What are the differences between Rust's `String` and `str`?
Rust中String和str有什么不同？
String?is the dynamic heap string type, like?Vec: use it when you need to own or modify your string data.
String 是一种动态堆字符串类型，像Vec类型一样，当你需要所有权或者修改你的字符串数据时使用它。
str?is an immutable1?sequence of UTF-8 bytes of dynamic length somewhere in memory. Since the size is unknown, one can only handle it behind a pointer. This means that?str?most commonly2?appears as?&str: a reference to some UTF-8 data, normally called a "string slice" or just a "slice".?A slice?is just a view onto some data, and that data can be anywhere, e.g.
str是一个不可变的UTF-8字节动态长度的序列，位于内存的某处。因为大小未知，所以只能使用一个指针来处理它。这意味着str通常以&str方式出现，一个UTF-8数据的引用，通常叫做string slice 或者 slice。一个slice是一些数据的视图，这些数据能够在任何地方。比如：
In static storage: a string literal?"foo"?is a?&'static str. The data is hardcoded into the executable and loaded into memory when the program runs.
静态存储中，一个字符串foo是一个&'static str类型。这个数据被硬编码到可执行文件中，并且当程序运行时加载进内存。
Inside a heap allocated?String:?String?dereferences to a?&str?view?of the?String's data.
在堆中分配的String，String解引用为一个&str 指向String数据的视图。
On the stack: e.g. the following creates a stack-allocated byte array, and then gets a?view of that data as a?&str:
在栈上，如下创建一个栈上分配的字节数组，然后使用&str获取这个数据的视图。
use std::str;
let x: &[u8] = &[b'a', b'b', b'c'];
let stack_str: &str = str::from_utf8(x).unwrap();
In summary, use?String?if you need owned string data (like passing strings to other threads, or building them at runtime), and use?&str?if you only need a view of a string。
综上，如果你需要获得字符串数据所有权，比如传递字符串给其他线程或者是在运行时构建它们，那么使用String，如果你仅仅需要一个字符串的视图，那么使用&str.
This is identical to the relationship between a vector?Vec<T>?and a slice?&[T], and is similar to the relationship between by-value?T?and by-reference?&T?for general types.
这与Vec<T>和&[T]切片的关系是相同的，与泛型中的通过值T和通过引用&T的关系是类似的。
1?A?str?is fixed-length; you cannot write bytes beyond the end, or leave trailing invalid bytes. Since UTF-8 is a variable-width encoding, this effectively forces all?strs to be immutable in many cases. In general, mutation requires writing more or fewer bytes than there were before (e.g. replacing an?a?(1 byte) with an???(2+ bytes) would require making more room in the?str). There are specific methods that can modify a?&mut str?in place, mostly those that handle only ASCII characters, like?make_ascii_uppercase.
1 str是固定长度，你不能再最后写入字节，或者去掉一些尾部的无效字节。UTF-8是一种变宽编码，在很多情况下这有效的强制了所有str为不可变。通常，可变要求比之前写入或多或少字节（比如，用?（两个字节）替换a（一个字节），要求在str中创建更多空间）。有一种特定的方法，在适当的位置，能够修改为 &mut str，这些大多数是只能处理assii编码字符，比如make_ascii_uppercase。
2?Dynamically sized types?allow things like?Rc<str>?for a sequence of reference counted UTF-8 bytes since Rust 1.2. Rust 1.21 allows easily creating these types.
2动态大小的类型允许使用Rc<str>引用序列计数这些UTF-8字节，在Rust1.2Rust1.2.1很容易创建这些类型。
