Extend the example code and assert the result

library/core/src/slice/raw.rs

@@ -92,11 +92,13 @@ use crate::ub_checks;
 /// ```
 /// use std::slice;
 ///
+/// /// Sum the elements of an FFI slice.
+/// ///
 /// /// # Safety
 /// ///
 /// /// If ptr is not NULL, it must be correctly aligned and
 /// /// point to `len` initialized items of type `f32`.
-/// unsafe extern "C" fn handle_slice(ptr: *const f32, len: usize) {
+/// unsafe extern "C" fn sum_slice(ptr: *const f32, len: usize) -> f32 {
 ///     let data = if ptr.is_null() {
 ///         // `len` is assumed to be 0.
 ///         &[]
@@ -104,9 +106,14 @@ use crate::ub_checks;
 ///         // SAFETY: see function docstring.
 ///         unsafe { slice::from_raw_parts(ptr, len) }
 ///     };
-///     dbg!(data);
-///     // ...
+///     data.sum()
 /// }
+///
+/// // This could be the result of C++'s std::vector::data():
+/// let ptr = std::ptr::null();
+/// // And this could be std::vector::size():
+/// let len = 0;
+/// assert_eq!(unsafe { sum_slice(ptr, len) }, 0.0);
 /// ```
 ///
 /// [valid]: ptr#safety