Rewrite/expand doc examples for `Vec::set_len`.

src/libcollections/vec.rs

@@ -532,9 +532,37 @@ impl<T> Vec<T> {
    /// # Examples
    ///
    /// ```
    /// let mut v = vec![1, 2, 3, 4];
    /// use std::ptr;
    ///
    /// let mut vec = vec!['r', 'u', 's', 't'];
    ///
    /// unsafe {
    ///     ptr::drop_in_place(&mut vec[3]);
    ///     vec.set_len(3);
    /// }
    /// assert_eq!(vec, ['r', 'u', 's']);
    /// ```
    ///
    /// In this example, there is a memory leak since the memory locations
    /// owned by the vector were not freed prior to the `set_len` call:
    ///
    /// ```
    /// let mut vec = vec!['r', 'u', 's', 't'];
    ///
    /// unsafe {
    ///     vec.set_len(0);
    /// }
    /// ```
    ///
    /// In this example, the vector gets expanded from zero to four items
    /// without any memory allocations occurring, resulting in vector
    /// values of unallocated memory:
    ///
    /// ```
    /// let mut vec: Vec<char> = Vec::new();
    ///
    /// unsafe {
    ///     v.set_len(1);
    ///     vec.set_len(4);
    /// }
    /// ```
    #[inline]