[PATCH v2 2/8] rust: add offset_of! macro

[Alice Ryhl <aliceryhl@google.com>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

This macro is used to compute the offset of a field in a struct.

This commit enables an unstable feature that is necessary for using
the macro in a constant. However, this is not a problem as the macro
will become available from the Rust standard library soon [1]. The
unstable feature can be disabled again once that happens.

The macro in this patch does not support sub-fields. That is, you cannot
write `offset_of!(MyStruct, field.sub_field)` to get the offset of
`sub_field` with `field`'s type being a struct with a field called
`sub_field`. This is because `field` might be a `Box<SubStruct>`, which
means that you would be trying to compute the offset to something in an
entirely different allocation. There's no easy way to fix the current
macro to support subfields, but the version being added to the standard
library should support it, so the limitation is temporary and not a big
deal.

Link: https://github.com/rust-lang/rust/issues/106655 [1]
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
---
 rust/kernel/lib.rs     | 35 +++++++++++++++++++++++++++++++++++
 scripts/Makefile.build |  2 +-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index eaded02ffb01..7ea777b731e6 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -14,6 +14,7 @@
 #![no_std]
 #![feature(allocator_api)]
 #![feature(coerce_unsized)]
+#![feature(const_refs_to_cell)]
 #![feature(dispatch_from_dyn)]
 #![feature(new_uninit)]
 #![feature(receiver_trait)]
@@ -98,3 +99,37 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
     // instead of `!`. See <https://github.com/rust-lang/rust-bindgen/issues/2094>.
     loop {}
 }
+
+/// Calculates the offset of a field from the beginning of the struct it belongs to.
+///
+/// # Examples
+///
+/// ```
+/// #[repr(C)]
+/// struct Test {
+///     a: u64,
+///     b: u32,
+/// }
+///
+/// assert_eq!(kernel::offset_of!(Test, b), 8);
+/// ```
+#[macro_export]
+macro_rules! offset_of {
+    ($type:path, $field:ident) => {{
+        let $type { $field: _, .. };
+        let tmp = ::core::mem::MaybeUninit::<$type>::uninit();
+        let outer = tmp.as_ptr();
+        // To avoid warnings when nesting `unsafe` blocks.
+        #[allow(unused_unsafe)]
+        // SAFETY: The pointer is valid and aligned, just not initialised; `addr_of` ensures that
+        // we don't actually read from `outer` (which would be UB) nor create an intermediate
+        // reference.
+        let inner = unsafe { ::core::ptr::addr_of!((*outer).$field) } as *const u8;
+        // To avoid warnings when nesting `unsafe` blocks.
+        #[allow(unused_unsafe)]
+        // SAFETY: The two pointers are within the same allocation block.
+        unsafe {
+            inner.offset_from(outer as *const u8) as usize
+        }
+    }};
+}
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 78175231c969..819510694769 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -277,7 +277,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit
+rust_allowed_features := const_refs_to_cell,new_uninit
 
 rust_common_cmd = \
 	RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) $(rust_flags) \
-- 
2.41.0.rc0.172.g3f132b7071-goog