Add tests for const_slice_from_ref and const_array_from_ref

Author: WaffleLapkin

library/core/tests/array.rs

@@ -7,6 +7,11 @@ fn array_from_ref() {
     let value: String = "Hello World!".into();
     let arr: &[String; 1] = array::from_ref(&value);
     assert_eq!(&[value.clone()], arr);
+
+    const VALUE: &&str = &"Hello World!";
+    const ARR: &[&str; 1] = array::from_ref(VALUE);
+    assert_eq!(&[*VALUE], ARR);
+    assert!(core::ptr::eq(VALUE, &ARR[0]));
 }
 
 #[test]

library/core/tests/lib.rs

@@ -71,6 +71,8 @@
 #![feature(trusted_random_access)]
 #![feature(unsize)]
 #![feature(unzip_option)]
+#![feature(const_array_from_ref)]
+#![feature(const_slice_from_ref)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;

library/core/tests/slice.rs

@@ -2146,6 +2146,14 @@ fn test_slice_run_destructors() {
     assert_eq!(x.get(), 1);
 }
 
+#[test]
+fn test_const_from_ref() {
+    const VALUE: &i32 = &1;
+    const SLICE: &[i32] = core::slice::from_ref(VALUE);
+
+    assert!(core::ptr::eq(VALUE, &SLICE[0]))
+}
+
 #[test]
 fn test_slice_fill_with_uninit() {
     // This should not UB. See #87891