rust: alloc: drain: Fix build with rust 1.87

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

rust/kernel/alloc/drain.rs

@@ -236,7 +236,10 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
             // it from the original vec but also avoid creating a &mut to the front since that could
             // invalidate raw pointers to it which some unsafe code might rely on.
             let vec_ptr = vec.as_mut().as_mut_ptr();
+            #[cfg(not(version("1.87")))]
             let drop_offset = drop_ptr.sub_ptr(vec_ptr);
+            #[cfg(version("1.87"))]
+            let drop_offset = drop_ptr.offset_from_unsigned(vec_ptr);
             let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
             ptr::drop_in_place(to_drop);
         }

rust/kernel/lib.rs

@@ -14,6 +14,7 @@
 #![no_std]
 #![feature(arbitrary_self_types)]
 #![feature(associated_type_defaults)]
+#![feature(cfg_version)]
 #![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
 #![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(pin_coerce_unsized_trait))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]