use const array repeat expressions for uninit_array

5 files changed, 26 insertions, 7 deletions

diff --git a/src/libcore/fmt/num.rs b/src/libcore/fmt/num.rs
index f9b4c26496c..e2d00e654dd 100644
--- a/src/libcore/fmt/num.rs
+++ b/src/libcore/fmt/num.rs
@@ -51,7 +51,7 @@ trait GenericRadix {
         // characters for a base 2 number.
         let zero = T::zero();
         let is_nonnegative = x >= zero;
-        let mut buf = uninitialized_array![u8; 128];
+        let mut buf = uninit_array![u8; 128];
         let mut curr = buf.len();
         let base = T::from_u8(Self::BASE);
         if is_nonnegative {
@@ -189,7 +189,7 @@ static DEC_DIGITS_LUT: &[u8; 200] =
 macro_rules! impl_Display {
     ($($t:ident),* as $u:ident via $conv_fn:ident named $name:ident) => {
         fn $name(mut n: $u, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            let mut buf = uninitialized_array![u8; 39];
+            let mut buf = uninit_array![u8; 39];
             let mut curr = buf.len() as isize;
             let buf_ptr = MaybeUninit::first_ptr_mut(&mut buf);
             let lut_ptr = DEC_DIGITS_LUT.as_ptr();
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index fe149d634e2..8d3c42cbf35 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -75,6 +75,7 @@
 #![feature(const_fn)]
 #![feature(const_fn_union)]
 #![cfg_attr(not(bootstrap), feature(const_generics))]
+#![cfg_attr(not(bootstrap), feature(const_in_array_repeat_expressions))]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]
diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index 293a2dd9492..9855c5fb9c3 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -626,20 +626,37 @@ macro_rules! todo {
 /// Creates an array of [`MaybeUninit`].
 ///
 /// This macro constructs an uninitialized array of the type `[MaybeUninit<K>; N]`.
+/// It exists solely because bootstrap does not yet support const array-init expressions.
 ///
 /// [`MaybeUninit`]: mem/union.MaybeUninit.html
+// FIXME: Remove both versions of this macro once bootstrap is 1.38.
 #[macro_export]
 #[unstable(feature = "maybe_uninit_array", issue = "53491")]
-macro_rules! uninitialized_array {
+#[cfg(bootstrap)]
+macro_rules! uninit_array {
     // This `assume_init` is safe because an array of `MaybeUninit` does not
     // require initialization.
-    // FIXME(#49147): Could be replaced by an array initializer, once those can
-    // be any const expression.
     ($t:ty; $size:expr) => (unsafe {
         MaybeUninit::<[MaybeUninit<$t>; $size]>::uninit().assume_init()
     });
 }
 
+/// Creates an array of [`MaybeUninit`].
+///
+/// This macro constructs an uninitialized array of the type `[MaybeUninit<K>; N]`.
+/// It exists solely because bootstrap does not yet support const array-init expressions.
+///
+/// [`MaybeUninit`]: mem/union.MaybeUninit.html
+// FIXME: Just inline this version of the macro once bootstrap is 1.38.
+#[macro_export]
+#[unstable(feature = "maybe_uninit_array", issue = "53491")]
+#[cfg(not(bootstrap))]
+macro_rules! uninit_array {
+    ($t:ty; $size:expr) => (
+        [MaybeUninit::<$t>::uninit(); $size]
+    );
+}
+
 /// Built-in macros to the compiler itself.
 ///
 /// These macros do not have any corresponding definition with a `macro_rules!`
diff --git a/src/libcore/mem/maybe_uninit.rs b/src/libcore/mem/maybe_uninit.rs
index f6f7ccffdb0..1c69e7f90f6 100644
--- a/src/libcore/mem/maybe_uninit.rs
+++ b/src/libcore/mem/maybe_uninit.rs
@@ -248,6 +248,7 @@ impl<T> MaybeUninit<T> {
     /// [type]: union.MaybeUninit.html
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[inline(always)]
+    #[rustc_promotable]
     pub const fn uninit() -> MaybeUninit<T> {
         MaybeUninit { uninit: () }
     }
diff --git a/src/libcore/slice/sort.rs b/src/libcore/slice/sort.rs
index c293b190018..fbf9caeaece 100644
--- a/src/libcore/slice/sort.rs
+++ b/src/libcore/slice/sort.rs
@@ -216,14 +216,14 @@ fn partition_in_blocks<T, F>(v: &mut [T], pivot: &T, is_less: &mut F) -> usize
     let mut block_l = BLOCK;
     let mut start_l = ptr::null_mut();
     let mut end_l = ptr::null_mut();
-    let mut offsets_l: [MaybeUninit<u8>; BLOCK] = uninitialized_array![u8; BLOCK];
+    let mut offsets_l: [MaybeUninit<u8>; BLOCK] = uninit_array![u8; BLOCK];
 
     // The current block on the right side (from `r.sub(block_r)` to `r`).
     let mut r = unsafe { l.add(v.len()) };
     let mut block_r = BLOCK;
     let mut start_r = ptr::null_mut();
     let mut end_r = ptr::null_mut();
-    let mut offsets_r: [MaybeUninit<u8>; BLOCK] = uninitialized_array![u8; BLOCK];
+    let mut offsets_r: [MaybeUninit<u8>; BLOCK] = uninit_array![u8; BLOCK];
 
     // FIXME: When we get VLAs, try creating one array of length `min(v.len(), 2 * BLOCK)` rather
     // than two fixed-size arrays of length `BLOCK`. VLAs might be more cache-efficient.