refactor: use #![feature(const_maybe_uninit_array_assume_init)]

This feature, which covers the `const fn` version of `MaybeUninit::
array_assume_init`, was added by [rust-lang/rust#96099][1].

[1]: rust-lang/rust#96099

Author: yvt

doc/toolchain_limitations.md

@@ -366,28 +366,6 @@ const _: () = { b"".iter(); };
 ```
 
 
-### `[tag:const_array_assume_init]` `MaybeUninit::array_assume_init` is not `const fn`
-
-```rust
-#![feature(maybe_uninit_array_assume_init)]
-use core::mem::MaybeUninit;
-assert!(matches!(
-    unsafe { MaybeUninit::array_assume_init([MaybeUninit::new(42)]) },
-    [42]
-));
-```
-
-```rust,compile_fail,E0015
-#![feature(maybe_uninit_array_assume_init)]
-use core::mem::MaybeUninit;
-const _: () = assert!(matches!(
-    // error[E0015]: cannot call non-const fn `MaybeUninit::<i32>::
-    // array_assume_init::<1_usize>` in constants
-    unsafe { MaybeUninit::array_assume_init([MaybeUninit::new(42)]) },
-    [42]
-));
-```
-
 ### `[tag:const_uninit_array]` `MaybeUninit::uninit_array` is unstable
 
 ```rust,compile_fail,E0658

src/r3_core/src/lib.rs

@@ -1,7 +1,9 @@
+#![feature(const_maybe_uninit_array_assume_init)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_nonnull_slice_from_raw_parts)]
 #![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_assume_init)]
+#![feature(maybe_uninit_array_assume_init)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(arbitrary_enum_discriminant)]

src/r3_core/src/utils/for_times.rs

@@ -232,21 +232,8 @@ macro_rules! const_array_from_fn {
             ))
         }
 
-        // `MaybeUninit::array_assume_init` is not `const fn` yet
-        // [ref:const_array_assume_init]
-        const unsafe fn __assume_init<
-            $($iter_gparam $($iter_gparam_bounds)*,)*
-            const LEN: usize
-        >(array: [MaybeUninit<$ty>; LEN]) -> [$ty; LEN] {
-            // Safety: This is equivalent to `transmute_copy(&array)`. The
-            // memory layout of `[MaybeUninit<T>; $len]` is identical to `[T; $len]`.
-            // We initialized all elements in `array[0..$len]`, so it's safe to
-            // reinterpret that range as `[T; $len]`.
-            unsafe { *(array.as_ptr() as *const _ as *const [$ty; LEN]) }
-        }
-
-        // Safety: See the body of `__assume_init`.
-        unsafe { __assume_init::<$($ctx_t,)* {$len_value}>(array) }
+        // Safety: All elements of `array` are initialized
+        unsafe { MaybeUninit::array_assume_init(array) }
     }};
 }
 

src/r3_core/src/utils/init.rs

@@ -44,12 +44,8 @@ impl<T: Init, const LEN: usize> Init for [T; LEN] {
             i += 1;
         }
 
-        // `MaybeUninit::array_assume_init` is not `const fn` yet
-        // [ref:const_array_assume_init]
-        // Safety: The memory layout of `[MaybeUninit<T>; LEN]` is
-        // identical to `[T; LEN]`. We initialized all elements, so it's
-        // safe to reinterpret that range as `[T; LEN]`.
-        unsafe { super::mem::transmute(array) }
+        // Safety: `array`'s elements are fully initialized
+        unsafe { mem::MaybeUninit::array_assume_init(array) }
     };
 }
 

src/r3_core/src/utils/mem.rs

@@ -44,9 +44,7 @@ mod tests {
                 MaybeUninit::new(2),
                 MaybeUninit::new(3),
             ];
-            // `MaybeUninit::array_assume_init` is not `const fn`
-            // [ref:const_array_assume_init]
-            unsafe { transmute(array) }
+            unsafe { MaybeUninit::array_assume_init(array) }
         };
         assert_eq!(ARRAY1, [1, 2, 3]);
     }

src/r3_kernel/src/lib.rs

@@ -1,5 +1,7 @@
+#![feature(const_maybe_uninit_array_assume_init)]
 #![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_assume_init)]
+#![feature(maybe_uninit_array_assume_init)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(const_precise_live_drops)]

src/r3_test_suite/src/kernel_benchmarks/timer_start.rs

@@ -49,9 +49,7 @@ impl<System: SupportedSystem> AppInner<System> {
                 i += 1;
             }
 
-            // `MaybeUninit::array_assume_init` is not `const fn` yet
-            // [ref:const_array_assume_init]
-            unsafe { core::mem::transmute_copy(&timers) }
+            unsafe { MaybeUninit::array_assume_init(timers) }
         };
 
         Self { timers }

src/r3_test_suite/src/lib.rs

@@ -1,4 +1,6 @@
+#![feature(const_maybe_uninit_array_assume_init)]
 #![feature(const_fn_floating_point_arithmetic)]
+#![feature(maybe_uninit_array_assume_init)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_transmute_copy)]
 #![feature(const_refs_to_cell)]