Rollup merge of rust-lang#77074 - lcnr:array-from-ref, r=SimonSapin

add array::from_ref

mirrors the methods in `std::slice` with the same name.

I guess this method previously didn't exist as there was close to no reason to create an array of size `1`.
This will change due to const generics in the near future.

Author: jonas-schievink

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 = "77101")]
+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 = "77101")]
+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/src/slice/raw.rs

@@ -1,5 +1,6 @@
 //! Free functions to create `&[T]` and `&mut [T]`.
 
+use crate::array;
 use crate::intrinsics::is_aligned_and_not_null;
 use crate::mem;
 use crate::ptr;
@@ -140,19 +141,11 @@ pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T]
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub fn from_ref<T>(s: &T) -> &[T] {
-    // SAFETY: a reference is guaranteed to be valid for reads. The returned
-    // reference cannot be mutated as it is an immutable reference.
-    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
-    // Thus the call to `from_raw_parts` is safe.
-    unsafe { from_raw_parts(s, 1) }
+    array::from_ref(s)
 }
 
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub fn from_mut<T>(s: &mut T) -> &mut [T] {
-    // SAFETY: a mutable reference is guaranteed to be valid for writes.
-    // The reference cannot be accessed by another pointer as it is an mutable reference.
-    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
-    // Thus the call to `from_raw_parts_mut` is safe.
-    unsafe { from_raw_parts_mut(s, 1) }
+    array::from_mut(s)
 }

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)]