TRPL copyedits: strings

Author: steveklabnik

src/doc/trpl/strings.md

@@ -1,36 +1,34 @@
 % Strings
 
-Strings are an important concept for any programmer to master. Rust's string
+Strings are an important concept for any programmer to master. Rust’s string
 handling system is a bit different from other languages, due to its systems
 focus. Any time you have a data structure of variable size, things can get
-tricky, and strings are a re-sizable data structure. That being said, Rust's
+tricky, and strings are a re-sizable data structure. That being said, Rust’s
 strings also work differently than in some other systems languages, such as C.
 
-Let's dig into the details. A *string* is a sequence of Unicode scalar values
-encoded as a stream of UTF-8 bytes. All strings are guaranteed to be
-validly encoded UTF-8 sequences. Additionally, strings are not null-terminated
-and can contain null bytes.
+Let’s dig into the details. A ‘string’ is a sequence of Unicode scalar values
+encoded as a stream of UTF-8 bytes. All strings are guaranteed to be a valid
+encoding of UTF-8 sequences. Additionally, unlike some systems languages,
+strings are not null-terminated and can contain null bytes.
 
-Rust has two main types of strings: `&str` and `String`.
+Rust has two main types of strings: `&str` and `String`. Let’s talk about
+`&str` first. These are called ‘string slices’. String literals are of the type
+`&'static str`:
 
-The first kind is a `&str`. These are called *string slices*. String literals
-are of the type `&str`:
-
-```{rust}
-let string = "Hello there."; // string: &str
+```rust
+let string = "Hello there."; // string: &'static str
 ```
 
-This string is statically allocated, meaning that it's saved inside our
+This string is statically allocated, meaning that it’s saved inside our
 compiled program, and exists for the entire duration it runs. The `string`
 binding is a reference to this statically allocated string. String slices
 have a fixed size, and cannot be mutated.
 
-A `String`, on the other hand, is a heap-allocated string. This string
-is growable, and is also guaranteed to be UTF-8. `String`s are
-commonly created by converting from a string slice using the
-`to_string` method.
+A `String`, on the other hand, is a heap-allocated string. This string is
+growable, and is also guaranteed to be UTF-8. `String`s are commonly created by
+converting from a string slice using the `to_string` method.
 
-```{rust}
+```rust
 let mut s = "Hello".to_string(); // mut s: String
 println!("{}", s);
 
@@ -54,8 +52,78 @@ fn main() {
 Viewing a `String` as a `&str` is cheap, but converting the `&str` to a
 `String` involves allocating memory. No reason to do that unless you have to!
 
-That's the basics of strings in Rust! They're probably a bit more complicated
-than you are used to, if you come from a scripting language, but when the
-low-level details matter, they really matter. Just remember that `String`s
-allocate memory and control their data, while `&str`s are a reference to
-another string, and you'll be all set.
+## Indexing
+
+Because strings are valid UTF-8, strings do not support indexing:
+
+```rust,ignore
+let s = "hello";
+
+println!("The first letter of s is {}", s[0]); // ERROR!!!
+```
+
+Usually, access to a vector with `[]` is very fast. But, because each character
+in a UTF-8 encoded string can be multiple bytes, you have to walk over the
+string to find the nᵗʰ letter of a string. This is a significantly more
+expensive operation, and we don’t want to be misleading. Furthermore, ‘letter’
+isn’t something defined in Unicode, exactly. We can choose to look at a string as
+individual bytes, or as codepoints:
+
+```rust
+let hachiko = "忠犬ハチ公";
+
+for b in hachiko.as_bytes() {
+print!("{}, ", b);
+}
+
+println!("");
+
+for c in hachiko.chars() {
+print!("{}, ", c);
+}
+
+println!("");
+```
+
+This prints:
+
+```text
+229, 191, 160, 231, 138, 172, 227, 131, 143, 227, 131, 129, 229, 133, 172, 
+忠, 犬, ハ, チ, 公, 
+```
+
+As you can see, there are more bytes than `char`s.
+
+You can get something similar to an index like this:
+
+```rust
+# let hachiko = "忠犬ハチ公";
+let dog = hachiko.chars().nth(1); // kinda like hachiko[1]
+```
+
+This emphasizes that we have to go through the whole list of `chars`.
+
+## Concatenation
+
+If you have a `String`, you can concatenate a `&str` to the end of it:
+
+```rust
+let hello = "Hello ".to_string();
+let world = "world!";
+
+let hello_world = hello + world;
+```
+
+But if you have two `String`s, you need an `&`:
+
+```rust
+let hello = "Hello ".to_string();
+let world = "world!".to_string();
+
+let hello_world = hello + &world;
+```
+
+This is because `&String` can automatically coerece to a `&str`. This is a
+feature called ‘[`Deref` coercions][dc]’.
+
+[dc]: deref-coercions.html