rust: add the container_of macro

This is the Rust version of the macro with the same name in C. It
produces a raw pointer to an outer type from a pointer to an inner
field.

Signed-off-by: Wedson Almeida Filho <[email protected]>

Author: wedsonaf

rust/kernel/lib.rs

@@ -16,6 +16,7 @@
 #![feature(coerce_unsized)]
 #![feature(dispatch_from_dyn)]
 #![feature(new_uninit)]
+#![feature(offset_of)]
 #![feature(receiver_trait)]
 #![feature(return_position_impl_trait_in_trait)]
 #![feature(unsize)]
@@ -113,6 +114,38 @@ impl ThisModule {
     }
 }
 
+/// Produces a pointer to an object from a pointer to one of its fields.
+///
+/// # Safety
+///
+/// Callers must ensure that the pointer to the field is in fact a pointer to the specified field,
+/// as opposed to a pointer to another object of the same type. If this condition is not met,
+/// any dereference of the resulting pointer is UB.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::container_of;
+/// struct Test {
+///     a: u64,
+///     b: u32,
+/// }
+///
+/// let test = Test { a: 10, b: 20 };
+/// let b_ptr = &test.b;
+/// let test_alias = container_of!(b_ptr, Test, b);
+/// assert!(core::ptr::eq(&test, test_alias));
+/// ```
+#[macro_export]
+macro_rules! container_of {
+    ($ptr:expr, $type:ty, $($f:tt)*) => {{
+        let ptr = $ptr as *const _ as *const u8;
+        let offset = ::core::mem::offset_of!($type, $($f)*);
+        $crate::build_assert!(offset <= isize::MAX as usize);
+        ptr.wrapping_sub(offset) as *const $type
+    }}
+}
+
 #[cfg(not(any(testlib, test)))]
 #[panic_handler]
 fn panic(info: &core::panic::PanicInfo<'_>) -> ! {

scripts/Makefile.build

@@ -262,7 +262,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit,return_position_impl_trait_in_trait
+rust_allowed_features := new_uninit,return_position_impl_trait_in_trait,offset_of
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree