测试用例：列表

为元素实现 fmt::Display，必须按顺序分别处理的结构是棘手的。问题是，每一个 write! 生成 fmt::Result。妥善处理这需要处理所有的结果。 Rust 提供 try! 宏正是为这个目的。

用 try! 在 write! 看起来是这样的：

// Try `write!` to see if it errors. If it errors, return
// the error. Otherwise continue.
try!(write!(f, "{}", value));

随着try! 可用，实现 fmt::Display 让 Vec 很简单：

use std::fmt; // Import the `fmt` module.

// Define a structure named `List` containing a `Vec`.
struct List(Vec
 
  );

impl fmt::Display for List {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Dereference `self` and create a reference to `vec`
        // via destructuring.
        let List(ref vec) = *self;
        let len = vec.len(); // Save the vector length in `len`.

        // Iterate over `vec` in `v` while enumerating the iteration
        // count in `count`.
        for (count, v) in vec.iter().enumerate() {
            // For every element except the last, format `write!`
            // with a comma. Use `try!` to return on errors.
            if count < len - 1 { try!(write!(f, "{}, ", v)) }
        }

        // `write!` the last value without special formatting.
        write!(f, "{}", vec[len-1])
    }
}

fn main() {
    let v = List(vec![1, 2, 3]);
    println!("{}", v);
}