[PATCH 8/8] rust: pin-init: add `MaybeZeroable` derive macro

[Benno Lossin <benno.lossin@proton.me>]

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>
---
 rust/pin-init/internal/src/lib.rs      |  5 +++
 rust/pin-init/internal/src/zeroable.rs | 27 +++++++++++-
 rust/pin-init/src/lib.rs               | 30 +++++++++++++
 rust/pin-init/src/macros.rs            | 59 ++++++++++++++++++++++++++
 4 files changed, 120 insertions(+), 1 deletion(-)

diff --git a/rust/pin-init/internal/src/lib.rs b/rust/pin-init/internal/src/lib.rs
index 56aa9ecc1e1a..297b0129a5bf 100644
--- a/rust/pin-init/internal/src/lib.rs
+++ b/rust/pin-init/internal/src/lib.rs
@@ -47,3 +47,8 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 pub fn derive_zeroable(input: TokenStream) -> TokenStream {
     zeroable::derive(input.into()).into()
 }
+
+#[proc_macro_derive(MaybeZeroable)]
+pub fn maybe_derive_zeroable(input: TokenStream) -> TokenStream {
+    zeroable::maybe_derive(input.into()).into()
+}
diff --git a/rust/pin-init/internal/src/zeroable.rs b/rust/pin-init/internal/src/zeroable.rs
index acc94008c152..e0ed3998445c 100644
--- a/rust/pin-init/internal/src/zeroable.rs
+++ b/rust/pin-init/internal/src/zeroable.rs
@@ -6,7 +6,14 @@
 use crate::helpers::{parse_generics, Generics};
 use proc_macro::{TokenStream, TokenTree};
 
-pub(crate) fn derive(input: TokenStream) -> TokenStream {
+pub(crate) fn parse_zeroable_derive_input(
+    input: TokenStream,
+) -> (
+    Vec<TokenTree>,
+    Vec<TokenTree>,
+    Vec<TokenTree>,
+    Option<TokenTree>,
+) {
     let (
         Generics {
             impl_generics,
@@ -64,6 +71,11 @@ pub(crate) fn derive(input: TokenStream) -> TokenStream {
     if in_generic && !inserted {
         new_impl_generics.extend(quote! { : ::pin_init::Zeroable });
     }
+    (rest, new_impl_generics, ty_generics, last)
+}
+
+pub(crate) fn derive(input: TokenStream) -> TokenStream {
+    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
     quote! {
         ::pin_init::__derive_zeroable!(
             parse_input:
@@ -74,3 +86,16 @@ pub(crate) fn derive(input: TokenStream) -> TokenStream {
         );
     }
 }
+
+pub(crate) fn maybe_derive(input: TokenStream) -> TokenStream {
+    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
+    quote! {
+        ::pin_init::__maybe_derive_zeroable!(
+            parse_input:
+                @sig(#(#rest)*),
+                @impl_generics(#(#new_impl_generics)*),
+                @ty_generics(#(#ty_generics)*),
+                @body(#last),
+        );
+    }
+}
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::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 @@ fn ensure_zeroable<$($impl_generics)*>()
         };
     };
 }
+
+#[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)*)?
+        {}
+    };
+}
-- 
2.48.1