zephyr: Add alignment helper

The Rust attribute that indicates structure alignment only supports
alignment values that are numeric constants.  This makes it difficult to
set alignment based on a value coming from the build environment, which
is commonly done.

Implement a bit of a workaround with an `AlignAs` type that is
parameterized with the alignment.  Parameters can be compile-time
constants not just numeric constants.  This works by having instances
for each of the desired alignments that each have the numeric constant
as the alignment.  The instances can be expanded if necessary.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

zephyr/src/align.rs

@@ -0,0 +1,36 @@
+//! Alignment
+//!
+//! Natively, the align attribute in rust does not allow anything other than an integer literal.
+//! However, Zephyr will define the external alignment based on numeric constants.  This defines a
+//! bit of a trick to enforce alignment of structs to values by defined constants.
+//!
+//! Thanks to Chayim Refael Friedman for help with this.
+
+pub struct AlignAsStruct;
+
+pub trait AlignAsTrait<const N: usize> {
+    type Aligned;
+}
+
+macro_rules! impl_alignas {
+    ( $($align:literal),* $(,)? ) => {
+        $(
+            const _: () = {
+                #[repr(align($align))]
+                pub struct Aligned;
+                impl AlignAsTrait<$align> for AlignAsStruct {
+                    type Aligned = Aligned;
+                }
+            };
+        )*
+    };
+}
+// This can be expanded as needed.
+impl_alignas!(1, 2, 4, 8, 16, 32, 64, 128, 256);
+
+/// Align a given struct to a given alignment.  To use this, just include `AlignAs<N>` as the first
+/// member of the struct.
+#[repr(transparent)]
+pub struct AlignAs<const N: usize>([<AlignAsStruct as AlignAsTrait<N>>::Aligned; 0])
+    where
+    AlignAsStruct: AlignAsTrait<N>;

zephyr/src/lib.rs

@@ -84,6 +84,7 @@
 #![no_std]
 #![allow(unexpected_cfgs)]
 
+pub mod align;
 pub mod object;
 pub mod sync;
 pub mod sys;