMStateFlags {
     }
 }
 
-impl_zeroable!(crate::bindings::Property__bindgen_ty_1);
+unsafe impl Zeroable for crate::bindings::Property__bindgen_ty_1 {}
-impl_zeroable!(crate::bindings::Property);
+unsafe impl Zeroable for crate::bindings::Property {}
-impl_zeroable!(crate::bindings::VMStateFlags);
+unsafe impl Zeroable for crate::bindings::VMStateFlags {}
-impl_zeroable!(crate::bindings::VMStateField);
+unsafe impl Zeroable for crate::bindings::VMStateField {}
-impl_zeroable!(crate::bindings::VMStateDescription);
+unsafe impl Zeroable for crate::bindings::VMStateDescription {}
-impl_zeroable!(crate::bindings::MemoryRegionOps__bindgen_ty_1);
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_1 {}
-impl_zeroable!(crate::bindings::MemoryRegionOps__bindgen_ty_2);
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps__bindgen_ty_2 {}
-impl_zeroable!(crate::bindings::MemoryRegionOps);
+unsafe impl Zeroable for crate::bindings::MemoryRegionOps {}
-impl_zeroable!(crate::bindings::MemTxAttrs);
+unsafe impl Zeroable for crate::bindings::MemTxAttrs {}
-impl_zeroable!(crate::bindings::CharBackend);
+unsafe impl Zeroable for crate::bindings::CharBackend {}