linux-kernel - [PATCH v2 06/13] rust: pin-init: implement `ZeroableOption` for function pointers with up to 20 arguments

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Message-ID: <20250523145125.523275-7-lossin@kernel.org>
Date: Fri, 23 May 2025 16:51:02 +0200
From: Benno Lossin <lossin@...nel.org>
To: Benno Lossin <lossin@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Fiona Behrens <me@...enk.dev>,
	Christian Schrefl <chrisi.schrefl@...il.com>
Cc: Lyude Paul <lyude@...hat.com>,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 06/13] rust: pin-init: implement `ZeroableOption` for function pointers with up to 20 arguments

`Option<[unsafe] [extern "abi"] fn(...args...) -> ret>` is documented
[1] to also have the `None` variant equal all zeroes.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Link: https://github.com/Rust-for-Linux/pin-init/pull/56/commits/b6c1ab4fb3699765f81ae512ecac5a2f032d8d51
Signed-off-by: Benno Lossin <lossin@...nel.org>
---
 rust/pin-init/src/lib.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index e46be80bd344..645633de0f6c 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -1638,6 +1638,22 @@ unsafe impl<$first: Zeroable, $($t: Zeroable),*> Zeroable for ($first, $($t),*)
 
 impl_tuple_zeroable!(A, B, C, D, E, F, G, H, I, J);
 
+macro_rules! impl_fn_zeroable_option {
+    ([$($abi:literal),* $(,)?] $args:tt) => {
+        $(impl_fn_zeroable_option!({extern $abi} $args);)*
+        $(impl_fn_zeroable_option!({unsafe extern $abi} $args);)*
+    };
+    ({$($prefix:tt)*} {$(,)?}) => {};
+    ({$($prefix:tt)*} {$ret:ident, $($rest:ident),* $(,)?}) => {
+        // SAFETY: function pointers are part of the option layout optimization:
+        // <https://doc.rust-lang.org/stable/std/option/index.html#representation>.
+        unsafe impl<$ret, $($rest),*> ZeroableOption for $($prefix)* fn($($rest),*) -> $ret {}
+        impl_fn_zeroable_option!({$($prefix)*} {$($rest),*,});
+    };
+}
+
+impl_fn_zeroable_option!(["Rust", "C"] { A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U });
+
 /// This trait allows creating an instance of `Self` which contains exactly one
 /// [structurally pinned value](https://doc.rust-lang.org/std/pin/index.html#projections-and-structural-pinning).
 ///

base-commit: ae8b3a83fb9de394f609035041cd7a668fda2ab3
prerequisite-patch-id: https://lore.kernel.org/all/20250523125424.192843-2-lossin@kernel.org
prerequisite-patch-id: https://lore.kernel.org/all/20250523125424.192843-3-lossin@kernel.org
prerequisite-patch-id: https://lore.kernel.org/all/20250523125424.192843-4-lossin@kernel.org
-- 
2.49.0