Auto merge of rust-lang#119127 - joboet:array_repeat, r=scottmcm

Implement `array::repeat`

See rust-lang/libs-team#310.

I've decided to make the function use the input value as last element instead of cloning it to every position and dropping it, and to make this part of the API so that callers are not surprised by this behaviour.

TODO: open a tracking issue. I'll wait for the ACP to be accepted, first.

`@rustbot` label +T-libs-api +T-libs
r? libs

library/core/src/array/mod.rs

@@ -10,7 +10,7 @@ use crate::convert::Infallible;
 use crate::error::Error;
 use crate::fmt;
 use crate::hash::{self, Hash};
-use crate::iter::UncheckedIterator;
+use crate::iter::{repeat_n, UncheckedIterator};
 use crate::mem::{self, MaybeUninit};
 use crate::ops::{
     ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
@@ -27,6 +27,33 @@ pub(crate) use drain::drain_array_with;
 #[stable(feature = "array_value_iter", since = "1.51.0")]
 pub use iter::IntoIter;
 
+/// Creates an array of type `[T; N]` by repeatedly cloning a value.
+///
+/// This is the same as `[val; N]`, but it also works for types that do not
+/// implement [`Copy`].
+///
+/// The provided value will be used as an element of the resulting array and
+/// will be cloned N - 1 times to fill up the rest. If N is zero, the value
+/// will be dropped.
+///
+/// # Example
+///
+/// Creating muliple copies of a `String`:
+/// ```rust
+/// #![feature(array_repeat)]
+///
+/// use std::array;
+///
+/// let string = "Hello there!".to_string();
+/// let strings = array::repeat(string);
+/// assert_eq!(strings, ["Hello there!", "Hello there!"]);
+/// ```
+#[inline]
+#[unstable(feature = "array_repeat", issue = "126695")]
+pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
+    from_trusted_iterator(repeat_n(val, N))
+}
+
 /// Creates an array of type [T; N], where each element `T` is the returned value from `cb`
 /// using that element's index.
 ///

library/core/src/iter/sources/repeat_n.rs

@@ -1,4 +1,4 @@
-use crate::iter::{FusedIterator, TrustedLen};
+use crate::iter::{FusedIterator, TrustedLen, UncheckedIterator};
 use crate::mem::ManuallyDrop;
 use crate::num::NonZero;
 
@@ -193,3 +193,5 @@ impl<A: Clone> FusedIterator for RepeatN<A> {}
 
 #[unstable(feature = "trusted_len", issue = "37572")]
 unsafe impl<A: Clone> TrustedLen for RepeatN<A> {}
+#[unstable(feature = "trusted_len_next_unchecked", issue = "37572")]
+impl<A: Clone> UncheckedIterator for RepeatN<A> {}

tests/codegen/array-repeat.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+#![feature(array_repeat)]
+
+use std::array::repeat;
+
+// CHECK-LABEL: @byte_repeat
+#[no_mangle]
+fn byte_repeat(b: u8) -> [u8; 1024] {
+    // CHECK-NOT: alloca
+    // CHECK-NOT: store
+    // CHECK: memset
+    repeat(b)
+}