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List.len by using "Deref".rs

// Define a structure named `List` containing a `Vec`.

use std::fmt;
use std::ops::Deref;

struct List(Vec<i32>);

impl Deref for List {
    type Target = Vec<i32>;
    
    fn deref(&self) -> &Vec<i32> {
        &self.0
    }
}

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;

        try!(write!(f, "["));

        // Iterate over `vec` in `v` while enumerating the iteration
        // count in `count`.
        for (count, v) in vec.iter().enumerate() {
            // For every element except the first, add a comma
            // before calling `write!`. Use `try!` to return on errors.
            if count != 0 { try!(write!(f, ", ")); }
            try!(write!(f, "{}", v));
        }

        // Close the opened bracket and return a fmt::Result value
        write!(f, "]")
    }
}

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