core: introduce split_at{,_mut}_checked

Introduce split_at_checked and split_at_mut_checked methods to slices
types (including str) which are non-panicking versions of split_at and
split_at_mut  respectively.  This is analogous to get method being
non-panicking version of indexing.

library/core/src/lib.rs

@@ -184,6 +184,7 @@
 #![feature(set_ptr_value)]
 #![feature(slice_ptr_get)]
 #![feature(slice_split_at_unchecked)]
+#![feature(split_at_checked)]
 #![feature(str_internals)]
 #![feature(str_split_inclusive_remainder)]
 #![feature(str_split_remainder)]

library/core/src/slice/mod.rs

@@ -1832,7 +1832,8 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics if `mid > len`.
+    /// Panics if `mid > len`.  For non-panicking alternative see
+    /// [`split_at_checked`](slice::split_at_checked).
     ///
     /// # Examples
     ///
@@ -1859,15 +1860,15 @@ impl<T> [T] {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_split_at_not_mut", since = "1.71.0")]
-    #[rustc_allow_const_fn_unstable(slice_split_at_unchecked)]
+    #[rustc_allow_const_fn_unstable(split_at_checked)]
     #[inline]
     #[track_caller]
     #[must_use]
     pub const fn split_at(&self, mid: usize) -> (&[T], &[T]) {
-        assert!(mid <= self.len());
-        // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
-        // fulfills the requirements of `split_at_unchecked`.
-        unsafe { self.split_at_unchecked(mid) }
+        match self.split_at_checked(mid) {
+            Some(pair) => pair,
+            None => panic!("mid > len"),
+        }
     }
 
     /// Divides one mutable slice into two at an index.
@@ -1878,7 +1879,8 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics if `mid > len`.
+    /// Panics if `mid > len`.  For non-panicking alternative see
+    /// [`split_at_mut_checked`](slice::split_at_mut_checked).
     ///
     /// # Examples
     ///
@@ -1897,10 +1899,10 @@ impl<T> [T] {
     #[must_use]
     #[rustc_const_unstable(feature = "const_slice_split_at_mut", issue = "101804")]
     pub const fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
-        assert!(mid <= self.len());
-        // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
-        // fulfills the requirements of `from_raw_parts_mut`.
-        unsafe { self.split_at_mut_unchecked(mid) }
+        match self.split_at_mut_checked(mid) {
+            Some(pair) => pair,
+            None => panic!("mid > len"),
+        }
     }
 
     /// Divides one slice into two at an index, without doing bounds checking.
@@ -2019,6 +2021,97 @@ 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
+    /// short.
+    ///
+    /// The first will contain all indices from `[0, mid)` (excluding
+    /// the index `mid` itself) and the second will contain all
+    /// indices from `[mid, len)` (excluding the index `len` itself).
+    ///
+    /// Returns `None` if `mid > len`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(split_at_checked)]
+    ///
+    /// let v = [1, 2, 3, 4, 5, 6];
+    ///
+    /// {
+    ///    let (left, right) = v.split_at_checked(0).unwrap();
+    ///    assert_eq!(left, []);
+    ///    assert_eq!(right, [1, 2, 3, 4, 5, 6]);
+    /// }
+    ///
+    /// {
+    ///     let (left, right) = v.split_at_checked(2).unwrap();
+    ///     assert_eq!(left, [1, 2]);
+    ///     assert_eq!(right, [3, 4, 5, 6]);
+    /// }
+    ///
+    /// {
+    ///     let (left, right) = v.split_at_checked(6).unwrap();
+    ///     assert_eq!(left, [1, 2, 3, 4, 5, 6]);
+    ///     assert_eq!(right, []);
+    /// }
+    ///
+    /// assert_eq!(None, v.split_at_checked(7));
+    /// ```
+    #[unstable(feature = "split_at_checked", reason = "new API", issue = "119128")]
+    #[rustc_const_unstable(feature = "split_at_checked", issue = "119128")]
+    #[inline]
+    #[track_caller]
+    #[must_use]
+    pub const fn split_at_checked(&self, mid: usize) -> Option<(&[T], &[T])> {
+        if mid <= self.len() {
+            // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
+            // fulfills the requirements of `split_at_unchecked`.
+            Some(unsafe { self.split_at_unchecked(mid) })
+        } else {
+            None
+        }
+    }
+
+    /// Divides one mutable slice into two at an index.
+    ///
+    /// The first will contain all indices from `[0, mid)` (excluding
+    /// the index `mid` itself) and the second will contain all
+    /// indices from `[mid, len)` (excluding the index `len` itself).
+    ///
+    /// Returns `None` if `mid > len`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(split_at_checked)]
+    ///
+    /// let mut v = [1, 0, 3, 0, 5, 6];
+    ///
+    /// if let Some((left, right)) = v.split_at_mut_checked(2) {
+    ///     assert_eq!(left, [1, 0]);
+    ///     assert_eq!(right, [3, 0, 5, 6]);
+    ///     left[1] = 2;
+    ///     right[1] = 4;
+    /// }
+    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
+    ///
+    /// assert_eq!(None, v.split_at_mut_checked(7));
+    /// ```
+    #[unstable(feature = "split_at_checked", reason = "new API", issue = "119128")]
+    #[rustc_const_unstable(feature = "split_at_checked", issue = "119128")]
+    #[inline]
+    #[track_caller]
+    #[must_use]
+    pub const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut [T], &mut [T])> {
+        if mid <= self.len() {
+            // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
+            // fulfills the requirements of `split_at_unchecked`.
+            Some(unsafe { self.split_at_mut_unchecked(mid) })
+        } else {
+            None
+        }
+    }
+
     /// Divides one slice into an array and a remainder slice at an index.
     ///
     /// The array will contain all indices from `[0, N)` (excluding

library/core/src/str/mod.rs

@@ -641,8 +641,9 @@ impl str {
     ///
     /// # Panics
     ///
-    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// past the end of the last code point of the string slice.
+    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is past
+    /// the end of the last code point of the string slice.  For non-panicking
+    /// alternative see [`split_at_checked`](str::split_at_checked).
     ///
     /// # Examples
     ///
@@ -658,13 +659,7 @@ impl str {
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at(&self, mid: usize) -> (&str, &str) {
-        // is_char_boundary checks that the index is in [0, .len()]
-        if self.is_char_boundary(mid) {
-            // SAFETY: just checked that `mid` is on a char boundary.
-            unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) }
-        } else {
-            slice_error_fail(self, 0, mid)
-        }
+        self.split_at_checked(mid).unwrap_or_else(|| slice_error_fail(self, 0, mid))
     }
 
     /// Divide one mutable string slice into two at an index.
@@ -681,8 +676,9 @@ impl str {
     ///
     /// # Panics
     ///
-    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// past the end of the last code point of the string slice.
+    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is past
+    /// the end of the last code point of the string slice.  For non-panicking
+    /// alternative see [`split_at_mut_checked`](str::split_at_mut_checked).
     ///
     /// # Examples
     ///
@@ -702,20 +698,114 @@ impl str {
     pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
-            let len = self.len();
-            let ptr = self.as_mut_ptr();
             // SAFETY: just checked that `mid` is on a char boundary.
-            unsafe {
-                (
-                    from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr, mid)),
-                    from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr.add(mid), len - mid)),
-                )
-            }
+            unsafe { self.split_at_mut_unchecked(mid) }
         } else {
             slice_error_fail(self, 0, mid)
         }
     }
 
+    /// Divide one string slice into two at an index.
+    ///
+    /// The argument, `mid`, should be a valid byte offset from the start of the
+    /// string. It must also be on the boundary of a UTF-8 code point. The
+    /// method returns `None` if that’s not the case.
+    ///
+    /// The two slices returned go from the start of the string slice to `mid`,
+    /// and from `mid` to the end of the string slice.
+    ///
+    /// To get mutable string slices instead, see the [`split_at_mut_checked`]
+    /// method.
+    ///
+    /// [`split_at_mut_checked`]: str::split_at_mut_checked
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(split_at_checked)]
+    ///
+    /// let s = "Per Martin-Löf";
+    ///
+    /// let (first, last) = s.split_at_checked(3).unwrap();
+    /// assert_eq!("Per", first);
+    /// assert_eq!(" Martin-Löf", last);
+    ///
+    /// assert_eq!(None, s.split_at_checked(13));  // Inside “ö”
+    /// assert_eq!(None, s.split_at_checked(16));  // Beyond the string length
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "split_at_checked", reason = "new API", issue = "119128")]
+    pub fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)> {
+        // is_char_boundary checks that the index is in [0, .len()]
+        if self.is_char_boundary(mid) {
+            // SAFETY: just checked that `mid` is on a char boundary.
+            Some(unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) })
+        } else {
+            None
+        }
+    }
+
+    /// Divide one mutable string slice into two at an index.
+    ///
+    /// The argument, `mid`, should be a valid byte offset from the start of the
+    /// string. It must also be on the boundary of a UTF-8 code point. The
+    /// method returns `None` if that’s not the case.
+    ///
+    /// The two slices returned go from the start of the string slice to `mid`,
+    /// and from `mid` to the end of the string slice.
+    ///
+    /// To get immutable string slices instead, see the [`split_at_checked`] method.
+    ///
+    /// [`split_at_checked`]: str::split_at_checked
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(split_at_checked)]
+    ///
+    /// let mut s = "Per Martin-Löf".to_string();
+    /// if let Some((first, last)) = s.split_at_mut_checked(3) {
+    ///     first.make_ascii_uppercase();
+    ///     assert_eq!("PER", first);
+    ///     assert_eq!(" Martin-Löf", last);
+    /// }
+    /// assert_eq!("PER Martin-Löf", s);
+    ///
+    /// assert_eq!(None, s.split_at_mut_checked(13));  // Inside “ö”
+    /// assert_eq!(None, s.split_at_mut_checked(16));  // Beyond the string length
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "split_at_checked", reason = "new API", issue = "119128")]
+    pub fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)> {
+        // is_char_boundary checks that the index is in [0, .len()]
+        if self.is_char_boundary(mid) {
+            // SAFETY: just checked that `mid` is on a char boundary.
+            Some(unsafe { self.split_at_mut_unchecked(mid) })
+        } else {
+            None
+        }
+    }
+
+    /// Divide one string slice into two at an index.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure `mid` is a byte offset at a UTF-8 character
+    /// boundary.
+    unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
+        let len = self.len();
+        let ptr = self.as_mut_ptr();
+        // SAFETY: caller guarantees `mid` is on a char boundary.
+        unsafe {
+            (
+                from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr, mid)),
+                from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr.add(mid), len - mid)),
+            )
+        }
+    }
+
     /// Returns an iterator over the [`char`]s of a string slice.
     ///
     /// As a string slice consists of valid UTF-8, we can iterate through a