3. 类型：type

// Meters 是 u32 的类型别名，支持 u32 类型的所有操作。
type Meters = u32;
let x: u32 = 5;
let y: Meters = 5;
println!("x + y = {}", x + y);

// 使用类型别名提升代码可读性。
type Thunk = Box<dyn Fn() + Send + 'static>;
let f: Thunk = Box::new(|| println!("hi"));
fn takes_long_type(f: Thunk) {}
fn returns_long_type() -> Thunk {}

// 类型别名支持泛型参数
type Result<T> = std::result::Result<T, std::io::Error>;

trait Iterator {
    // 声明关联类型时，可以对类型进行限界。
    type Item: std::fmt::Display;

    fn next(&mut self) -> Option<Self::Item>;
}

// 等效于 fn print_message() -> () {
fn print_message() {
    println!("Hello, world!");
}

fn main() {
    // 打印地址相同, 说明是唯一类型值
    println!("{:p}, {:p}", &(), &());

    print_message();

    // 显式使用单元类型和单元值
    let my_unit: () = ();

    // 函数参数接受单元类型值
    take_unit(());

    // 泛型类型也可以使用单元类型
    let result: Result<(), &str> = Ok(());
    match result {
        Ok(_) => println!("Operation was successful."),
        Err(e) => println!("Error occurred: {}", e),
    }
}

fn main() {
    // 浮点取模运算, 截断除法
    let remainder = 43.0 % 5.0;

    let logical: bool = true;
    let a_float: f64 = 1.0;
    let an_integer   = 5i32;

    // 缺省推断：整型推断为 i32，浮点数推断为 f64
    let default_float   = 3.0; // f64
    let default_integer = 7;   // i32

    // 类型推导（从后续的赋值语句推导出类型为 i64）。
    let mut inferred_type = 12;
    inferred_type = 4294967296i64;

    let mut mutable = 12;
    mutable = 21;

    // Error! The type of a variable can't be changed.
    //mutable = true;

    // 变量可以被 shodow（重新定义类型和可变性）
    let mutable = true;
}

fn main() {
    let x: u8 = 255;
    // 使用 wrapping_add 可以防止 panic
    let y: u8 = x.wrapping_add(1);
    println!("y: {}", y);
    // 输出: y: 0
}

fn main() {
    let decimal = 97.123_f32;
    let integer: u8 = decimal as u8;
    let c1: char = decimal as char;
    let c2 = integer as char;

    let integer: u32 = 5;
    let float: f64 = 3.0;
    let int_to_float = integer as f64; // 5.0

    // 浮点数转换为整数，小数部分被截断
    let float_to_int = float as u32; // 3
}

// let val: Box<dyn Any + 'a> = *val as Box<dyn Any + 'a>; // 错误

// OK： circle 可以 usized 协变到 dyn Circle
let circle = Box::new(circle) as Box<dyn Circle>;
let a = *const [u16] as *const [u8]

// From/Into 由标准库 prelude 自动导入，不需要手动导入。
// 其它 Trait 需要显式导入，如 TryInto。
use std::convert::TryInto;

fn main() {
    let decimal = 65.4321_f64;

    // 使用 TryInto trait 的 try_into 方法进行安全转换。
    let integer: u8 = decimal.try_into().unwrap_or_default();

    // 使用 From trait 进行结果检查的安全转换。
    let integer_from = u8::from(42);
    let string_from = String::from("just for test");

    // 字符串的 parse::<T>() 方法可以将字符串解析转换为其它类型值，parse() 方法基于 FromStr trait 实现。
    let i77 = "123".parse::<i32>().unwrap();
    let i77: i32 = "123".parse().unwrap();
}

fn main(){
  // 需注意，下面的数值都加上了类型后缀。
  // 这是因为在调用方法的时候，需要知道值的所属类型才能找到这种类型具有的方法
  println!("{}", 3_u8.pow(2));      // 9
  println!("{}", (-3_i32).abs());   // 3
  // 4，计算45的二进制中有多少个1
  println!("{}", 45i32.count_ones());  // 4
}

fn main() {
    let emoji: char = '😂';
    let chinese_character: char = '中';

    let word = "Rust语言";
    for ch in word.chars() {
        println!("{}", ch);
    }

    // 将字符转换为对应的 Unicode 代码点
    let unicode_codepoint = '🦀' as u32;
    println!("The Unicode code point of '🦀' is: U+{:X}", unicode_codepoint);

    let character_from_codepoint = std::char::from_u32(unicode_codepoint).unwrap_or_default();
    println!("The character from code point U+{:X} is: '{}'", unicode_codepoint, character_from_codepoint);
}

//let s: str = "hello, world"; // 错误，str 不能直接作为类型
let s: &str = "hello, world"; // OK

let hello_world = "Hello, World!";
// 等效于
let hello_world: &'static str = "Hello, world!";

// 使用 &'static str 可以避免为 struct 指定 lifetime 参数
struct Anime {
    name: &'static str,
    bechdel_pass: bool
};

let aria = Anime {
    name: "Aria: The Animation",
    bechdel_pass: true
};

// &str 不能自动协变到 &[u8], 可以使用 as_bytes() 转换为 &[u8]
let bytes = "bors".as_bytes();
assert_eq!(b"bors", bytes);

// 字符串字面量是 &'static str 类型
let story = "Once upon a time...";
let ptr = story.as_ptr(); // 返回指向连续内存区域的 raw pointer：*const u8
let len = story.len(); // byte 数量
assert_eq!(19, len);

// 从 raw pointer 构建 slice
let s = unsafe {
    // 由于 ptr 是 *const u8 类型，所以返回的 slice 是 &[u8] 类型
    let slice = slice::from_raw_parts(ptr, len);
    // 从 &[u8] 创建 &str
    str::from_utf8(slice)
};
assert_eq!(s, Ok(story));

// 使用智能指针保存 str
let boxed: Box<str> = Box::from("hello");
assert_eq!(Cow::from("eggplant"), Cow::Borrowed("eggplant"));
let shared: Rc<str> = Rc::from("statue");

// 从 &str 创建 Vec<u8>
assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);

// 在堆上分配字符串内存，s 拥有该对象，当 s 被 drop 时，堆内存也被释放
let s: Box<str> = "hello, world".into();
// Box<str> 实现了 Deref<Target=str>, 所以 &Box<str> 等效于 &str
greetings(&s);

let method = b"GET";
assert_eq!(method, &[b'G', b'E', b'T']);
// 错误的情况
assert_eq!(method, &['G', 'E', 'T']); // 类型是 &[char] 而非 &[u8]
assert_eq!(method, &[b"G", b"E", b"T"]); // 类型是 &[&[u8]] 而非 &[u8]

let s1 = String::from("hello,");
println!("#{:20.20}#", s1); // 字符串显示默认左对齐(数字是右对齐),显示: #hello,              #

println!("{}", "a\t
      b  \
       c d
      ef
      ");

let s = String::from("hello world");
let hello = &s[0..5]; // &str 类型
println!("{}", hello);

let s = "hello";
// println!("The first letter of s is {}", s[0]); // 错误，&str 不支持 s[i];

// 使用 as_bytes() 方法将 String/&str 转换为 &[u8], index 操作返回 u8:
let s = "hello";
assert_eq!(s.as_bytes()[0], 104);
assert_eq!(s.as_bytes()[0], b'h');

let s = "💖💖💖💖💖";
assert_eq!(s.as_bytes()[0], 240);

use std::ffi::OsStr;
let os_str = OsStr::new("foo");

// OsStr 的方法:
pub fn as_encoded_bytes(&self) -> &[u8]
pub fn into_os_string(self: Box<OsStr>) -> OsString // 消耗自身
pub fn make_ascii_lowercase(&mut self)

pub fn to_os_string(&self) -> OsString
pub fn to_str(&self) -> Option<&str>
pub fn to_string_lossy(&self) -> Cow<'_, str>

use std::ffi::CString;
use std::ffi::c_char;

fn main() {
    let s = String::from("Hello, world!");
    let cs = CString::new(s).unwrap();

    let p = cs.as_ptr() as *const c_char;
    println!("Address: {:?}", p);
}

// 返回字符串的 bytes（而非字符）数量
pub const fn len(&self) -> usize
    let len = "foo".len();
    assert_eq!(3, len); // 字节长度
    assert_eq!("ƒoo".chars().count(), 3); // 字符数量

pub const fn is_empty(&self) -> bool

pub fn is_char_boundary(&self, index: usize) -> bool
// Finds the closest x not exceeding index where is_char_boundary(x) is true.
pub fn floor_char_boundary(&self, index: usize) -> usize
pub fn ceil_char_boundary(&self, index: usize) -> usize
    #![feature(round_char_boundary)]
    let s = "❤️🧡💛💚💙💜";
    assert_eq!(s.len(), 26);
    assert!(!s.is_char_boundary(13));
    let closest = s.floor_char_boundary(13);
    assert_eq!(closest, 10);
    assert_eq!(&s[..closest], "❤️🧡");

// 转换为 slice
pub const fn as_bytes(&self) -> &[u8]
pub unsafe fn as_bytes_mut(&mut self) -> &mut [u8]
    let bytes = "bors".as_bytes();
    assert_eq!(b"bors", bytes);
    let mut s = String::from("Hello");
    let bytes = unsafe { s.as_bytes_mut() };
    assert_eq!(b"Hello", bytes);

// 转换为 raw pointer
pub const fn as_ptr(&self) -> *const u8
pub fn as_mut_ptr(&mut self) -> *mut u8
    let s = "Hello";
    let ptr = s.as_ptr(); // *const u8

// 安全返回一个子字符串 &str（不 panic），如果不在字符串边界则返回 None
pub fn get<I>(&self, i: I) -> Option<&<I as SliceIndex<str>>::Output> where I: SliceIndex<str>
pub fn get_mut<I>( &mut self, i: I) -> Option<&mut <I as SliceIndex<str>>::Output> where I: SliceIndex<str>
    let v = String::from("🗻∈🌏");
    assert_eq!(Some("🗻"), v.get(0..4));
    assert!(v.get(1..).is_none());
    assert!(v.get(..8).is_none());
    assert!(v.get(..42).is_none());

// 返回一个子字符串 &str，调用者确保传入的 index 范围是有效的。
pub unsafe fn get_unchecked<I>(&self, i: I) -> &<I as SliceIndex<str>>::Output where I: SliceIndex<str>
pub unsafe fn get_unchecked_mut<I>( &mut self, i: I ) -> &mut <I as SliceIndex<str>>::Output where I: SliceIndex<str>

// 分割字符串
pub fn split_at(&self, mid: usize) -> (&str, &str)
pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str)
pub fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)>
pub fn split_at_mut_checked( &mut self, mid: usize ) -> Option<(&mut str, &mut str)>

// 返回字符串的 char 或 byte 迭代器
pub fn chars(&self) -> Chars<'_>
pub fn char_indices(&self) -> CharIndices<'_>
pub fn bytes(&self) -> Bytes<'_>

// 返回空白字符分割的子字符串迭代器, 多个连续空白字符视为一个
pub fn split_whitespace(&self) -> SplitWhitespace<'_>
pub fn split_ascii_whitespace(&self) -> SplitAsciiWhitespace<'_>
    let mut iter = " Mary   had\ta\u{2009}little  \n\t lamb".split_whitespace();
    assert_eq!(Some("Mary"), iter.next());
    assert_eq!(Some("had"), iter.next());
    assert_eq!(Some("a"), iter.next());
    assert_eq!(Some("little"), iter.next());
    assert_eq!(Some("lamb"), iter.next());
    assert_eq!(None, iter.next());
    assert_eq!("".split_whitespace().next(), None);
    assert_eq!("   ".split_whitespace().next(), None);

// 返回行迭代器，如果是空行则返回空字符串，不包括行尾的换行
pub fn lines(&self) -> Lines<'_>
pub fn lines_any(&self) -> LinesAny<'_>
    let text = "foo\nbar\n\r\nbaz";
    // 返回的迭代器对象必须是 mut 类型，因为迭代时会修改它的内部状态
    let mut lines = text.lines();
    assert_eq!(Some("foo"), lines.next());
    assert_eq!(Some("bar"), lines.next());
    assert_eq!(Some(""), lines.next());
    assert_eq!(Some("baz"), lines.next());
    assert_eq!(None, lines.next());

pub fn encode_utf16(&self) -> EncodeUtf16<'_>

// 是否包含 pattern
pub fn contains<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>
// 是否以 pattern 开始或结束
pub fn starts_with<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>
pub fn ends_with<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>

// find 返回匹配 pattern 的 index，如果为找到则返回 None
pub fn find<'a, P>(&'a self, pat: P) -> Option<usize> where P: Pattern<'a>
pub fn rfind<'a, P>(&'a self, pat: P) -> Option<usize> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>

// 拆分字符串为可迭代的子串 &str
pub fn split<'a, P>(&'a self, pat: P) -> Split<'a, P> where P: Pattern<'a>
pub fn split_inclusive<'a, P>(&'a self, pat: P) -> SplitInclusive<'a, P> where P: Pattern<'a>
pub fn split_terminator<'a, P>(&'a self, pat: P) -> SplitTerminator<'a, P> where P: Pattern<'a>
pub fn splitn<'a, P>(&'a self, n: usize, pat: P) -> SplitN<'a, P> where    P: Pattern<'a>
pub fn split_once<'a, P>(&'a self, delimiter: P) -> Option<(&'a str, &'a str)> where P: Pattern<'a>
pub fn rsplit<'a, P>(&'a self, pat: P) -> RSplit<'a, P> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
pub fn rsplit_terminator<'a, P>(&'a self, pat: P) -> RSplitTerminator<'a, P> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
pub fn rsplitn<'a, P>(&'a self, n: usize, pat: P) -> RSplitN<'a, P> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
pub fn rsplit_once<'a, P>(&'a self, delimiter: P) -> Option<(&'a str, &'a str)> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
    let v: Vec<&str> = "Mary had a little lamb".split(' ').collect();
    assert_eq!(v, ["Mary", "had", "a", "little", "lamb"]);
    let v: Vec<&str> = "".split('X').collect();
    assert_eq!(v, [""]);
    let v: Vec<&str> = "lionXXtigerXleopard".split('X').collect();
    assert_eq!(v, ["lion", "", "tiger", "leopard"]);

// 返回匹配 pattern 的子字符串迭代器
pub fn matches<'a, P>(&'a self, pat: P) -> Matches<'a, P> where P: Pattern<'a>
pub fn match_indices<'a, P>(&'a self, pat: P) -> MatchIndices<'a, P> where P: Pattern<'a>
pub fn rmatches<'a, P>(&'a self, pat: P) -> RMatches<'a, P> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
pub fn rmatch_indices<'a, P>(&'a self, pat: P) -> RMatchIndices<'a, P> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>
    let v: Vec<&str> = "abcXXXabcYYYabc".matches("abc").collect();
    assert_eq!(v, ["abc", "abc", "abc"]);
    let v: Vec<&str> = "1abc2abc3".matches(char::is_numeric).collect();
    assert_eq!(v, ["1", "2", "3"]);

// 删除（执行多次）start/end 两端的空格或两端匹配的 pattern
pub fn trim(&self) -> &str
pub fn trim_start(&self) -> &str
pub fn trim_end(&self) -> &str
pub fn trim_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>
pub fn trim_start_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>
pub fn trim_end_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>

// 删除开头的前缀，最多删除一次
pub fn strip_prefix<'a, P>(&'a self, prefix: P) -> Option<&'a str> where P: Pattern<'a>
pub fn strip_suffix<'a, P>(&'a self, suffix: P) -> Option<&'a str> where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>

// 将字符串转换为其他类型 F，F 类型需要实现 FromStr trait。
// Rust 的基本类型都实现了该 trait。
pub fn parse<F>(&self) -> Result<F, <F as FromStr>::Err> where F: FromStr

pub const fn is_ascii(&self) -> bool
pub const fn as_ascii(&self) -> Option<&[AsciiChar]>
pub fn eq_ignore_ascii_case(&self, other: &str) -> bool
pub fn make_ascii_uppercase(&mut self)
pub fn make_ascii_lowercase(&mut self)
pub const fn trim_ascii_start(&self) -> &str
pub const fn trim_ascii_end(&self) -> &str
pub const fn trim_ascii(&self) -> &str
pub fn escape_debug(&self) -> EscapeDebug<'_>
pub fn escape_default(&self) -> EscapeDefault<'_>
pub fn escape_unicode(&self) -> EscapeUnicode<'_>

pub fn into_boxed_bytes(self: Box<str>) -> Box<[u8]>
pub fn into_string(self: Box<str>) -> String // &str 转为 String

pub fn replace<'a, P>(&'a self, from: P, to: &str) -> String where P: Pattern<'a>
pub fn replacen<'a, P>(&'a self, pat: P, to: &str, count: usize) -> String where P: Pattern<'a>

pub fn repeat(&self, n: usize) -> String
pub fn to_lowercase(&self) -> String
pub fn to_uppercase(&self) -> String
pub fn to_ascii_uppercase(&self) -> String
pub fn to_ascii_lowercase(&self) -> String

// &str
assert_eq!("abaaa".find("ba"), Some(1));
assert_eq!("abaaa".find("bac"), None);

// char
assert_eq!("abaaa".find('a'), Some(0));
assert_eq!("abaaa".find('b'), Some(1));
assert_eq!("abaaa".find('c'), None);

// &[char; N]，不是 &[u8; N]
assert_eq!("ab".find(&['b', 'a']), Some(0));
assert_eq!("abaaa".find(&['a', 'z']), Some(0));
assert_eq!("abaaa".find(&['c', 'd']), None);

// &[char]
assert_eq!("ab".find(&['b', 'a'][..]), Some(0));
assert_eq!("abaaa".find(&['a', 'z'][..]), Some(0));
assert_eq!("abaaa".find(&['c', 'd'][..]), None);

// FnMut(char) -> bool
assert_eq!("abcdef_z".find(|ch| ch > 'd' && ch < 'y'), Some(4));
assert_eq!("abcddd_z".find(|ch| ch > 'd' && ch < 'y'), None);

pub trait FromStr: Sized {
    type Err;

    // Required method
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

pub fn parse<F>(&self) -> Result<F, <F as FromStr>::Err> where F: FromStr

let four: u32 = "4".parse().unwrap();
assert_eq!(4, four);

let four = "4".parse::<u32>();
assert_eq!(Ok(4), four);

// Error
let nope = "j".parse::<u32>();
assert!(nope.is_err());

let error_message = "too many pets".to_string();
assert_eq!(format!("{}°{:02}′{:02}′′N", 24, 5, 23), "24°05′23′′N".to_string());

let bits = vec!["veni", "vidi", "vici"];
assert_eq!(bits.concat(), "venividivici");
assert_eq!(bits.join(", "), "veni, vidi, vici");

let hello = String::from("Hello, world!");
let mut hello = String::from("Hello, ");
hello.push('w');
hello.push_str("orld!");

let sparkle_heart = vec![240, 159, 146, 150];
let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
assert_eq!("💖", sparkle_heart);

let s = "hello";
let third_character = s.chars().nth(2);
assert_eq!(third_character, Some('l'));

let noodles = "noodles".to_string();
let oodles = &noodles[1..]; // String/str 的 slice 操作返回 &str

let mut ss = String::from("abcd");
ss += " def"; // OK: String + &str

// 错误：`+` cannot be used to concatenate a `&str` with a `String`
// " def" + ss;

" def".to_owned() + &ss;   // OK

let s1 = String::from("hello,");
let s2 = String::from("world!");
let s3 = s1 + &s2;   // let s3 = s1.clone() +&s2;
assert_eq!(s3, "hello,world!");
//println!("{}", s1); // s1 已经在上面的 + 操作被 move, 导致继续使用 s1 出错。

let story = String::from("Once upon a time...");

// Prevent automatically dropping the String's data
let mut story = std::mem::ManuallyDrop::new(story);

let ptr = story.as_mut_ptr();
let len = story.len();
let capacity = story.capacity();
assert_eq!(19, len);

let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;
assert_eq!(String::from("Once upon a time..."), s);

// 空 String
pub const fn new() -> String
// 指定初始容量的空 String
pub fn with_capacity(capacity: usize) -> String

// 从 Vec<u8> 创建 String
pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>
pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String
    let sparkle_heart = vec![240, 159, 146, 150];
    let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
    assert_eq!("💖", sparkle_heart);

// 从 &[u8] 创建 String
pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str>
pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error>
pub fn from_utf16_lossy(v: &[u16]) -> String
pub fn from_utf16le(v: &[u8]) -> Result<String, FromUtf16Error>
pub fn from_utf16le_lossy(v: &[u8]) -> String
pub fn from_utf16be(v: &[u8]) -> Result<String, FromUtf16Error>
pub fn from_utf16be_lossy(v: &[u8]) -> String

// raw pointer 互操作
pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize ) -> String
pub fn into_raw_parts(self) -> (*mut u8, usize, usize)

// 转换为 Vec<u8>, &[u8], &str
pub fn into_bytes(self) -> Vec<u8>  // 消耗自身
pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8>
pub fn into_boxed_str(self) -> Box<str>
pub fn as_bytes(&self) -> &[u8]
pub fn as_str(&self) -> &str
pub fn as_mut_str(&mut self) -> &mut str

// 添加或指定位置插入 char 或 &str，自动调整容量。
pub fn push(&mut self, ch: char)
pub fn push_str(&mut self, string: &str)
pub fn extend_from_within<R>(&mut self, src: R) where R: RangeBounds<usize>
pub fn insert(&mut self, idx: usize, ch: char)
pub fn insert_str(&mut self, idx: usize, string: &str)

// 返回容量和长度
pub fn capacity(&self) -> usize
pub fn len(&self) -> usize
pub fn is_empty(&self) -> bool

// 修改长度
pub fn reserve(&mut self, additional: usize)
pub fn reserve_exact(&mut self, additional: usize)
pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>
pub fn try_reserve_exact( &mut self,    additional: usize) -> Result<(), TryReserveError>
pub fn shrink_to_fit(&mut self)
pub fn shrink_to(&mut self, min_capacity: usize)
pub fn truncate(&mut self, new_len: usize)
pub fn clear(&mut self)

// 删除 char
pub fn pop(&mut self) -> Option<char> // 为空时返回 None
pub fn remove(&mut self, idx: usize) -> char
pub fn remove_matches<P, 'a>(&'a mut self, pat: P) where P: for<'x> Pattern<'x>

// 保留 f 返回 true 的字符
pub fn retain<F>(&mut self, f: F) where F: FnMut(char) -> bool
    let mut s = String::from("f_o_ob_ar");
    s.retain(|c| c != '_');
    assert_eq!(s, "foobar");

pub fn split_off(&mut self, at: usize) -> String

// 删除指定范围的字符，返回删除字符串的迭代器
pub fn drain<R>(&mut self, range: R) -> Drain<'_> where R: RangeBounds<usize>
    let mut s = String::from("α is alpha, β is beta");
    let beta_offset = s.find('β').unwrap_or(s.len());
    let t: String = s.drain(..beta_offset).collect();
    assert_eq!(t, "α is alpha, ");
    assert_eq!(s, "β is beta");
    s.drain(..);
    assert_eq!(s, "");

pub fn replace_range<R>(&mut self, range: R, replace_with: &str) where R: RangeBounds<usize>
pub fn leak<'a>(self) -> &'a mut str

impl [u8]

// 检查 [u8] 各元素是否是 ascii
pub const fn is_ascii(&self) -> bool
pub const fn as_ascii(&self) -> Option<&[AsciiChar]>
pub const unsafe fn as_ascii_unchecked(&self) -> &[AsciiChar]
pub fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool
pub fn make_ascii_uppercase(&mut self)
pub fn make_ascii_lowercase(&mut self)
pub fn escape_ascii(&self) -> EscapeAscii<'_>
    let s = b"0\t\r\n'\"\\\x9d";
    let escaped = s.escape_ascii().to_string();
    assert_eq!(escaped, "0\\t\\r\\n\\'\\\"\\\\\\x9d");

pub const fn trim_ascii_start(&self) -> &[u8]
pub const fn trim_ascii_end(&self) -> &[u8]
pub const fn trim_ascii(&self) -> &[u8]
    #![feature(byte_slice_trim_ascii)]
    assert_eq!(b" \t hello world\n".trim_ascii_start(), b"hello world\n");
    assert_eq!(b"  ".trim_ascii_start(), b"");
    assert_eq!(b"".trim_ascii_start(), b"");