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>

1 files changed, 20 insertions, 0 deletions

diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index b4bc44c27bd4..c42105c2ff96 100644
--- a/rust/macros/lib.rs
+++ b/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)
+}