add split_at_unchecked

Author: KGrewal1

library/core/src/str/mod.rs

@@ -766,6 +766,33 @@ impl str {
         }
     }
 
+    /// Divides one string slice into two immutable slices at an index.
+    ///
+    /// # Safety
+    ///
+    /// 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.
+    #[inline]
+    #[must_use]
+    unsafe const fn split_at_unchecked(&self, mid: usize) -> (&str, &str) {
+        let len = self.len();
+        let ptr = self.as_ptr();
+
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "str::split_at_unchecked requires the index to be within the slice",
+            (mid: usize = mid, len: usize = len) => mid <= len,
+        );
+
+        // SAFETY: caller guarantees `mid` is on a char boundary.
+        unsafe {
+            (
+                from_utf8_unchecked_mut(slice::from_raw_parts(ptr, mid)),
+                from_utf8_unchecked_mut(slice::from_raw_parts(ptr.add(mid), len - mid)),
+            )
+        }
+    }
+
     /// Divides one string slice into two mutable slices at an index.
     ///
     /// # Safety
@@ -780,7 +807,7 @@ impl str {
 
         assert_unsafe_precondition!(
             check_library_ub,
-            "slice::split_at_mut_unchecked requires the index to be within the slice",
+            "str::split_at_mut_unchecked requires the index to be within the slice",
             (mid: usize = mid, len: usize = len) => mid <= len,
         );