review

library/core/src/slice/mod.rs

@@ -1832,7 +1832,7 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics if `mid > len`.  For non-panicking alternative see
+    /// Panics if `mid > len`.  For a non-panicking alternative see
     /// [`split_at_checked`](slice::split_at_checked).
     ///
     /// # Examples
@@ -1879,7 +1879,7 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics if `mid > len`.  For non-panicking alternative see
+    /// Panics if `mid > len`.  For a non-panicking alternative see
     /// [`split_at_mut_checked`](slice::split_at_mut_checked).
     ///
     /// # Examples
@@ -2021,7 +2021,7 @@ impl<T> [T] {
         unsafe { (from_raw_parts_mut(ptr, mid), from_raw_parts_mut(ptr.add(mid), len - mid)) }
     }
 
-    /// Divides one slice into two at an index returning `None` if slice is too
+    /// Divides one slice into two at an index returning, `None` if slice is too
     /// short.
     ///
     /// The first will contain all indices from `[0, mid)` (excluding
@@ -2072,7 +2072,8 @@ impl<T> [T] {
         }
     }
 
-    /// Divides one mutable slice into two at an index.
+    /// Divides one mutable slice into two at an index, returning `None` if
+    /// slice is too short.
     ///
     /// The first will contain all indices from `[0, mid)` (excluding
     /// the index `mid` itself) and the second will contain all

library/core/src/str/mod.rs

@@ -659,7 +659,10 @@ impl str {
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at(&self, mid: usize) -> (&str, &str) {
-        self.split_at_checked(mid).unwrap_or_else(|| slice_error_fail(self, 0, mid))
+        match self.split_at_checked(mid) {
+            None => slice_error_fail(self, 0, mid),
+            Some(pair) => pair,
+        }
     }
 
     /// Divide one mutable string slice into two at an index.
@@ -792,8 +795,8 @@ impl str {
     ///
     /// # Safety
     ///
-    /// The caller must ensure `mid` is a byte offset at a UTF-8 character
-    /// boundary.
+    /// The caller must ensure that `mid` is a valid byte offset from the start
+    /// of the string and falls on the boundary of a UTF-8 code point.
     unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
         let len = self.len();
         let ptr = self.as_mut_ptr();