Remove use of slice_ptr_get unstable feature

**Description**
Replace use of `NonNull::<[T]>::get_unchecked_mut` function call with
a copy from the standard library.

This function is used to index into a pointer to a slice (`*const [T]`)
and return a pointer to an element (`*const T`) without doing bounds
checking.

**Motivation**
This change is a partial solution towards issue #20. The only remaining
unstable feature used is `hasher_prefixfree_extras`.

**Testing Done**
 - `cargo test`
 - `cargo miri test`
 - `cargo fmt`
 - `cargo clippy`

src/lib.rs

@@ -1,5 +1,5 @@
 // TODO(#20): Use rust stable distribution, remove usage of nightly features
-#![feature(slice_ptr_get, hasher_prefixfree_extras)]
+#![feature(hasher_prefixfree_extras)]
 #![allow(unstable_name_collisions)]
 #![deny(
     missing_docs,

src/nightly_rust_apis.rs

@@ -99,3 +99,21 @@ pub fn non_null_slice_from_raw_parts<T>(data: NonNull<T>, len: usize) -> NonNull
     // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
     unsafe { NonNull::<[T]>::new_unchecked(slice_from_raw_parts_mut(data.as_ptr(), len)) }
 }
+
+/// Returns a raw pointer to an element, without doing bounds checking.
+///
+/// Calling this method with an out-of-bounds index or when `data` is not
+/// dereferenceable is *[undefined behavior]* even if the resulting pointer is
+/// not used.
+///
+/// **This is a unstable API copied from the Rust standard library, tracking
+/// issue is [#74265][issue-74265]**
+///
+/// [issue-74265]: https://github.com/rust-lang/rust/issues/74265
+/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+pub unsafe fn non_null_get_unchecked_mut<T>(data: NonNull<[T]>, index: usize) -> NonNull<T> {
+    // SAFETY: the caller ensures that `self` is dereferenceable and `index`
+    // in-bounds. As a consequence, the resulting pointer cannot be null.
+
+    unsafe { NonNull::new_unchecked(data.as_ptr().cast::<T>().add(index)) }
+}

src/nodes/representation/iterators.rs

@@ -568,10 +568,12 @@ impl<V> Iterator for InnerNode48Iter<V> {
                 //        `initialized_child_pointers`, which returns the initialized portion of
                 //        the `child_pointers` array.
                 let child_pointer = unsafe {
-                    self.child_pointers_ptr
-                        .get_unchecked_mut(usize::from(u8::from(next_index)))
-                        .as_ptr()
-                        .read()
+                    crate::nightly_rust_apis::non_null_get_unchecked_mut(
+                        self.child_pointers_ptr,
+                        usize::from(u8::from(next_index)),
+                    )
+                    .as_ptr()
+                    .read()
                 };
 
                 // SAFETY:
@@ -642,10 +644,12 @@ impl<V> DoubleEndedIterator for InnerNode48Iter<V> {
                 //        `initialized_child_pointers`, which returns the initialized portion of
                 //        the `child_pointers` array.
                 let child_pointer = unsafe {
-                    self.child_pointers_ptr
-                        .get_unchecked_mut(usize::from(u8::from(next_index)))
-                        .as_ptr()
-                        .read()
+                    crate::nightly_rust_apis::non_null_get_unchecked_mut(
+                        self.child_pointers_ptr,
+                        usize::from(u8::from(next_index)),
+                    )
+                    .as_ptr()
+                    .read()
                 };
 
                 // SAFETY: