rust: pin-init: add `MaybeZeroable` derive macro

This derive macro implements `Zeroable` for structs & unions precisely
if all fields also implement `Zeroable` and does nothing otherwise. The
plain `Zeroable` derive macro instead errors when it cannot derive
`Zeroable` safely. The `MaybeZeroable` derive macro is useful in cases
where manual checking is infeasible such as with the bindings crate.

Move the zeroable generics parsing into a standalone function in order
to avoid code duplication between the two derive macros.

Link: https://github.com/Rust-for-Linux/pin-init/pull/42/commits/1165cdad1a391b923efaf30cf76bc61e38da022e
Signed-off-by: Benno Lossin <benno.lossin@proton.me>

2 files changed, 89 insertions, 0 deletions

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 774f8ca033bc..05a0cd6ad8f4 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -413,6 +413,36 @@ pub use ::pin_init_internal::pinned_drop;
 /// ```
 pub use ::pin_init_internal::Zeroable;
 
+/// Derives the [`Zeroable`] trait for the given struct if all fields implement [`Zeroable`].
+///
+/// Contrary to the derive macro named [`macro@Zeroable`], this one silently fails when a field
+/// doesn't implement [`Zeroable`].
+///
+/// # Examples
+///
+/// ```
+/// use pin_init::MaybeZeroable;
+///
+/// // implmements `Zeroable`
+/// #[derive(MaybeZeroable)]
+/// pub struct DriverData {
+///     id: i64,
+///     buf_ptr: *mut u8,
+///     len: usize,
+/// }
+///
+/// // does not implmement `Zeroable`
+/// #[derive(MaybeZeroable)]
+/// pub struct DriverData2 {
+///     id: i64,
+///     buf_ptr: *mut u8,
+///     len: usize,
+///     // this field doesn't implement `Zeroable`
+///     other_data: &'static i32,
+/// }
+/// ```
+pub use ::pin_init_internal::MaybeZeroable;
+
 /// Initialize and pin a type directly on the stack.
 ///
 /// # Examples
diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
index 332d7e08925b..935d77745d1d 100644
--- a/rust/pin-init/src/macros.rs
+++ b/rust/pin-init/src/macros.rs
@@ -1443,3 +1443,62 @@ macro_rules! __derive_zeroable {
         };
     };
 }
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __maybe_derive_zeroable {
+    (parse_input:
+        @sig(
+            $(#[$($struct_attr:tt)*])*
+            $vis:vis struct $name:ident
+            $(where $($whr:tt)*)?
+        ),
+        @impl_generics($($impl_generics:tt)*),
+        @ty_generics($($ty_generics:tt)*),
+        @body({
+            $(
+                $(#[$($field_attr:tt)*])*
+                $field_vis:vis $field:ident : $field_ty:ty
+            ),* $(,)?
+        }),
+    ) => {
+        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
+        #[automatically_derived]
+        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
+        where
+            $(
+                // the `for<'__dummy>` HRTB makes this not error without the `trivial_bounds`
+                // feature <https://github.com/rust-lang/rust/issues/48214#issuecomment-2557829956>.
+                $field_ty: for<'__dummy> $crate::Zeroable,
+            )*
+            $($($whr)*)?
+        {}
+    };
+    (parse_input:
+        @sig(
+            $(#[$($struct_attr:tt)*])*
+            $vis:vis union $name:ident
+            $(where $($whr:tt)*)?
+        ),
+        @impl_generics($($impl_generics:tt)*),
+        @ty_generics($($ty_generics:tt)*),
+        @body({
+            $(
+                $(#[$($field_attr:tt)*])*
+                $field_vis:vis $field:ident : $field_ty:ty
+            ),* $(,)?
+        }),
+    ) => {
+        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
+        #[automatically_derived]
+        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
+        where
+            $(
+                // the `for<'__dummy>` HRTB makes this not error without the `trivial_bounds`
+                // feature <https://github.com/rust-lang/rust/issues/48214#issuecomment-2557829956>.
+                $field_ty: for<'__dummy> $crate::Zeroable,
+            )*
+            $($($whr)*)?
+        {}
+    };
+}