[PATCH v2 4/9] rust: list: add macro for implementing ListItem

[Alice Ryhl <aliceryhl@google.com>]

Adds a macro for safely implementing the ListItem trait. As part of the
implementation of the macro, we also provide a HasListLinks trait
similar to the workqueue's HasWorkItem trait.

The HasListLinks trait is only necessary if you are implementing
ListItem using the impl_list_item macro.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/list.rs                    |  3 ++
 rust/kernel/list/impl_list_item_mod.rs | 99 ++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index b5cfbb96a392..f2eca542e090 100644
--- a/rust/kernel/list.rs
+++ b/rust/kernel/list.rs
@@ -8,6 +8,9 @@
 use crate::types::Opaque;
 use core::ptr;
 
+mod impl_list_item_mod;
+pub use self::impl_list_item_mod::{impl_has_list_links, impl_list_item, HasListLinks};
+
 mod arc;
 pub use self::arc::{
     impl_list_arc_safe, AtomicListArcTracker, ListArc, ListArcSafe, TryNewListArc,
diff --git a/rust/kernel/list/impl_list_item_mod.rs b/rust/kernel/list/impl_list_item_mod.rs
new file mode 100644
index 000000000000..3ff483be89d1
--- /dev/null
+++ b/rust/kernel/list/impl_list_item_mod.rs
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Helpers for implementing list traits safely.
+
+use crate::list::ListLinks;
+
+/// Declares that this type has a `ListLinks<ID>` field at a fixed offset.
+///
+/// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented
+/// manually, then this trait is not needed.
+///
+/// # Safety
+///
+/// All values of this type must have a `ListLinks<ID>` field at the given offset.
+pub unsafe trait HasListLinks<const ID: u64 = 0> {
+    /// The offset of the `ListLinks` field.
+    const OFFSET: usize;
+
+    /// Returns a pointer to the [`ListLinks<T, ID>`] field.
+    ///
+    /// # Safety
+    ///
+    /// The provided pointer must point at a valid struct of type `Self`.
+    ///
+    /// [`ListLinks<T, ID>`]: ListLinks
+    // We don't really need this method, but it's necessary for the implementation of
+    // `impl_has_work!` to be correct.
+    #[inline]
+    unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> {
+        // SAFETY: The caller promises that the pointer is valid. The implementer promises that the
+        // `OFFSET` constant is correct.
+        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> }
+    }
+}
+
+/// Implements the [`HasListLinks`] trait for the given type.
+#[macro_export]
+macro_rules! impl_has_list_links {
+    ($(impl$(<$($implarg:ident),*>)?
+       HasListLinks$(<$id:tt>)?
+       for $self:ident $(<$($selfarg:ty),*>)?
+       { self$(.$field:ident)* }
+    )*) => {$(
+        // SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
+        // right type.
+        unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for
+            $self $(<$($selfarg),*>)?
+        {
+            const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize;
+
+            #[inline]
+            unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? {
+                // SAFETY: The caller promises that the pointer is not dangling.
+                unsafe {
+                    ::core::ptr::addr_of_mut!((*ptr)$(.$field)*)
+                }
+            }
+        }
+    )*};
+}
+pub use impl_has_list_links;
+
+/// Implements the [`ListItem`] trait for the given type.
+///
+/// Assumes that the type implements [`HasListLinks`].
+///
+/// [`ListItem`]: crate::list::ListItem
+#[macro_export]
+macro_rules! impl_list_item {
+    (
+        impl$({$($generics:tt)*})? $crate::list::ListItem<$num:tt> for $t:ty {
+            using ListLinks;
+        } $($rest:tt)*
+    ) => {
+        unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
+            unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
+                unsafe {
+                    <Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
+                }
+            }
+
+            unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
+                let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
+                unsafe { (me as *const u8).sub(offset) as *const Self }
+            }
+
+            unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
+                unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) }
+            }
+
+            unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
+                unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
+            }
+        }
+    };
+}
+pub use impl_list_item;

-- 
2.45.0.rc1.225.g2a3ae87e7f-goog