Rollup merge of rust-lang#125082 - kpreid:const-uninit, r=dtolnay

Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks.

\[This PR originally contained the changes in rust-lang#125995 too. See edit history for the original PR description.]

The documentation of `MaybeUninit::uninit_array()` says:

> Note: in a future Rust version this method may become unnecessary when Rust allows [inline const expressions](rust-lang#76001). The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.

The PR adding it also said: <rust-lang#65580 (comment)>

> if it’s stabilized soon enough maybe it’s not worth having a standard library method that will be replaceable with `let buffer = [MaybeUninit::<T>::uninit(); $N];`

That time has come to pass — inline const expressions are stable — so `MaybeUninit::uninit_array()` is now unnecessary. The only remaining question is whether it is an important enough *convenience* to keep it around.

I believe it is net good to remove this function, on the principle that it is better to compose two orthogonal features (`MaybeUninit` and array construction) than to have a specific function for the specific combination, now that that is possible.

alloc/src/collections/vec_deque/into_iter.rs

@@ -132,7 +132,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn next_chunk<const N: usize>(
         &mut self,
     ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
-        let mut raw_arr = MaybeUninit::uninit_array();
+        let mut raw_arr = [const { MaybeUninit::uninit() }; N];
         let raw_arr_ptr = raw_arr.as_mut_ptr().cast();
         let (head, tail) = self.inner.as_slices();
 

alloc/src/lib.rs

@@ -135,7 +135,6 @@
 #![feature(layout_for_ptr)]
 #![feature(local_waker)]
 #![feature(maybe_uninit_slice)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(panic_internals)]
 #![feature(pattern)]

alloc/src/vec/into_iter.rs

@@ -254,7 +254,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn next_chunk<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
-        let mut raw_ary = MaybeUninit::uninit_array();
+        let mut raw_ary = [const { MaybeUninit::uninit() }; N];
 
         let len = self.len();
 

core/src/array/iter.rs

@@ -101,7 +101,6 @@ impl<T, const N: usize> IntoIter<T, N> {
     /// ```
     /// #![feature(array_into_iter_constructors)]
     /// #![feature(maybe_uninit_uninit_array_transpose)]
-    /// #![feature(maybe_uninit_uninit_array)]
     /// use std::array::IntoIter;
     /// use std::mem::MaybeUninit;
     ///
@@ -111,7 +110,7 @@ impl<T, const N: usize> IntoIter<T, N> {
     /// fn next_chunk<T: Copy, const N: usize>(
     ///     it: &mut impl Iterator<Item = T>,
     /// ) -> Result<[T; N], IntoIter<T, N>> {
-    ///     let mut buffer = MaybeUninit::uninit_array();
+    ///     let mut buffer = [const { MaybeUninit::uninit() }; N];
     ///     let mut i = 0;
     ///     while i < N {
     ///         match it.next() {
@@ -203,7 +202,7 @@ impl<T, const N: usize> IntoIter<T, N> {
     #[unstable(feature = "array_into_iter_constructors", issue = "91583")]
     #[rustc_const_unstable(feature = "const_array_into_iter_constructors", issue = "91583")]
     pub const fn empty() -> Self {
-        let buffer = MaybeUninit::uninit_array();
+        let buffer = [const { MaybeUninit::uninit() }; N];
         let initialized = 0..0;
 
         // SAFETY: We're telling it that none of the elements are initialized,
@@ -405,7 +404,8 @@ impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
     fn clone(&self) -> Self {
         // Note, we don't really need to match the exact same alive range, so
         // we can just clone into offset 0 regardless of where `self` is.
-        let mut new = Self { data: MaybeUninit::uninit_array(), alive: IndexRange::zero_to(0) };
+        let mut new =
+            Self { data: [const { MaybeUninit::uninit() }; N], alive: IndexRange::zero_to(0) };
 
         // Clone all alive elements.
         for (src, dst) in iter::zip(self.as_slice(), &mut new.data) {

core/src/array/mod.rs

@@ -127,7 +127,7 @@ where
     R: Try,
     R::Residual: Residual<[R::Output; N]>,
 {
-    let mut array = MaybeUninit::uninit_array::<N>();
+    let mut array = [const { MaybeUninit::uninit() }; N];
     match try_from_fn_erased(&mut array, cb) {
         ControlFlow::Break(r) => FromResidual::from_residual(r),
         ControlFlow::Continue(()) => {
@@ -918,7 +918,7 @@ impl<T> Drop for Guard<'_, T> {
 pub(crate) fn iter_next_chunk<T, const N: usize>(
     iter: &mut impl Iterator<Item = T>,
 ) -> Result<[T; N], IntoIter<T, N>> {
-    let mut array = MaybeUninit::uninit_array::<N>();
+    let mut array = [const { MaybeUninit::uninit() }; N];
     let r = iter_next_chunk_erased(&mut array, iter);
     match r {
         Ok(()) => {

core/src/fmt/float.rs

@@ -35,8 +35,8 @@ fn float_to_decimal_common_exact<T>(
 where
     T: flt2dec::DecodableFloat,
 {
-    let mut buf: [MaybeUninit<u8>; 1024] = MaybeUninit::uninit_array(); // enough for f32 and f64
-    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 4] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; 1024] = [MaybeUninit::uninit(); 1024]; // enough for f32 and f64
+    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 4] = [MaybeUninit::uninit(); 4];
     let formatted = flt2dec::to_exact_fixed_str(
         flt2dec::strategy::grisu::format_exact,
         *num,
@@ -62,8 +62,9 @@ where
     T: flt2dec::DecodableFloat,
 {
     // enough for f32 and f64
-    let mut buf: [MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] = MaybeUninit::uninit_array();
-    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 4] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] =
+        [MaybeUninit::uninit(); flt2dec::MAX_SIG_DIGITS];
+    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 4] = [MaybeUninit::uninit(); 4];
     let formatted = flt2dec::to_shortest_str(
         flt2dec::strategy::grisu::format_shortest,
         *num,
@@ -107,8 +108,8 @@ fn float_to_exponential_common_exact<T>(
 where
     T: flt2dec::DecodableFloat,
 {
-    let mut buf: [MaybeUninit<u8>; 1024] = MaybeUninit::uninit_array(); // enough for f32 and f64
-    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 6] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; 1024] = [MaybeUninit::uninit(); 1024]; // enough for f32 and f64
+    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 6] = [MaybeUninit::uninit(); 6];
     let formatted = flt2dec::to_exact_exp_str(
         flt2dec::strategy::grisu::format_exact,
         *num,
@@ -135,8 +136,9 @@ where
     T: flt2dec::DecodableFloat,
 {
     // enough for f32 and f64
-    let mut buf: [MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] = MaybeUninit::uninit_array();
-    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 6] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] =
+        [MaybeUninit::uninit(); flt2dec::MAX_SIG_DIGITS];
+    let mut parts: [MaybeUninit<numfmt::Part<'_>>; 6] = [MaybeUninit::uninit(); 6];
     let formatted = flt2dec::to_shortest_exp_str(
         flt2dec::strategy::grisu::format_shortest,
         *num,

core/src/iter/adapters/copied.rs

@@ -202,7 +202,7 @@ where
     T: Copy,
 {
     fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>> {
-        let mut raw_array = MaybeUninit::uninit_array();
+        let mut raw_array = [const { MaybeUninit::uninit() }; N];
 
         let len = self.len();
 

core/src/iter/adapters/filter.rs

@@ -64,7 +64,7 @@ where
     fn next_chunk<const N: usize>(
         &mut self,
     ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
-        let mut array: [MaybeUninit<Self::Item>; N] = MaybeUninit::uninit_array();
+        let mut array: [MaybeUninit<Self::Item>; N] = [const { MaybeUninit::uninit() }; N];
 
         struct Guard<'a, T> {
             array: &'a mut [MaybeUninit<T>],

core/src/iter/adapters/filter_map.rs

@@ -68,7 +68,7 @@ where
     fn next_chunk<const N: usize>(
         &mut self,
     ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
-        let mut array: [MaybeUninit<Self::Item>; N] = MaybeUninit::uninit_array();
+        let mut array: [MaybeUninit<Self::Item>; N] = [const { MaybeUninit::uninit() }; N];
 
         struct Guard<'a, T> {
             array: &'a mut [MaybeUninit<T>],

core/src/iter/adapters/map_windows.rs

@@ -110,7 +110,8 @@ impl<I: Iterator, const N: usize> MapWindowsInner<I, N> {
 impl<T, const N: usize> Buffer<T, N> {
     fn try_from_iter(iter: &mut impl Iterator<Item = T>) -> Option<Self> {
         let first_half = crate::array::iter_next_chunk(iter).ok()?;
-        let buffer = [MaybeUninit::new(first_half).transpose(), MaybeUninit::uninit_array()];
+        let buffer =
+            [MaybeUninit::new(first_half).transpose(), [const { MaybeUninit::uninit() }; N]];
         Some(Self { buffer, start: 0 })
     }
 
@@ -204,7 +205,7 @@ impl<T, const N: usize> Buffer<T, N> {
 impl<T: Clone, const N: usize> Clone for Buffer<T, N> {
     fn clone(&self) -> Self {
         let mut buffer = Buffer {
-            buffer: [MaybeUninit::uninit_array(), MaybeUninit::uninit_array()],
+            buffer: [[const { MaybeUninit::uninit() }; N], [const { MaybeUninit::uninit() }; N]],
             start: self.start,
         };
         buffer.as_uninit_array_mut().write(self.as_array_ref().clone());

core/src/lib.rs

@@ -140,7 +140,6 @@
 #![feature(const_likely)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
-#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_midpoint)]
 #![feature(const_option)]
@@ -177,7 +176,6 @@
 #![feature(is_ascii_octdigit)]
 #![feature(isqrt)]
 #![feature(link_cfg)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(offset_of_enum)]
 #![feature(offset_of_nested)]
 #![feature(panic_internals)]

core/src/mem/maybe_uninit.rs

@@ -917,11 +917,10 @@ impl<T> MaybeUninit<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(maybe_uninit_uninit_array)]
     /// #![feature(maybe_uninit_array_assume_init)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
+    /// let mut array: [MaybeUninit<i32>; 3] = [MaybeUninit::uninit(); 3];
     /// array[0].write(0);
     /// array[1].write(1);
     /// array[2].write(2);

core/src/net/display_buffer.rs

@@ -11,7 +11,7 @@ pub struct DisplayBuffer<const SIZE: usize> {
 impl<const SIZE: usize> DisplayBuffer<SIZE> {
     #[inline]
     pub const fn new() -> Self {
-        Self { buf: MaybeUninit::uninit_array(), len: 0 }
+        Self { buf: [MaybeUninit::uninit(); SIZE], len: 0 }
     }
 
     #[inline]

core/src/slice/sort/stable/mod.rs

@@ -104,7 +104,7 @@ struct AlignedStorage<T, const N: usize> {
 
 impl<T, const N: usize> AlignedStorage<T, N> {
     fn new() -> Self {
-        Self { _align: [], storage: MaybeUninit::uninit_array() }
+        Self { _align: [], storage: [const { MaybeUninit::uninit() }; N] }
     }
 
     fn as_uninit_slice_mut(&mut self) -> &mut [MaybeUninit<T>] {

core/tests/lib.rs

@@ -54,8 +54,6 @@
 #![feature(slice_split_once)]
 #![feature(split_as_slice)]
 #![feature(maybe_uninit_fill)]
-#![feature(maybe_uninit_slice)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_write_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(min_specialization)]

std/src/fs/tests.rs

@@ -406,7 +406,7 @@ fn file_test_read_buf() {
     let filename = &tmpdir.join("test");
     check!(fs::write(filename, &[1, 2, 3, 4]));
 
-    let mut buf: [MaybeUninit<u8>; 128] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; 128] = [MaybeUninit::uninit(); 128];
     let mut buf = BorrowedBuf::from(buf.as_mut_slice());
     let mut file = check!(File::open(filename));
     check!(file.read_buf(buf.unfilled()));

std/src/lib.rs

@@ -337,7 +337,6 @@
 #![feature(hint_assert_unchecked)]
 #![feature(ip)]
 #![feature(maybe_uninit_slice)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_write_slice)]
 #![feature(panic_can_unwind)]
 #![feature(panic_info_message)]
@@ -407,7 +406,6 @@
 #![feature(const_ip)]
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
-#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_waker)]
 #![feature(thread_local_internals)]
 // tidy-alphabetical-end

std/src/net/tcp/tests.rs

@@ -301,7 +301,7 @@ fn read_buf() {
         });
 
         let mut s = t!(srv.accept()).0;
-        let mut buf: [MaybeUninit<u8>; 128] = MaybeUninit::uninit_array();
+        let mut buf: [MaybeUninit<u8>; 128] = [MaybeUninit::uninit(); 128];
         let mut buf = BorrowedBuf::from(buf.as_mut_slice());
         t!(s.read_buf(buf.unfilled()));
         assert_eq!(buf.filled(), &[1, 2, 3, 4]);

std/src/process/tests.rs

@@ -137,7 +137,7 @@ fn child_stdout_read_buf() {
     let child = cmd.spawn().unwrap();
 
     let mut stdout = child.stdout.unwrap();
-    let mut buf: [MaybeUninit<u8>; 128] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; 128] = [MaybeUninit::uninit(); 128];
     let mut buf = BorrowedBuf::from(buf.as_mut_slice());
     stdout.read_buf(buf.unfilled()).unwrap();
 

std/src/sys/pal/windows/mod.rs

@@ -225,7 +225,7 @@ where
     // This initial size also works around `GetFullPathNameW` returning
     // incorrect size hints for some short paths:
     // https://github.com/dylni/normpath/issues/5
-    let mut stack_buf: [MaybeUninit<u16>; 512] = MaybeUninit::uninit_array();
+    let mut stack_buf: [MaybeUninit<u16>; 512] = [MaybeUninit::uninit(); 512];
     let mut heap_buf: Vec<MaybeUninit<u16>> = Vec::new();
     unsafe {
         let mut n = stack_buf.len();