[PATCH v2] rust: mem: add `Align` type

[Gary Guo <gary@garyguo.net>]

Rust's `repr(align())` only works with integers. Add an `Align` type so it
can also be specified via const generics.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
Andreas, could you see if this patch meets your need for cache-line
aligning?

Changes in v2:
- Added `Clone`, `Copy`, `Debug` impl. (Sashiko)
- Expand impl to 65536, i.e. PAGE_SIZE_64K. Kernel technically support
  PAGE_SIZE_256K, but it's uncommon enough so I think there's no need to
  support it unless a need arises. (Sashiko)
- Use `_align` in example, as usually the field is not going to be
  referenced.
- Link to v1: https://patch.msgid.link/20260609120325.233721-1-gary@kernel.org
---
 rust/kernel/lib.rs |  1 +
 rust/kernel/mem.rs | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9512af7156df..0a12c3011c61 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -92,6 +92,7 @@
 pub mod kunit;
 pub mod list;
 pub mod maple_tree;
+pub mod mem;
 pub mod miscdevice;
 pub mod mm;
 pub mod module_param;
diff --git a/rust/kernel/mem.rs b/rust/kernel/mem.rs
new file mode 100644
index 000000000000..7f7545c6e545
--- /dev/null
+++ b/rust/kernel/mem.rs
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Utilities handling things related to memory layouts.
+
+// TODO: `Alignment` should be moved here as well, Rust moved it upstream in 1.96.
+pub use crate::ptr::Alignment;
+
+/// Trait indicating that [`Align<N>`] is a valid alignment.
+pub trait ValidAlign {
+    #[doc(hidden)]
+    type Repr: Copy;
+}
+
+/// Zero-sized type that provides the alignment specified via the generic parameter.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::mem::Align;
+/// const ALIGN: usize = 128;
+/// struct MyStruct {
+///     _align: Align<ALIGN>,
+/// }
+/// assert_eq!(align_of::<MyStruct>(), ALIGN);
+/// ```
+#[repr(transparent)]
+#[derive(Default, Clone, Copy)]
+pub struct Align<const N: usize>([<Self as ValidAlign>::Repr; 0])
+where
+    Self: ValidAlign;
+
+impl<const N: usize> Align<N>
+where
+    Self: ValidAlign,
+{
+    /// Create a new [`Align<N>`].
+    #[inline]
+    pub const fn new() -> Self {
+        Align([])
+    }
+}
+
+macro_rules! impl_align {
+    () => {};
+    ($a:literal $($rest:literal)*) => {
+        const _: () = {
+            #[repr(align($a))]
+            #[derive(Clone, Copy)]
+            pub struct Repr;
+
+            impl ValidAlign for Align<$a> {
+                type Repr = Repr;
+            }
+        };
+
+        impl_align!($($rest)*);
+    }
+}
+
+impl_align!(1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768 65536);
+
+impl<const N: usize> core::fmt::Debug for Align<N>
+where
+    Self: ValidAlign,
+{
+    #[inline]
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "Align<{N}>")
+    }
+}

---
base-commit: a87737435cfa134f9cdcc696ba3080759d04cf72
change-id: 20260610-const-align-7020a76367ce

Best regards,
--  
Gary Guo <gary@garyguo.net>