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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>

authored by

Gary Guo and committed by
Danilo Krummrich 08da98f1 11439c46

+27 -1
+1
rust/kernel/lib.rs
··· 20 20 #![feature(generic_nonzero)] 21 21 #![feature(inline_const)] 22 22 #![feature(pointer_is_aligned)] 23 + #![feature(slice_ptr_len)] 23 24 // 24 25 // Stable since Rust 1.80.0. 25 26 #![feature(slice_flatten)]
+26 -1
rust/kernel/ptr.rs
··· 2 2 3 3 //! Types and functions to work with pointers and addresses. 4 4 5 - use core::mem::align_of; 5 + use core::mem::{ 6 + align_of, 7 + size_of, // 8 + }; 6 9 use core::num::NonZero; 7 10 8 11 /// Type representing an alignment, which is always a power of two. ··· 228 225 } 229 226 230 227 impl_alignable_uint!(u8, u16, u32, u64, usize); 228 + 229 + /// Trait to represent compile-time known size information. 230 + /// 231 + /// This is a generalization of [`size_of`] that works for dynamically sized types. 232 + pub trait KnownSize { 233 + /// Get the size of an object of this type in bytes, with the metadata of the given pointer. 234 + fn size(p: *const Self) -> usize; 235 + } 236 + 237 + impl<T> KnownSize for T { 238 + #[inline(always)] 239 + fn size(_: *const Self) -> usize { 240 + size_of::<T>() 241 + } 242 + } 243 + 244 + impl<T> KnownSize for [T] { 245 + #[inline(always)] 246 + fn size(p: *const Self) -> usize { 247 + p.len() * size_of::<T>() 248 + } 249 + }