Add basic doc example for core::ptr::write_bytes.

src/libcore/intrinsics.rs

@@ -596,6 +596,19 @@ extern "rust-intrinsic" {
 
     /// Invokes memset on the specified pointer, setting `count * size_of::<T>()`
     /// bytes of memory starting at `dst` to `val`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ptr;
+    ///
+    /// let mut vec = vec![0; 4];
+    /// unsafe {
+    ///     let vec_ptr = vec.as_mut_ptr();
+    ///     ptr::write_bytes(vec_ptr, b'a', 2);
+    /// }
+    /// assert_eq!(vec, [b'a', b'a', 0, 0]);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);