rust/kernel: add field_size macro

Add a macro to determine the size of a structure field at compile time.
This is used by the CCA abstractions to ensure that the private data of
every CCA will fit into the space that the kernel provides for it.

Signed-off-by: Valentin Obst <kernel@valentinobst.de>

Author: Valentin Obst

rust/kernel/types.rs

@@ -398,3 +398,36 @@ pub enum Either<L, R> {
     /// Constructs an instance of [`Either`] containing a value of type `R`.
     Right(R),
 }
+
+/// Returns the size of a struct field in bytes.
+///
+/// This macro can be used in const contexts.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::field_size;
+///
+/// struct Foo {
+///     bar: u64,
+///     baz: [i8; 100],
+/// }
+///
+/// assert_eq!(field_size!(Foo, bar), 8);
+/// assert_eq!(field_size!(Foo, baz), 100);
+/// ```
+// Link: https://stackoverflow.com/a/70222282
+#[macro_export]
+macro_rules! field_size {
+    ($t:ty, $field:ident) => {{
+        let m = core::mem::MaybeUninit::<$t>::uninit();
+        // SAFETY: It is OK to dereference invalid pointers inside of
+        // `addr_of!`.
+        let p = unsafe { core::ptr::addr_of!((*m.as_ptr()).$field) };
+
+        const fn size_of_raw<T>(_: *const T) -> usize {
+            core::mem::size_of::<T>()
+        }
+        size_of_raw(p)
+    }};
+}