rust: add derive macro for `Zeroable`

Add a derive proc-macro for the `Zeroable` trait. The macro supports
structs where every field implements the `Zeroable` trait. This way
`unsafe` implementations can be avoided.

The macro is split into two parts:
- a proc-macro to parse generics into impl and ty generics,
- a declarative macro that expands to the impl block.

Suggested-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230814084602.25699-4-benno.lossin@proton.me
[ Added `ignore` to the `lib.rs` example and cleaned trivial nit. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/macros.rs

@@ -1215,3 +1215,38 @@ macro_rules! __init_internal {
         );
     };
 }
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __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:ident : $field_ty:ty
+            ),* $(,)?
+        }),
+    ) => {
+        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
+        #[automatically_derived]
+        unsafe impl<$($impl_generics)*> $crate::init::Zeroable for $name<$($ty_generics)*>
+        where
+            $($($whr)*)?
+        {}
+        const _: () = {
+            fn assert_zeroable<T: ?::core::marker::Sized + $crate::init::Zeroable>() {}
+            fn ensure_zeroable<$($impl_generics)*>()
+                where $($($whr)*)?
+            {
+                $(assert_zeroable::<$field_ty>();)*
+            }
+        };
+    };
+}

rust/kernel/prelude.rs

@@ -18,7 +18,7 @@ pub use core::pin::Pin;
 pub use alloc::{boxed::Box, vec::Vec};
 
 #[doc(no_inline)]
-pub use macros::{module, pin_data, pinned_drop, vtable};
+pub use macros::{module, pin_data, pinned_drop, vtable, Zeroable};
 
 pub use super::build_assert;
 

rust/macros/lib.rs

@@ -11,6 +11,7 @@ mod paste;
 mod pin_data;
 mod pinned_drop;
 mod vtable;
+mod zeroable;
 
 use proc_macro::TokenStream;
 
@@ -343,3 +344,22 @@ pub fn paste(input: TokenStream) -> TokenStream {
     paste::expand(&mut tokens);
     tokens.into_iter().collect()
 }
+
+/// Derives the [`Zeroable`] trait for the given struct.
+///
+/// This can only be used for structs where every field implements the [`Zeroable`] trait.
+///
+/// # Examples
+///
+/// ```rust,ignore
+/// #[derive(Zeroable)]
+/// pub struct DriverData {
+///     id: i64,
+///     buf_ptr: *mut u8,
+///     len: usize,
+/// }
+/// ```
+#[proc_macro_derive(Zeroable)]
+pub fn derive_zeroable(input: TokenStream) -> TokenStream {
+    zeroable::derive(input)
+}

rust/macros/quote.rs

@@ -124,6 +124,18 @@ macro_rules! quote_spanned {
         ));
         quote_spanned!(@proc $v $span $($tt)*);
     };
+    (@proc $v:ident $span:ident ; $($tt:tt)*) => {
+        $v.push(::proc_macro::TokenTree::Punct(
+                ::proc_macro::Punct::new(';', ::proc_macro::Spacing::Alone)
+        ));
+        quote_spanned!(@proc $v $span $($tt)*);
+    };
+    (@proc $v:ident $span:ident + $($tt:tt)*) => {
+        $v.push(::proc_macro::TokenTree::Punct(
+                ::proc_macro::Punct::new('+', ::proc_macro::Spacing::Alone)
+        ));
+        quote_spanned!(@proc $v $span $($tt)*);
+    };
     (@proc $v:ident $span:ident $id:ident $($tt:tt)*) => {
         $v.push(::proc_macro::TokenTree::Ident(::proc_macro::Ident::new(stringify!($id), $span)));
         quote_spanned!(@proc $v $span $($tt)*);

rust/macros/zeroable.rs

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use crate::helpers::{parse_generics, Generics};
+use proc_macro::{TokenStream, TokenTree};
+
+pub(crate) fn derive(input: TokenStream) -> TokenStream {
+    let (
+        Generics {
+            impl_generics,
+            ty_generics,
+        },
+        mut rest,
+    ) = parse_generics(input);
+    // This should be the body of the struct `{...}`.
+    let last = rest.pop();
+    // Now we insert `Zeroable` as a bound for every generic parameter in `impl_generics`.
+    let mut new_impl_generics = Vec::with_capacity(impl_generics.len());
+    // Are we inside of a generic where we want to add `Zeroable`?
+    let mut in_generic = !impl_generics.is_empty();
+    // Have we already inserted `Zeroable`?
+    let mut inserted = false;
+    // Level of `<>` nestings.
+    let mut nested = 0;
+    for tt in impl_generics {
+        match &tt {
+            // If we find a `,`, then we have finished a generic/constant/lifetime parameter.
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == ',' => {
+                if in_generic && !inserted {
+                    new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
+                }
+                in_generic = true;
+                inserted = false;
+                new_impl_generics.push(tt);
+            }
+            // If we find `'`, then we are entering a lifetime.
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == '\'' => {
+                in_generic = false;
+                new_impl_generics.push(tt);
+            }
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == ':' => {
+                new_impl_generics.push(tt);
+                if in_generic {
+                    new_impl_generics.extend(quote! { ::kernel::init::Zeroable + });
+                    inserted = true;
+                }
+            }
+            TokenTree::Punct(p) if p.as_char() == '<' => {
+                nested += 1;
+                new_impl_generics.push(tt);
+            }
+            TokenTree::Punct(p) if p.as_char() == '>' => {
+                assert!(nested > 0);
+                nested -= 1;
+                new_impl_generics.push(tt);
+            }
+            _ => new_impl_generics.push(tt),
+        }
+    }
+    assert_eq!(nested, 0);
+    if in_generic && !inserted {
+        new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
+    }
+    quote! {
+        ::kernel::__derive_zeroable!(
+            parse_input:
+                @sig(#(#rest)*),
+                @impl_generics(#(#new_impl_generics)*),
+                @ty_generics(#(#ty_generics)*),
+                @body(#last),
+        );
+    }
+}