rustlings

---

时间轴

时间轴

2025-10-27

init

本文介绍了 Rust 编程语言练习项目 rustlings 的学习笔记，涵盖了环境搭建、变量定义、move 语义、结构体更新语法、枚举类型、字符串处理、HashMap 的 entry 方法、Option 与 match 模式匹配、错误处理（包括 Box 类型与 ? 运算符）、trait 默认实现、迭代器的惰性特性及 map 方法、智能指针（如 Cow）以及多线程中共享可变状态的安全方案。文章总结了 Rust 中常见易错点与实用技巧，例如常量定义需显式标注类型、vec 与参数需声明为 mut、match 可通过引用绑定避免所有权转移、RefCell 不能跨线程使用，而应使用 Arc 结合其他同步类型实现线程安全的数据共享。

---

环境搭建

# Installation
cargo install rustlings
# Initialization
rustlings init
# Moving into new directory
cd rustlings
# 进入rustlings命令行模式
rustlings
# 检查所有题目
rustlings check-all
# 运行某一个题目
rustlings run primitive_types2

variable

常量定义要加上类型！
variable6

// TODO: Change the line below to fix the compiler error.
const NUMBER: i32 = 3;

fn main() {
    println!("Number: {NUMBER}");
}

move_semantics

vec 声明需要为 mut，参数也得为 mut

fn fill_vec(mut vec: Vec<i32>) -> Vec<i32> {
    vec.push(88);

    vec
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn move_semantics3() {
        let mut vec0 = vec![22, 44, 66];
        let vec1 = fill_vec(vec0);
        assert_eq!(vec1, [22, 44, 66, 88]);
    }
}

struct

…更新语法

#[derive(Debug)]
struct Order {
    name: String,
    year: u32,
    made_by_phone: bool,
    made_by_mobile: bool,
    made_by_email: bool,
    item_number: u32,
    count: u32,
}

fn create_order_template() -> Order {
    Order {
        name: String::from("Bob"),
        year: 2019,
        made_by_phone: false,
        made_by_mobile: false,
        made_by_email: true,
        item_number: 123,
        count: 0,
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn your_order() {
        let order_template = create_order_template();

        // TODO: Create your own order using the update syntax and template above!
        // let your_order =
        let your_order = Order {
            name: String::from("Hacker in Rust"),
            count: 1,
            ..order_template
        };
        assert_eq!(your_order.name, "Hacker in Rust");
        assert_eq!(your_order.year, order_template.year);
        assert_eq!(your_order.made_by_phone, order_template.made_by_phone);
        assert_eq!(your_order.made_by_mobile, order_template.made_by_mobile);
        assert_eq!(your_order.made_by_email, order_template.made_by_email);
        assert_eq!(your_order.item_number, order_template.item_number);
        assert_eq!(your_order.count, 1);
    }
}

enum

enum 可以存放的 variants 类型

#[derive(Debug)]
struct Point {
    x: u64,
    y: u64,
}

#[derive(Debug)]
enum Message {
    // TODO: Define the different variants used below.
    Resize { width: u32, height: u32 },
    Move(Point),
    Echo(String),
    ChangeColor(u8, u8, u8),
    Quit,
}

impl Message {
    fn call(&self) {
        println!("{self:?}");
    }
}

fn main() {
    let messages = [
        Message::Resize {
            width: 10,
            height: 30,
        },
        Message::Move(Point { x: 10, y: 15 }),
        Message::Echo(String::from("hello world")),
        Message::ChangeColor(200, 255, 255),
        Message::Quit,
    ];

    for message in &messages {
        message.call();
    }
}

string

quiz2

// This is a quiz for the following sections:
// - Strings
// - Vecs
// - Move semantics
// - Modules
// - Enums
//
// Let's build a little machine in the form of a function. As input, we're going
// to give a list of strings and commands. These commands determine what action
// is going to be applied to the string. It can either be:
// - Uppercase the string
// - Trim the string
// - Append "bar" to the string a specified amount of times
//
// The exact form of this will be:
// - The input is going to be a Vector of 2-length tuples,
//   the first element is the string, the second one is the command.
// - The output element is going to be a vector of strings.

enum Command {
    Uppercase,
    Trim,
    Append(usize),
}

mod my_module {
    use super::Command;

    // TODO: Complete the function as described above.
    // pub fn transformer(input: ???) -> ??? { ??? }
    pub fn transformer(input: Vec<(String, Command)>) -> Vec<String> {
        let mut output = vec![];
        for (s, cmd) in input.iter() {
            match cmd {
                Command::Uppercase => {
                    output.push(s.to_uppercase());
                }
                Command::Trim => {
                    output.push(s.trim().to_string());
                }
                Command::Append(times) => {
                    output.push(format!("{}{}", s, "bar".repeat(*times)));
                }
            }
        }
        output
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    // TODO: What do we need to import to have `transformer` in scope?
    // use ???;
    use super::my_module::transformer;
    use super::Command;

    #[test]
    fn it_works() {
        let input = vec![
            ("hello".to_string(), Command::Uppercase),
            (" all roads lead to rome! ".to_string(), Command::Trim),
            ("foo".to_string(), Command::Append(1)),
            ("bar".to_string(), Command::Append(5)),
        ];
        let output = transformer(input);

        assert_eq!(
            output,
            [
                "HELLO",
                "all roads lead to rome!",
                "foobar",
                "barbarbarbarbarbar",
            ]
        );
    }
}

hashmap

entry 方法非常好用

// We're collecting different fruits to bake a delicious fruit cake. For this,
// we have a basket, which we'll represent in the form of a hash map. The key
// represents the name of each fruit we collect and the value represents how
// many of that particular fruit we have collected. Three types of fruits -
// Apple (4), Mango (2) and Lychee (5) are already in the basket hash map. You
// must add fruit to the basket so that there is at least one of each kind and
// more than 11 in total - we have a lot of mouths to feed. You are not allowed
// to insert any more of the fruits that are already in the basket (Apple,
// Mango, and Lychee).

use std::collections::HashMap;

#[derive(Hash, PartialEq, Eq, Debug)]
enum Fruit {
    Apple,
    Banana,
    Mango,
    Lychee,
    Pineapple,
}

fn fruit_basket(basket: &mut HashMap<Fruit, u32>) {
    let fruit_kinds = [
        Fruit::Apple,
        Fruit::Banana,
        Fruit::Mango,
        Fruit::Lychee,
        Fruit::Pineapple,
    ];

    for fruit in fruit_kinds {
        // TODO: Insert new fruits if they are not already present in the
        // basket. Note that you are not allowed to put any type of fruit that's
        // already present!
        basket.entry(fruit).or_insert(8);
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    // Don't modify this function!
    fn get_fruit_basket() -> HashMap<Fruit, u32> {
        let content = [(Fruit::Apple, 4), (Fruit::Mango, 2), (Fruit::Lychee, 5)];
        HashMap::from_iter(content)
    }

    #[test]
    fn test_given_fruits_are_not_modified() {
        let mut basket = get_fruit_basket();
        fruit_basket(&mut basket);
        assert_eq!(*basket.get(&Fruit::Apple).unwrap(), 4);
        assert_eq!(*basket.get(&Fruit::Mango).unwrap(), 2);
        assert_eq!(*basket.get(&Fruit::Lychee).unwrap(), 5);
    }

    #[test]
    fn at_least_five_types_of_fruits() {
        let mut basket = get_fruit_basket();
        fruit_basket(&mut basket);
        let count_fruit_kinds = basket.len();
        assert!(count_fruit_kinds >= 5);
    }

    #[test]
    fn greater_than_eleven_fruits() {
        let mut basket = get_fruit_basket();
        fruit_basket(&mut basket);
        let count = basket.values().sum::<u32>();
        assert!(count > 11);
    }

    #[test]
    fn all_fruit_types_in_basket() {
        let fruit_kinds = [
            Fruit::Apple,
            Fruit::Banana,
            Fruit::Mango,
            Fruit::Lychee,
            Fruit::Pineapple,
        ];

        let mut basket = get_fruit_basket();
        fruit_basket(&mut basket);

        for fruit_kind in fruit_kinds {
            let Some(amount) = basket.get(&fruit_kind) else {
                panic!("Fruit kind {fruit_kind:?} was not found in basket");
            };
            assert!(*amount > 0);
        }
    }
}

entry().and_modify().or_insert()

// A list of scores (one per line) of a soccer match is given. Each line is of
// the form "<team_1_name>,<team_2_name>,<team_1_goals>,<team_2_goals>"
// Example: "England,France,4,2" (England scored 4 goals, France 2).
//
// You have to build a scores table containing the name of the team, the total
// number of goals the team scored, and the total number of goals the team
// conceded.

use std::collections::HashMap;

// A structure to store the goal details of a team.
#[derive(Default)]
struct TeamScores {
    goals_scored: u8,
    goals_conceded: u8,
}

fn build_scores_table(results: &str) -> HashMap<&str, TeamScores> {
    // The name of the team is the key and its associated struct is the value.
    let mut scores = HashMap::<&str, TeamScores>::new();

    for line in results.lines() {
        let mut split_iterator = line.split(',');
        // NOTE: We use `unwrap` because we didn't deal with error handling yet.
        let team_1_name = split_iterator.next().unwrap();
        let team_2_name = split_iterator.next().unwrap();
        let team_1_score: u8 = split_iterator.next().unwrap().parse().unwrap();
        let team_2_score: u8 = split_iterator.next().unwrap().parse().unwrap();

        // TODO: Populate the scores table with the extracted details.
        // Keep in mind that goals scored by team 1 will be the number of goals
        // conceded by team 2. Similarly, goals scored by team 2 will be the
        // number of goals conceded by team 1.
        scores
            .entry(team_1_name)
            .and_modify(|teamscore| {
                teamscore.goals_scored += team_1_score;
                teamscore.goals_conceded += team_2_score;
            })
            .or_insert(TeamScores {
                goals_scored: team_1_score,
                goals_conceded: team_2_score,
            });
        scores
            .entry(team_2_name)
            .and_modify(|teamscore| {
                teamscore.goals_scored += team_2_score;
                teamscore.goals_conceded += team_1_score;
            })
            .or_insert(TeamScores {
                goals_scored: team_2_score,
                goals_conceded: team_1_score,
            });
    }

    scores
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    const RESULTS: &str = "England,France,4,2
France,Italy,3,1
Poland,Spain,2,0
Germany,England,2,1
England,Spain,1,0";

    #[test]
    fn build_scores() {
        let scores = build_scores_table(RESULTS);

        assert!(["England", "France", "Germany", "Italy", "Poland", "Spain"]
            .into_iter()
            .all(|team_name| scores.contains_key(team_name)));
    }

    #[test]
    fn validate_team_score_1() {
        let scores = build_scores_table(RESULTS);
        let team = scores.get("England").unwrap();
        assert_eq!(team.goals_scored, 6);
        assert_eq!(team.goals_conceded, 4);
    }

    #[test]
    fn validate_team_score_2() {
        let scores = build_scores_table(RESULTS);
        let team = scores.get("Spain").unwrap();
        assert_eq!(team.goals_scored, 0);
        assert_eq!(team.goals_conceded, 3);
    }
}

option

match 取得所有权，可以用ref绑定取得引用而非值

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let optional_point = Some(Point { x: 100, y: 200 });

    // TODO: Fix the compiler error by adding something to this match statement.
    match optional_point {
        Some(ref p) => println!("Coordinates are {},{}", p.x, p.y),
        _ => panic!("No match!"),
    }

    println!("{optional_point:?}"); // Don't change this line.
}

也可以直接match引用类型

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let optional_point = Some(Point { x: 100, y: 200 });

    // TODO: Fix the compiler error by adding something to this match statement.
    match &optional_point {
        Some(p) => println!("Coordinates are {},{}", p.x, p.y),
        _ => panic!("No match!"),
    }

    println!("{optional_point:?}"); // Don't change this line.
}

error_handling

返回Box类型，?运算符会自动包装返回的错误类型

// This exercise is an altered version of the `errors4` exercise. It uses some
// concepts that we won't get to until later in the course, like `Box` and the
// `From` trait. It's not important to understand them in detail right now, but
// you can read ahead if you like. For now, think of the `Box<dyn ???>` type as
// an "I want anything that does ???" type.
//
// In short, this particular use case for boxes is for when you want to own a
// value and you care only that it is a type which implements a particular
// trait. To do so, the `Box` is declared as of type `Box<dyn Trait>` where
// `Trait` is the trait the compiler looks for on any value used in that
// context. For this exercise, that context is the potential errors which
// can be returned in a `Result`.

use std::error::Error;
use std::fmt;

#[derive(PartialEq, Debug)]
enum CreationError {
    Negative,
    Zero,
}

// This is required so that `CreationError` can implement `Error`.
impl fmt::Display for CreationError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let description = match *self {
            CreationError::Negative => "number is negative",
            CreationError::Zero => "number is zero",
        };
        f.write_str(description)
    }
}

impl Error for CreationError {}

#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
        match value {
            x if x < 0 => Err(CreationError::Negative),
            0 => Err(CreationError::Zero),
            x => Ok(PositiveNonzeroInteger(x as u64)),
        }
    }
}

// TODO: Add the correct return type `Result<(), Box<dyn ???>>`. What can we
// use to describe both errors? Is there a trait which both errors implement?
fn main() -> Result<(), Box<dyn Error>> {
    let pretend_user_input = "42";
    let x: i64 = pretend_user_input.parse()?;
    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
    Ok(())
}

map_error

// Using catch-all error types like `Box<dyn Error>` isn't recommended for
// library code where callers might want to make decisions based on the error
// content instead of printing it out or propagating it further. Here, we define
// a custom error type to make it possible for callers to decide what to do next
// when our function returns an error.

use std::num::ParseIntError;

#[derive(PartialEq, Debug)]
enum CreationError {
    Negative,
    Zero,
}

// A custom error type that we will be using in `PositiveNonzeroInteger::parse`.
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
    Creation(CreationError),
    ParseInt(ParseIntError),
}

impl ParsePosNonzeroError {
    fn from_creation(err: CreationError) -> Self {
        Self::Creation(err)
    }

    // TODO: Add another error conversion function here.
    // fn from_parse_int(???) -> Self { ??? }
    fn from_parse_int(err: ParseIntError) -> Self {
        Self::ParseInt(err)
    }
}

#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<Self, CreationError> {
        match value {
            x if x < 0 => Err(CreationError::Negative),
            0 => Err(CreationError::Zero),
            x => Ok(Self(x as u64)),
        }
    }

    fn parse(s: &str) -> Result<Self, ParsePosNonzeroError> {
        // TODO: change this to return an appropriate error instead of panicking
        // when `parse()` returns an error.
        let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parse_int)?;
        Self::new(x).map_err(ParsePosNonzeroError::from_creation)
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_parse_error() {
        assert!(matches!(
            PositiveNonzeroInteger::parse("not a number"),
            Err(ParsePosNonzeroError::ParseInt(_)),
        ));
    }

    #[test]
    fn test_negative() {
        assert_eq!(
            PositiveNonzeroInteger::parse("-555"),
            Err(ParsePosNonzeroError::Creation(CreationError::Negative)),
        );
    }

    #[test]
    fn test_zero() {
        assert_eq!(
            PositiveNonzeroInteger::parse("0"),
            Err(ParsePosNonzeroError::Creation(CreationError::Zero)),
        );
    }

    #[test]
    fn test_positive() {
        let x = PositiveNonzeroInteger::new(42).unwrap();
        assert_eq!(x.0, 42);
        assert_eq!(PositiveNonzeroInteger::parse("42"), Ok(x));
    }
}

trait

trait default implementation

trait Licensed {
    // TODO: Add a default implementation for `licensing_info` so that
    // implementors like the two structs below can share that default behavior
    // without repeating the function.
    // The default license information should be the string "Default license".
    fn licensing_info(&self) -> String {
        String::from("Default license")
    }
}

struct SomeSoftware {
    version_number: i32,
}

struct OtherSoftware {
    version_number: String,
}

impl Licensed for SomeSoftware {} // Don't edit this line.
impl Licensed for OtherSoftware {} // Don't edit this line.

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn is_licensing_info_the_same() {
        let licensing_info = "Default license";
        let some_software = SomeSoftware { version_number: 1 };
        let other_software = OtherSoftware {
            version_number: "v2.0.0".to_string(),
        };
        assert_eq!(some_software.licensing_info(), licensing_info);
        assert_eq!(other_software.licensing_info(), licensing_info);
    }
}

iterator

iterator是惰性的，除非调用next或collect否则什么也没发生

#[derive(Debug, PartialEq, Eq)]
enum DivisionError {
    // Example: 42 / 0
    DivideByZero,
    // Only case for `i64`: `i64::MIN / -1` because the result is `i64::MAX + 1`
    IntegerOverflow,
    // Example: 5 / 2 = 2.5
    NotDivisible,
}

// TODO: Calculate `a` divided by `b` if `a` is evenly divisible by `b`.
// Otherwise, return a suitable error.
fn divide(a: i64, b: i64) -> Result<i64, DivisionError> {
    if b == 0 {
        return Err(DivisionError::DivideByZero);
    }
    if a == i64::MIN && b == -1 {
        return Err(DivisionError::IntegerOverflow);
    }
    if a % b == 0 {
        return Ok(a / b);
    } else {
        return Err(DivisionError::NotDivisible);
    }
}

// TODO: Add the correct return type and complete the function body.
// Desired output: `Ok([1, 11, 1426, 3])`
fn result_with_list() -> Result<Vec<i64>, DivisionError> {
    let numbers = [27, 297, 38502, 81];
    let division_results = numbers.into_iter().map(|n| divide(n, 27));
    division_results.collect()
}

// TODO: Add the correct return type and complete the function body.
// Desired output: `[Ok(1), Ok(11), Ok(1426), Ok(3)]`
fn list_of_results() -> Vec<Result<i64, DivisionError>> {
    let numbers = [27, 297, 38502, 81];
    let division_results = numbers.into_iter().map(|n| divide(n, 27));
    division_results.collect()
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_success() {
        assert_eq!(divide(81, 9), Ok(9));
        assert_eq!(divide(81, -1), Ok(-81));
        assert_eq!(divide(i64::MIN, i64::MIN), Ok(1));
    }

    #[test]
    fn test_divide_by_0() {
        assert_eq!(divide(81, 0), Err(DivisionError::DivideByZero));
    }

    #[test]
    fn test_integer_overflow() {
        assert_eq!(divide(i64::MIN, -1), Err(DivisionError::IntegerOverflow));
    }

    #[test]
    fn test_not_divisible() {
        assert_eq!(divide(81, 6), Err(DivisionError::NotDivisible));
    }

    #[test]
    fn test_divide_0_by_something() {
        assert_eq!(divide(0, 81), Ok(0));
    }

    #[test]
    fn test_result_with_list() {
        assert_eq!(result_with_list().unwrap(), [1, 11, 1426, 3]);
    }

    #[test]
    fn test_list_of_results() {
        assert_eq!(list_of_results(), [Ok(1), Ok(11), Ok(1426), Ok(3)]);
    }
}

计算阶乘

fn factorial(num: u64) -> u64 {
    // TODO: Complete this function to return the factorial of `num` which is
    // defined as `1 * 2 * 3 * … * num`.
    // https://en.wikipedia.org/wiki/Factorial
    //
    // Do not use:
    // - early returns (using the `return` keyword explicitly)
    // Try not to use:
    // - imperative style loops (for/while)
    // - additional variables
    // For an extra challenge, don't use:
    // - recursion
    if num == 0 { 1 } else { (1..=num).product() }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn factorial_of_0() {
        assert_eq!(factorial(0), 1);
    }

    #[test]
    fn factorial_of_1() {
        assert_eq!(factorial(1), 1);
    }
    #[test]
    fn factorial_of_2() {
        assert_eq!(factorial(2), 2);
    }

    #[test]
    fn factorial_of_4() {
        assert_eq!(factorial(4), 24);
    }
}

map可以把iter所要迭代的类型转换成要迭代的另一种类型

// Let's define a simple model to track Rustlings' exercise progress. Progress
// will be modelled using a hash map. The name of the exercise is the key and
// the progress is the value. Two counting functions were created to count the
// number of exercises with a given progress. Recreate this counting
// functionality using iterators. Try to not use imperative loops (for/while).

use std::collections::HashMap;

#[derive(Clone, Copy, PartialEq, Eq)]
enum Progress {
    None,
    Some,
    Complete,
}

fn count_for(map: &HashMap<String, Progress>, value: Progress) -> usize {
    let mut count = 0;
    for val in map.values() {
        if *val == value {
            count += 1;
        }
    }
    count
}

// TODO: Implement the functionality of `count_for` but with an iterator instead
// of a `for` loop.
fn count_iterator(map: &HashMap<String, Progress>, value: Progress) -> usize {
    // `map` is a hash map with `String` keys and `Progress` values.
    // map = { "variables1": Complete, "from_str": None, … }
    map.values().filter(|&&val| val == value).count()
}

fn count_collection_for(collection: &[HashMap<String, Progress>], value: Progress) -> usize {
    let mut count = 0;
    for map in collection {
        for val in map.values() {
            if *val == value {
                count += 1;
            }
        }
    }
    count
}

// TODO: Implement the functionality of `count_collection_for` but with an
// iterator instead of a `for` loop.
fn count_collection_iterator(collection: &[HashMap<String, Progress>], value: Progress) -> usize {
    // `collection` is a slice of hash maps.
    // collection = [{ "variables1": Complete, "from_str": None, … },
    //               { "variables2": Complete, … }, … ]
    collection
        .iter()
        .map(|map| count_iterator(map, value))
        .sum()
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    fn get_map() -> HashMap<String, Progress> {
        use Progress::*;

        let mut map = HashMap::new();
        map.insert(String::from("variables1"), Complete);
        map.insert(String::from("functions1"), Complete);
        map.insert(String::from("hashmap1"), Complete);
        map.insert(String::from("arc1"), Some);
        map.insert(String::from("as_ref_mut"), None);
        map.insert(String::from("from_str"), None);

        map
    }

    fn get_vec_map() -> Vec<HashMap<String, Progress>> {
        use Progress::*;

        let map = get_map();

        let mut other = HashMap::new();
        other.insert(String::from("variables2"), Complete);
        other.insert(String::from("functions2"), Complete);
        other.insert(String::from("if1"), Complete);
        other.insert(String::from("from_into"), None);
        other.insert(String::from("try_from_into"), None);

        vec![map, other]
    }

    #[test]
    fn count_complete() {
        let map = get_map();
        assert_eq!(count_iterator(&map, Progress::Complete), 3);
    }

    #[test]
    fn count_some() {
        let map = get_map();
        assert_eq!(count_iterator(&map, Progress::Some), 1);
    }

    #[test]
    fn count_none() {
        let map = get_map();
        assert_eq!(count_iterator(&map, Progress::None), 2);
    }

    #[test]
    fn count_complete_equals_for() {
        let map = get_map();
        let progress_states = [Progress::Complete, Progress::Some, Progress::None];
        for progress_state in progress_states {
            assert_eq!(
                count_for(&map, progress_state),
                count_iterator(&map, progress_state),
            );
        }
    }

    #[test]
    fn count_collection_complete() {
        let collection = get_vec_map();
        assert_eq!(
            count_collection_iterator(&collection, Progress::Complete),
            6,
        );
    }

    #[test]
    fn count_collection_some() {
        let collection = get_vec_map();
        assert_eq!(count_collection_iterator(&collection, Progress::Some), 1);
    }

    #[test]
    fn count_collection_none() {
        let collection = get_vec_map();
        assert_eq!(count_collection_iterator(&collection, Progress::None), 4);
    }

    #[test]
    fn count_collection_equals_for() {
        let collection = get_vec_map();
        let progress_states = [Progress::Complete, Progress::Some, Progress::None];

        for progress_state in progress_states {
            assert_eq!(
                count_collection_for(&collection, progress_state),
                count_collection_iterator(&collection, progress_state),
            );
        }
    }
}

smart pointers

std::borrow::Cow 智能指针

// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
// enclose and provide immutable access to borrowed data and clone the data
// lazily when mutation or ownership is required. The type is designed to work
// with general borrowed data via the `ToOwned` trait.

use std::borrow::Cow;

fn abs_all(input: &mut Cow<[i32]>) {
    for ind in 0..input.len() {
        let value = input[ind];
        if value < 0 {
            // Clones into a vector if not already owned.
            input.to_mut()[ind] = -value;
        }
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn reference_mutation() {
        // Clone occurs because `input` needs to be mutated.
        let vec = vec![-1, 0, 1];
        let mut input = Cow::from(&vec);
        abs_all(&mut input);
        assert!(matches!(input, Cow::Owned(_)));
    }

    #[test]
    fn reference_no_mutation() {
        // No clone occurs because `input` doesn't need to be mutated.
        let vec = vec![0, 1, 2];
        let mut input = Cow::from(&vec);
        abs_all(&mut input);
        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
        assert!(matches!(input, Cow::Borrowed(_)));
    }

    #[test]
    fn owned_no_mutation() {
        // We can also pass `vec` without `&` so `Cow` owns it directly. In this
        // case, no mutation occurs (all numbers are already absolute) and thus
        // also no clone. But the result is still owned because it was never
        // borrowed or mutated.
        let vec = vec![0, 1, 2];
        let mut input = Cow::from(vec);
        abs_all(&mut input);
        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
        assert!(matches!(input, Cow::Owned(_)));
    }

    #[test]
    fn owned_mutation() {
        // Of course this is also the case if a mutation does occur (not all
        // numbers are absolute). In this case, the call to `to_mut()` in the
        // `abs_all` function returns a reference to the same data as before.
        let vec = vec![-1, 0, 1];
        let mut input = Cow::from(vec);
        abs_all(&mut input);
        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
        assert!(matches!(input, Cow::Owned(_)));
    }
}

thread

收集线程返回值

// This program spawns multiple threads that each runs for at least 250ms, and
// each thread returns how much time it took to complete. The program should
// wait until all the spawned threads have finished and should collect their
// return values into a vector.

use std::{
    thread,
    time::{Duration, Instant},
};

fn main() {
    let mut handles = Vec::new();
    for i in 0..10 {
        let handle = thread::spawn(move || {
            let start = Instant::now();
            thread::sleep(Duration::from_millis(250));
            println!("Thread {i} done");
            start.elapsed().as_millis()
        });
        handles.push(handle);
    }

    let mut results = Vec::new();
    for handle in handles {
        // TODO: Collect the results of all threads into the `results` vector.
        // Use the `JoinHandle` struct which is returned by `thread::spawn`.
        results.push(handle.join().unwrap());
    }

    if results.len() != 10 {
        panic!("Oh no! Some thread isn't done yet!");
    }

    println!();
    for (i, result) in results.into_iter().enumerate() {
        println!("Thread {i} took {result}ms");
    }
}

线程共享可变状态。在 Rust 中，RefCell 提供的是 单线程下的内部可变性，它不能跨线程安全使用。所以把 RefCell 包在 Arc 里，是 不安全的，会在编译时或者运行时出错。要在多线程中安全地共享和修改数据，需要用 Arc<Mutex>

// Building on the last exercise, we want all of the threads to complete their
// work. But this time, the spawned threads need to be in charge of updating a
// shared value: `JobStatus.jobs_done`

use std::{sync::Mutex, sync::Arc, thread, time::Duration};

struct JobStatus {
    jobs_done: u32,
}

fn main() {
    // TODO: `Arc` isn't enough if you want a **mutable** shared state.
    let status = Arc::new(Mutex::new(JobStatus { jobs_done: 0 }));

    let mut handles = Vec::new();
    for _ in 0..10 {
        let status_shared = Arc::clone(&status);
        let handle = thread::spawn(move || {
            thread::sleep(Duration::from_millis(250));

            // TODO: You must take an action before you update a shared value.
            status_shared.lock().unwrap().jobs_done += 1;
        });
        handles.push(handle);
    }

    // Waiting for all jobs to complete.
    for handle in handles {
        handle.join().unwrap();
    }

    // TODO: Print the value of `JobStatus.jobs_done`.
    println!("Jobs done: {}", status.lock().unwrap().jobs_done);
}

arc1 练习运行结果，输出 Jobs done: 10