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

Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks.
\[This PR originally contained the changes in #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](#76001). The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
The PR adding it also said: <#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.

Author: compiler-errors

compiler/rustc_data_structures/src/lib.rs

@@ -27,7 +27,6 @@
 #![feature(lint_reasons)]
 #![feature(macro_metavar_expr)]
 #![feature(map_try_insert)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]

compiler/rustc_data_structures/src/sip128.rs

@@ -188,7 +188,7 @@ impl SipHasher128 {
 pubfnnew_with_keys(key0:u64,key1:u64) -> SipHasher128{
 letmut hasher = SipHasher128{
 nbuf:0,
-buf:MaybeUninit::uninit_array(),
+buf:[MaybeUninit::uninit();BUFFER_WITH_SPILL_CAPACITY],
 state:State{
 v0: key0 ^ 0x736f6d6570736575,
 // The XOR with 0xee is only done on 128-bit algorithm version.

library/alloc/src/collections/vec_deque/into_iter.rs

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

library/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)]

library/alloc/src/vec/into_iter.rs

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

library/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")]
 pubconstfnempty() -> 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> {
 fnclone(&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.
-letmut new = Self{data:MaybeUninit::uninit_array(),alive:IndexRange::zero_to(0)};
+letmut 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){

library/core/src/array/mod.rs

@@ -127,7 +127,7 @@ where
 R:Try,
 R::Residual:Residual<[R::Output;N]>,
 {
-letmut array = MaybeUninit::uninit_array::<N>();
+letmut array = [const{MaybeUninit::uninit()};N];
 matchtry_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)fniter_next_chunk<T,constN:usize>(
 iter:&mutimplIterator<Item = T>,
 ) -> Result<[T;N],IntoIter<T,N>>{
-letmut array = MaybeUninit::uninit_array::<N>();
+letmut array = [const{MaybeUninit::uninit()};N];
 let r = iter_next_chunk_erased(&mut array, iter);
 match r {
 Ok(()) => {

library/core/src/fmt/float.rs

@@ -35,8 +35,8 @@ fn float_to_decimal_common_exact<T>(
 where
 T: flt2dec::DecodableFloat,
 {
-letmut buf:[MaybeUninit<u8>;1024] = MaybeUninit::uninit_array();// enough for f32 and f64
-letmut parts:[MaybeUninit<numfmt::Part<'_>>;4] = MaybeUninit::uninit_array();
+letmut buf:[MaybeUninit<u8>;1024] = [MaybeUninit::uninit();1024];// enough for f32 and f64
+letmut 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
-letmut buf:[MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] = MaybeUninit::uninit_array();
-letmut parts:[MaybeUninit<numfmt::Part<'_>>;4] = MaybeUninit::uninit_array();
+letmut buf:[MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] =
+[MaybeUninit::uninit(); flt2dec::MAX_SIG_DIGITS];
+letmut 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,
 {
-letmut buf:[MaybeUninit<u8>;1024] = MaybeUninit::uninit_array();// enough for f32 and f64
-letmut parts:[MaybeUninit<numfmt::Part<'_>>;6] = MaybeUninit::uninit_array();
+letmut buf:[MaybeUninit<u8>;1024] = [MaybeUninit::uninit();1024];// enough for f32 and f64
+letmut 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
-letmut buf:[MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] = MaybeUninit::uninit_array();
-letmut parts:[MaybeUninit<numfmt::Part<'_>>;6] = MaybeUninit::uninit_array();
+letmut buf:[MaybeUninit<u8>; flt2dec::MAX_SIG_DIGITS] =
+[MaybeUninit::uninit(); flt2dec::MAX_SIG_DIGITS];
+letmut parts:[MaybeUninit<numfmt::Part<'_>>;6] = [MaybeUninit::uninit();6];
 let formatted = flt2dec::to_shortest_exp_str(
  flt2dec::strategy::grisu::format_shortest,
 *num,

library/core/src/iter/adapters/copied.rs

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

library/core/src/iter/adapters/filter.rs

@@ -64,7 +64,7 @@ where
 fnnext_chunk<constN:usize>(
 &mutself,
 ) -> Result<[Self::Item;N], array::IntoIter<Self::Item,N>>{
-letmut array:[MaybeUninit<Self::Item>;N] = MaybeUninit::uninit_array();
+letmut array:[MaybeUninit<Self::Item>;N] = [const{MaybeUninit::uninit()};N];
 
 structGuard<'a,T>{
 array:&'amut[MaybeUninit<T>],