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
这与Vec
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 ä (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ä（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
2动态大小的类型允许使用Rc