[PATCH v4 1/3] rust: ptr: add `KnownSize` trait to support DST size info extraction

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

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>
---
 rust/kernel/lib.rs |  1 +
 rust/kernel/ptr.rs | 27 ++++++++++++++++++++++++++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 3da92f18f4ee..510cc7fe4961 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -20,6 +20,7 @@
 #![feature(generic_nonzero)]
 #![feature(inline_const)]
 #![feature(pointer_is_aligned)]
+#![feature(slice_ptr_len)]
 //
 // Stable since Rust 1.80.0.
 #![feature(slice_flatten)]
diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
index 5b6a382637fe..cf980a103acf 100644
--- a/rust/kernel/ptr.rs
+++ b/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 @@ fn align_up(self, alignment: Alignment) -> Option<Self> {
 }
 
 impl_alignable_uint!(u8, u16, u32, u64, usize);
+
+/// Trait to represent compile-time known size information.
+///
+/// This is a generalization of what [`size_of`] which 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>()
+    }
+}
-- 
2.51.2