rust: ptr: add KnownSize trait to support DST size info extraction

Add a `KnownSize` trait which is used obtain a size from a raw pointer's
metadata. This makes it possible to obtain size information on a raw slice
pointer. This is similar to Rust `core::mem::size_of_val_raw` which is not
yet stable.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20260302164239.284084-2-gary@kernel.org
[ Fix wording in doc-comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/ptr.rs

@@ -2,7 +2,10 @@
 
 //! Types and functions to work with pointers and addresses.
 
-use core::mem::align_of;
+use core::mem::{
+    align_of,
+    size_of, //
+};
 use core::num::NonZero;
 
 /// Type representing an alignment, which is always a power of two.
@@ -225,3 +228,25 @@ macro_rules! impl_alignable_uint {
 }
 
 impl_alignable_uint!(u8, u16, u32, u64, usize);
+
+/// Trait to represent compile-time known size information.
+///
+/// This is a generalization of [`size_of`] that works for dynamically sized types.
+pub trait KnownSize {
+    /// Get the size of an object of this type in bytes, with the metadata of the given pointer.
+    fn size(p: *const Self) -> usize;
+}
+
+impl<T> KnownSize for T {
+    #[inline(always)]
+    fn size(_: *const Self) -> usize {
+        size_of::<T>()
+    }
+}
+
+impl<T> KnownSize for [T] {
+    #[inline(always)]
+    fn size(p: *const Self) -> usize {
+        p.len() * size_of::<T>()
+    }
+}