diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..ebf5faa
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,562 @@
+use core::fmt;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::fmt::{Display, Formatter};
+use std::fs::File;
+use std::io::{self, Read};
+use std::ops::Deref;
+use std::rc::{Rc, Weak};
+
+#[derive(Debug)]
+pub struct Point1<T, U> {
+    pub x: T,
+    pub y: U,
+}
+
+impl<D, Z> Point1<D, Z> {
+    pub fn x(&self) -> &D {
+        &self.x
+    }
+}
+
+impl Point1<i32, i32> {
+    pub fn y(&self) -> i32 {
+        self.y
+    }
+}
+
+impl<T, U> Point1<T, U> {
+    pub fn mixup<V, W>(self, p: Point1<V, W>) -> Point1<T, W> {
+        Point1 { x: self.x, y: p.y }
+    }
+}
+
+pub fn test_tuple(t: (i32, &str)) {
+    println!("{} {}", t.0, t.1);
+}
+
+pub static mut COUNTER: u32 = 0;
+
+pub fn add_to_count(inc: u32) {
+    unsafe {
+        COUNTER += inc;
+    }
+}
+
+#[derive(Debug)]
+pub struct Bt {
+    pub height: u32,
+    pub label: Option<String>,
+}
+
+#[derive(Debug)]
+pub struct SelectBox {
+    pub width: u32,
+    pub height: u32,
+    pub options: Vec<String>,
+}
+
+impl Draw for SelectBox {
+    fn draw(&self) {
+        // code to actually draw a select box
+        println!("{:?}", self);
+    }
+}
+
+#[derive(Debug)]
+pub struct Button {
+    pub width: u32,
+    pub height: u32,
+    pub label: String,
+}
+
+impl Draw for Button {
+    fn draw(&self) {
+        // 实际绘制按钮的代码
+        println!("{:?}", self);
+    }
+}
+
+pub struct Screen {
+    pub components: Vec<Box<dyn Draw>>,
+}
+
+impl Screen {
+    pub fn run(&self) {
+        for component in self.components.iter() {
+            component.draw();
+        }
+    }
+}
+
+pub trait Draw {
+    fn draw(&self);
+}
+
+pub struct MyBox<T>(T);
+
+impl<T> MyBox<T> {
+    pub fn new(x: T) -> MyBox<T> {
+        MyBox(x)
+    }
+}
+
+impl<T> Deref for MyBox<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+pub fn hello_str(name: &str) {
+    println!("Hello, {}!", name);
+}
+
+#[derive(Debug)]
+pub enum List {
+    Cons(i32, RefCell<Rc<List>>),
+    Nil,
+}
+
+impl List {
+    pub fn tail(&self) -> Option<&RefCell<Rc<List>>> {
+        use crate::List::*;
+        match self {
+            Cons(_, item) => Some(item),
+            Nil => None,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Node {
+    pub value: i32,
+    pub parent: RefCell<Weak<Node>>,
+    pub children: RefCell<Vec<Rc<Node>>>,
+}
+
+pub fn change(some_string: &mut String) {
+    some_string.push_str(", world");
+}
+
+//enum IpAddrKind {
+//    V4,
+//    V6,
+//}
+
+#[derive(Debug)]
+pub enum IpAddr {
+    V4(u8, u8, u8, u8),
+    V6(String),
+}
+
+pub enum Color {
+    _Rgb(i32, i32, i32),
+    Hsv(i32, i32, i32),
+}
+
+pub enum Msg {
+    _Quit,
+    ChangeColor(Color),
+}
+
+// Quit 没有关联任何数据。
+// Move 包含一个匿名结构体。
+// Write 包含单独一个 String。
+// ChangeColor 包含三个 i32。
+#[derive(Debug)]
+pub enum Message {
+    Quit,
+    Move { x: i32, y: i32 },
+    Write(String),
+    ChangeColor(i32, i32, i32),
+}
+
+impl Message {
+    pub fn call(&self) {
+        println!("this message call.");
+    }
+}
+
+#[derive(Debug)]
+pub struct Rectangle {
+    pub width: u32,
+    pub height: u32,
+}
+
+impl Rectangle {
+    pub fn area(&self) -> u32 {
+        self.width * self.height
+    }
+
+    pub fn can_hold(&self, other: &Rectangle) -> bool {
+        self.width > other.width && self.height > other.height
+    }
+
+    pub fn square(size: u32) -> Rectangle {
+        Rectangle {
+            width: size,
+            height: size,
+        }
+    }
+}
+
+mod sound {
+    pub mod instrument {
+        pub fn clarinet() {
+            println!("this clarinet");
+        }
+    }
+}
+
+pub mod performance_group {
+    pub use crate::sound::instrument;
+
+    pub fn clarinet_trio() {
+        instrument::clarinet();
+        instrument::clarinet();
+        instrument::clarinet();
+    }
+}
+
+///  传播错误
+pub fn read_username_from_file(path: &str) -> Result<String, io::Error> {
+    //    1.
+    // let f = File::open(path);
+    //
+    // let mut f = match f {
+    //     Ok(file) => file,
+    //     Err(error) => return Err(error),
+    // };
+    //
+    // let mut s = String::new();
+    // match f.read_to_string(&mut s) {
+    //     Ok(_) => Ok(s),
+    //     Err(e) => Err(e),
+    // }
+
+    //    2.
+    let mut f = File::open(path)?;
+
+    let mut s = String::new();
+    f.read_to_string(&mut s)?;
+    Ok(s)
+
+    //    3.
+    // let mut s = String::new();
+    // File::open(path)?.read_to_string(&mut s)?;
+    // Ok(s)
+
+    //    4.
+    // fs::read_to_string(path)
+}
+
+pub trait Summary {
+    fn summarize_author(&self) -> String;
+
+    fn summarize(&self) -> String {
+        format!("(Read more from {}...)", self.summarize_author())
+    }
+}
+
+#[derive(Debug)]
+pub struct Tweet {
+    pub username: String,
+    pub content: String,
+    pub reply: bool,
+    pub retweet: bool,
+}
+
+impl Summary for Tweet {
+    fn summarize_author(&self) -> String {
+        format!("@{}", self.username)
+    }
+}
+
+// impl ToString for Tweet {
+//     fn to_string(&self) -> String {
+//         format!("{:?}", self)
+//     }
+// }
+
+impl Display for Tweet {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Tweet username: {}, content: {}",
+            self.username, self.content
+        )
+    }
+}
+
+//fn notify<T: Summary + ToString, U: Summary>(t: &T, u: &U) -> String {
+//    format!("{}, {}", u.summarize(), t.to_string())
+//}
+
+//fn notify(t: &(impl Summary + ToString), u: &(impl Summary)) -> String {
+//    format!("{}, {}", u.summarize(), t.to_string())
+//}
+
+pub fn notify<T, U>(t: &T, u: &U) -> String
+where
+    T: Summary + ToString,
+    U: Summary,
+{
+    format!("{}, {}", u.summarize(), t.to_string())
+}
+
+// 返回实现了指定 trait 的方法
+pub fn returns_summarizable() -> impl Summary {
+    Tweet {
+        username: String::from("horse_ebooks"),
+        content: String::from("of course, as you probably already know, people"),
+        reply: false,
+        retweet: false,
+    }
+}
+
+pub fn largest<T: PartialOrd + Copy>(list: &[T]) -> T {
+    //fn largest(list: &[impl PartialOrd + Copy]) -> impl PartialOrd + Copy {
+    //fn largest<T>(list: &[T]) -> T
+    //    where T: PartialOrd + Copy {
+    let mut largest = list[0];
+
+    for &item in list.iter() {
+        if item > largest {
+            largest = item;
+        }
+    }
+
+    // for item in list.iter() {
+    //     if *item > largest {
+    //         largest = *item;
+    //     }
+    // }
+
+    return largest;
+}
+
+pub fn largest2<T: PartialOrd>(list: &[T]) -> &T {
+    let mut largest = &list[0];
+
+    for item in list.iter() {
+        if item > largest {
+            largest = item;
+        }
+    }
+
+    return largest;
+}
+
+#[derive(Debug)]
+pub struct Pair<T> {
+    x: T,
+    y: T,
+}
+
+impl<T> Pair<T> {
+    pub fn new(x: T, y: T) -> Self {
+        Self { x, y }
+    }
+}
+
+// 只有那些为 T 类型实现了 PartialOrd trait（来允许比较）和 Display trait（来启用打印）的 Pair<T> 才会实现 cmp_display 方法
+impl<T: Display + PartialOrd> Pair<T> {
+    pub fn cmp_display(&self) {
+        if self.x >= self.y {
+            println!("The largest member is x = {}", self.x);
+        } else {
+            println!("The largest member is y = {}", self.y);
+        }
+    }
+}
+
+pub fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
+    if x.len() > y.len() {
+        x
+    } else {
+        y
+    }
+}
+
+// struct Cacher<T>
+// where
+//     T: Fn(u32) -> u32,
+// {
+//     calculation: T,
+//     value: Option<u32>,
+// }
+//
+// impl<T> Cacher<T>
+// where
+//     T: Fn(u32) -> u32,
+// {
+//     fn new(calculation: T) -> Cacher<T> {
+//         Self {
+//             calculation,
+//             value: None,
+//         }
+//     }
+//
+//     fn value(&mut self, args: u32) -> u32 {
+//         match self.value {
+//             Some(&v) => v,
+//             None => {
+//                 let v = (self.calculation)(args);
+//                 v
+//             }
+//         }
+//     }
+// }
+
+pub struct Cacher<T, K, V>
+where
+    T: Fn(&K) -> V,
+{
+    pub calculation: T,
+    pub value: HashMap<K, V>,
+}
+use core::hash::Hash;
+impl<T, K, V> Cacher<T, K, V>
+where
+    V: Copy,
+    K: Hash + Eq,
+    T: Fn(&K) -> V,
+{
+    pub fn new(calculation: T) -> Cacher<T, K, V> {
+        Self {
+            calculation,
+            value: HashMap::<K, V>::new(),
+        }
+    }
+
+    pub fn value(&mut self, args: K) -> V {
+        let value = &mut self.value;
+
+        match value.get(&args) {
+            Some(&v) => v,
+            None => {
+                let x = (self.calculation)(&args);
+                let x1 = x.clone();
+                value.insert(args, x);
+                x1
+            }
+        }
+
+        // match self.value.get(&args) {
+        //     Some(&v) => v,
+        //     None => {
+        //         let x = (self.calculation)(&args);
+        //         let x1 = x.clone();
+        //         self.value.insert(args, x);
+        //         x1
+        //     }
+        // }
+    }
+}
+
+#[derive(Debug)]
+pub struct Shoe {
+    pub size: u32,
+    pub style: String,
+}
+
+pub fn shoes_in_my_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
+    shoes
+        // .iter() //  a collection of type `std::vec::Vec<Shoe>` cannot be built from `std::iter::Iterator<Item=&Shoe>`
+        .into_iter() //获取 vector 所有权的迭代器
+        .filter(|s| s.size == shoe_size)
+        .collect()
+}
+
+pub struct Counter {
+    count: u32,
+}
+
+impl Counter {
+    pub fn new() -> Counter {
+        Counter { count: 0 }
+    }
+}
+
+impl Iterator for Counter {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.count += 1;
+
+        if self.count < 6 {
+            Some(self.count)
+        } else {
+            None
+        }
+    }
+}
+
+pub trait Messenger {
+    fn send(&self, msg: &str);
+}
+
+pub struct LimitTracker<'a, T: Messenger> {
+    messenger: &'a T,
+    value: usize,
+    max: usize,
+}
+
+impl<'a, T> LimitTracker<'a, T>
+where
+    T: Messenger,
+{
+    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<T> {
+        LimitTracker {
+            messenger,
+            value: 0,
+            max,
+        }
+    }
+
+    pub fn set_value(&mut self, value: usize) {
+        self.value = value;
+
+        let percentage_of_max = self.value as f64 / self.max as f64;
+
+        if percentage_of_max >= 1.0 {
+            self.messenger.send("Error: You are over your quota!");
+        } else if percentage_of_max >= 0.9 {
+            self.messenger
+                .send("Urgent warning: You've used up over 90% of your quota!");
+        } else if percentage_of_max >= 0.75 {
+            self.messenger
+                .send("Warning: You've used up over 75% of your quota!");
+        }
+    }
+}
+
+pub trait Pilot {
+    fn fly(&self);
+}
+
+pub trait Wizard {
+    fn fly(&self);
+}
+
+pub struct Human;
+
+impl Pilot for Human {
+    fn fly(&self) {
+        println!("This is your captain speaking.");
+    }
+}
+
+impl Wizard for Human {
+    fn fly(&self) {
+        println!("Up!");
+    }
+}
+
+impl Human {
+    pub fn fly(&self) {
+        println!("*waving arms furiously*");
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index d4f0b22..ec3ea67 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,12 +4,12 @@ mod tests;
 mod user;
 
 use std::cell::RefCell;
-use std::hash::Hash;
-use std::ops::Deref;
 use std::rc::{Rc, Weak};
 use std::sync::{mpsc, Arc, Mutex};
 use std::time::Duration;
-use std::{collections::HashMap, fmt::Display, fs::File, io, io::ErrorKind, thread};
+use std::{collections::HashMap, fs::File, io::ErrorKind, thread};
+
+use rust_study::*;
 
 fn main() {
     let mut s = String::from("Hello");
@@ -1084,561 +1084,3 @@ fn main() {
     // person.fly();
     Human::fly(&person);
 }
-
-#[derive(Debug)]
-struct Point1<T, U> {
-    x: T,
-    y: U,
-}
-
-impl<D, Z> Point1<D, Z> {
-    fn x(&self) -> &D {
-        &self.x
-    }
-}
-
-impl Point1<i32, i32> {
-    fn y(&self) -> i32 {
-        self.y
-    }
-}
-
-impl<T, U> Point1<T, U> {
-    fn mixup<V, W>(self, p: Point1<V, W>) -> Point1<T, W> {
-        Point1 { x: self.x, y: p.y }
-    }
-}
-
-fn test_tuple(t: (i32, &str)) {
-    println!("{} {}", t.0, t.1);
-}
-
-static mut COUNTER: u32 = 0;
-
-fn add_to_count(inc: u32) {
-    unsafe {
-        COUNTER += inc;
-    }
-}
-
-use core::fmt;
-use std::fmt::Formatter;
-use std::io::Read;
-
-#[derive(Debug)]
-struct Bt {
-    height: u32,
-    label: Option<String>,
-}
-
-#[derive(Debug)]
-struct SelectBox {
-    width: u32,
-    height: u32,
-    options: Vec<String>,
-}
-
-impl Draw for SelectBox {
-    fn draw(&self) {
-        // code to actually draw a select box
-        println!("{:?}", self);
-    }
-}
-
-#[derive(Debug)]
-pub struct Button {
-    pub width: u32,
-    pub height: u32,
-    pub label: String,
-}
-
-impl Draw for Button {
-    fn draw(&self) {
-        // 实际绘制按钮的代码
-        println!("{:?}", self);
-    }
-}
-
-pub struct Screen {
-    pub components: Vec<Box<dyn Draw>>,
-}
-
-impl Screen {
-    pub fn run(&self) {
-        for component in self.components.iter() {
-            component.draw();
-        }
-    }
-}
-
-pub trait Draw {
-    fn draw(&self);
-}
-
-struct MyBox<T>(T);
-
-impl<T> MyBox<T> {
-    fn new(x: T) -> MyBox<T> {
-        MyBox(x)
-    }
-}
-
-impl<T> Deref for MyBox<T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        &self.0
-    }
-}
-
-fn hello_str(name: &str) {
-    println!("Hello, {}!", name);
-}
-
-#[derive(Debug)]
-enum List {
-    Cons(i32, RefCell<Rc<List>>),
-    Nil,
-}
-
-impl List {
-    fn tail(&self) -> Option<&RefCell<Rc<List>>> {
-        use crate::List::*;
-        match self {
-            Cons(_, item) => Some(item),
-            Nil => None,
-        }
-    }
-}
-
-#[derive(Debug)]
-struct Node {
-    value: i32,
-    parent: RefCell<Weak<Node>>,
-    children: RefCell<Vec<Rc<Node>>>,
-}
-
-fn change(some_string: &mut String) {
-    some_string.push_str(", world");
-}
-
-//enum IpAddrKind {
-//    V4,
-//    V6,
-//}
-
-#[derive(Debug)]
-enum IpAddr {
-    V4(u8, u8, u8, u8),
-    V6(String),
-}
-
-enum Color {
-    _Rgb(i32, i32, i32),
-    Hsv(i32, i32, i32),
-}
-
-enum Msg {
-    _Quit,
-    ChangeColor(Color),
-}
-
-// Quit 没有关联任何数据。
-// Move 包含一个匿名结构体。
-// Write 包含单独一个 String。
-// ChangeColor 包含三个 i32。
-#[derive(Debug)]
-enum Message {
-    Quit,
-    Move { x: i32, y: i32 },
-    Write(String),
-    ChangeColor(i32, i32, i32),
-}
-
-impl Message {
-    fn call(&self) {
-        println!("this message call.");
-    }
-}
-
-#[derive(Debug)]
-struct Rectangle {
-    width: u32,
-    height: u32,
-}
-
-impl Rectangle {
-    fn area(&self) -> u32 {
-        self.width * self.height
-    }
-
-    fn can_hold(&self, other: &Rectangle) -> bool {
-        self.width > other.width && self.height > other.height
-    }
-
-    fn square(size: u32) -> Rectangle {
-        Rectangle {
-            width: size,
-            height: size,
-        }
-    }
-}
-
-mod sound {
-    pub mod instrument {
-        pub fn clarinet() {
-            println!("this clarinet");
-        }
-    }
-}
-
-mod performance_group {
-    pub use crate::sound::instrument;
-
-    pub fn clarinet_trio() {
-        instrument::clarinet();
-        instrument::clarinet();
-        instrument::clarinet();
-    }
-}
-
-///  传播错误
-fn read_username_from_file(path: &str) -> Result<String, io::Error> {
-    //    1.
-    // let f = File::open(path);
-    //
-    // let mut f = match f {
-    //     Ok(file) => file,
-    //     Err(error) => return Err(error),
-    // };
-    //
-    // let mut s = String::new();
-    // match f.read_to_string(&mut s) {
-    //     Ok(_) => Ok(s),
-    //     Err(e) => Err(e),
-    // }
-
-    //    2.
-    let mut f = File::open(path)?;
-
-    let mut s = String::new();
-    f.read_to_string(&mut s)?;
-    Ok(s)
-
-    //    3.
-    // let mut s = String::new();
-    // File::open(path)?.read_to_string(&mut s)?;
-    // Ok(s)
-
-    //    4.
-    // fs::read_to_string(path)
-}
-
-trait Summary {
-    fn summarize_author(&self) -> String;
-
-    fn summarize(&self) -> String {
-        format!("(Read more from {}...)", self.summarize_author())
-    }
-}
-
-#[derive(Debug)]
-struct Tweet {
-    pub username: String,
-    pub content: String,
-    pub reply: bool,
-    pub retweet: bool,
-}
-
-impl Summary for Tweet {
-    fn summarize_author(&self) -> String {
-        format!("@{}", self.username)
-    }
-}
-
-// impl ToString for Tweet {
-//     fn to_string(&self) -> String {
-//         format!("{:?}", self)
-//     }
-// }
-
-impl Display for Tweet {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(
-            f,
-            "Tweet username: {}, content: {}",
-            self.username, self.content
-        )
-    }
-}
-
-//fn notify<T: Summary + ToString, U: Summary>(t: &T, u: &U) -> String {
-//    format!("{}, {}", u.summarize(), t.to_string())
-//}
-
-//fn notify(t: &(impl Summary + ToString), u: &(impl Summary)) -> String {
-//    format!("{}, {}", u.summarize(), t.to_string())
-//}
-
-fn notify<T, U>(t: &T, u: &U) -> String
-where
-    T: Summary + ToString,
-    U: Summary,
-{
-    format!("{}, {}", u.summarize(), t.to_string())
-}
-
-// 返回实现了指定 trait 的方法
-fn returns_summarizable() -> impl Summary {
-    Tweet {
-        username: String::from("horse_ebooks"),
-        content: String::from("of course, as you probably already know, people"),
-        reply: false,
-        retweet: false,
-    }
-}
-
-fn largest<T: PartialOrd + Copy>(list: &[T]) -> T {
-    //fn largest(list: &[impl PartialOrd + Copy]) -> impl PartialOrd + Copy {
-    //fn largest<T>(list: &[T]) -> T
-    //    where T: PartialOrd + Copy {
-    let mut largest = list[0];
-
-    for &item in list.iter() {
-        if item > largest {
-            largest = item;
-        }
-    }
-
-    // for item in list.iter() {
-    //     if *item > largest {
-    //         largest = *item;
-    //     }
-    // }
-
-    return largest;
-}
-
-fn largest2<T: PartialOrd>(list: &[T]) -> &T {
-    let mut largest = &list[0];
-
-    for item in list.iter() {
-        if item > largest {
-            largest = item;
-        }
-    }
-
-    return largest;
-}
-
-#[derive(Debug)]
-struct Pair<T> {
-    x: T,
-    y: T,
-}
-
-impl<T> Pair<T> {
-    fn new(x: T, y: T) -> Self {
-        Self { x, y }
-    }
-}
-
-// 只有那些为 T 类型实现了 PartialOrd trait（来允许比较）和 Display trait（来启用打印）的 Pair<T> 才会实现 cmp_display 方法
-impl<T: Display + PartialOrd> Pair<T> {
-    fn cmp_display(&self) {
-        if self.x >= self.y {
-            println!("The largest member is x = {}", self.x);
-        } else {
-            println!("The largest member is y = {}", self.y);
-        }
-    }
-}
-
-fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
-    if x.len() > y.len() {
-        x
-    } else {
-        y
-    }
-}
-
-// struct Cacher<T>
-// where
-//     T: Fn(u32) -> u32,
-// {
-//     calculation: T,
-//     value: Option<u32>,
-// }
-//
-// impl<T> Cacher<T>
-// where
-//     T: Fn(u32) -> u32,
-// {
-//     fn new(calculation: T) -> Cacher<T> {
-//         Self {
-//             calculation,
-//             value: None,
-//         }
-//     }
-//
-//     fn value(&mut self, args: u32) -> u32 {
-//         match self.value {
-//             Some(&v) => v,
-//             None => {
-//                 let v = (self.calculation)(args);
-//                 v
-//             }
-//         }
-//     }
-// }
-
-struct Cacher<T, K, V>
-where
-    T: Fn(&K) -> V,
-{
-    calculation: T,
-    value: HashMap<K, V>,
-}
-
-impl<T, K, V> Cacher<T, K, V>
-where
-    V: Copy,
-    K: Hash + Eq,
-    T: Fn(&K) -> V,
-{
-    fn new(calculation: T) -> Cacher<T, K, V> {
-        Self {
-            calculation,
-            value: HashMap::<K, V>::new(),
-        }
-    }
-
-    fn value(&mut self, args: K) -> V {
-        let value = &mut self.value;
-
-        match value.get(&args) {
-            Some(&v) => v,
-            None => {
-                let x = (self.calculation)(&args);
-                let x1 = x.clone();
-                value.insert(args, x);
-                x1
-            }
-        }
-
-        // match self.value.get(&args) {
-        //     Some(&v) => v,
-        //     None => {
-        //         let x = (self.calculation)(&args);
-        //         let x1 = x.clone();
-        //         self.value.insert(args, x);
-        //         x1
-        //     }
-        // }
-    }
-}
-
-#[derive(Debug)]
-struct Shoe {
-    size: u32,
-    style: String,
-}
-
-fn shoes_in_my_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
-    shoes
-        // .iter() //  a collection of type `std::vec::Vec<Shoe>` cannot be built from `std::iter::Iterator<Item=&Shoe>`
-        .into_iter() //获取 vector 所有权的迭代器
-        .filter(|s| s.size == shoe_size)
-        .collect()
-}
-
-struct Counter {
-    count: u32,
-}
-
-impl Counter {
-    fn new() -> Counter {
-        Counter { count: 0 }
-    }
-}
-
-impl Iterator for Counter {
-    type Item = u32;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.count += 1;
-
-        if self.count < 6 {
-            Some(self.count)
-        } else {
-            None
-        }
-    }
-}
-
-pub trait Messenger {
-    fn send(&self, msg: &str);
-}
-
-pub struct LimitTracker<'a, T: Messenger> {
-    messenger: &'a T,
-    value: usize,
-    max: usize,
-}
-
-impl<'a, T> LimitTracker<'a, T>
-where
-    T: Messenger,
-{
-    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<T> {
-        LimitTracker {
-            messenger,
-            value: 0,
-            max,
-        }
-    }
-
-    pub fn set_value(&mut self, value: usize) {
-        self.value = value;
-
-        let percentage_of_max = self.value as f64 / self.max as f64;
-
-        if percentage_of_max >= 1.0 {
-            self.messenger.send("Error: You are over your quota!");
-        } else if percentage_of_max >= 0.9 {
-            self.messenger
-                .send("Urgent warning: You've used up over 90% of your quota!");
-        } else if percentage_of_max >= 0.75 {
-            self.messenger
-                .send("Warning: You've used up over 75% of your quota!");
-        }
-    }
-}
-
-trait Pilot {
-    fn fly(&self);
-}
-
-trait Wizard {
-    fn fly(&self);
-}
-
-struct Human;
-
-impl Pilot for Human {
-    fn fly(&self) {
-        println!("This is your captain speaking.");
-    }
-}
-
-impl Wizard for Human {
-    fn fly(&self) {
-        println!("Up!");
-    }
-}
-
-impl Human {
-    fn fly(&self) {
-        println!("*waving arms furiously*");
-    }
-}