add array from_ref

Author: lcnr

library/core/src/array/mod.rs

@@ -19,6 +19,20 @@ mod iter;
 #[unstable(feature = "array_value_iter", issue = "65798")]
 pub use iter::IntoIter;
 
+/// Converts a reference to `T` into a reference to an array of length 1 (without copying).
+#[unstable(feature = "array_from_ref", issue = "none")]
+pub fn from_ref<T>(s: &T) -> &[T; 1] {
+    // SAFETY: Converting `&T` to `&[T; 1]` is sound.
+    unsafe { &*(s as *const T).cast::<[T; 1]>() }
+}
+
+/// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
+#[unstable(feature = "array_from_ref", issue = "none")]
+pub fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
+    // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
+    unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
+}
+
 /// Utility trait implemented only on arrays of fixed size
 ///
 /// This trait can be used to implement other traits on fixed-size arrays

library/core/tests/array.rs

@@ -1,4 +1,4 @@
-use core::array::{FixedSizeArray, IntoIter};
+use core::array::{self, FixedSizeArray, IntoIter};
 use core::convert::TryFrom;
 
 #[test]
@@ -19,6 +19,21 @@ fn fixed_size_array() {
     assert_eq!(FixedSizeArray::as_mut_slice(&mut empty_zero_sized).len(), 0);
 }
 
+#[test]
+fn array_from_ref() {
+    let value: String = "Hello World!".into();
+    let arr: &[String; 1] = array::from_ref(&value);
+    assert_eq!(&[value.clone()], arr);
+}
+
+#[test]
+fn array_from_mut() {
+    let mut value: String = "Hello World".into();
+    let arr: &mut [String; 1] = array::from_mut(&mut value);
+    arr[0].push_str("!");
+    assert_eq!(&value, "Hello World!");
+}
+
 #[test]
 fn array_try_from() {
     macro_rules! test {

library/core/tests/lib.rs

@@ -1,5 +1,6 @@
 #![feature(alloc_layout_extra)]
 #![feature(array_chunks)]
+#![feature(array_from_ref)]
 #![feature(array_methods)]
 #![feature(array_map)]
 #![feature(array_windows)]