測試用例：列表

測試用例：列表

爲元素實現 fmt::Display，必須按順序分別處理的結構是棘手的。問題是，每一個 write! 生成 fmt::Result。妥善處理這需要處理所有的結果。 Rust 提供 try! 宏正是爲這個目的。

用 try! 在 write! 看起來是這樣的：

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

隨着try! 可用，實現 fmt::Display 讓 Vec 很簡單：

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);
}