refactor(kernel): use Range: const Iterator and const_for

Author: yvt

src/r3_kernel/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(maybe_uninit_array_assume_init)]
+#![feature(const_intoiterator_identity)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(const_precise_live_drops)]
 #![feature(const_raw_ptr_comparison)]
@@ -24,9 +25,11 @@
 #![feature(const_option)]
 #![feature(const_deref)]
 #![feature(const_heap)]
+#![feature(const_iter)]
 #![feature(const_swap)]
 #![feature(never_type)] // `!`
 #![feature(decl_macro)]
+#![feature(const_for)]
 #![feature(doc_cfg)] // `#[doc(cfg(...))]`
 #![deny(unsafe_op_in_unsafe_fn)]
 #![cfg_attr(

src/r3_kernel/src/utils/ctz.rs

@@ -209,11 +209,10 @@ fn ctz_array_lut<const LEN: usize>(x: usize) -> u32 {
     impl<const LEN: usize> LutTrait for Lut<LEN> {
         const LUT: &'static [u8] = &{
             let mut array = [0u8; LEN];
-            // `for` is unusable in `const fn` [ref:const_for]
-            let mut i = 0;
-            while i < array.len() {
+            // `[T]::iter_mut` is unusable in `const fn` [ref:const_slice_iter]
+            // `core::array::from_fn` is not `const fn` [ref:const_array_from_fn]
+            for i in 0..array.len() {
                 array[i] = i.trailing_zeros() as u8;
-                i += 1;
             }
             array
         };