The Rust Programming Language

We talked about strings in Chapter 4, but we’ll look at them in more depth now. New Rustaceans commonly get stuck on strings due to a combination of three concepts: Rust’s propensity for exposing possible errors, strings being a more complicated data structure than many programmers give them credit for, and UTF-8. These concepts combine in a way that can seem difficult when you’re coming from other programming languages.

This discussion of strings is in the collections chapter because strings are implemented as a collection of bytes plus some methods to provide useful functionality when those bytes are interpreted as text. In this section, we’ll talk about the operations on String that every collection type has, such as creating, updating, and reading. We’ll also discuss the ways in which String is different than the other collections, namely how indexing into a String is complicated by the differences between how people and computers interpret String data.

We’ll first define what we mean by the term string. Rust has only one string type in the core language, which is the string slice str that is usually seen in its borrowed form &str. In Chapter 4, we talked about string slices, which are references to some UTF-8 encoded string data stored elsewhere. String literals, for example, are stored in the binary output of the program and are therefore string slices.

The String type is provided in Rust’s standard library rather than coded into the core language and is a growable, mutable, owned, UTF-8 encoded string type. When Rustaceans refer to “strings” in Rust, they usually mean the String and the string slice &str types, not just one of those types. Although this section is largely about String, both types are used heavily in Rust’s standard library and both String and string slices are UTF-8 encoded.

Rust’s standard library also includes a number of other string types, such as OsString, OsStr, CString, and CStr. Library crates can provide even more options for storing string data. Similar to the *String/*Str naming, they often provide an owned and borrowed variant, just like String/&str. These string types can store text in different encodings or be represented in memory in a different way, for example. We won’t discuss these other string types in this chapter; see their API documentation for more about how to use them and when each is appropriate.

Many of the same operations available with Vec<T> are available with String as well, starting with the new function to create a string, shown in Listing 8-11:

# #![allow(unused_variables)]
#fn main() {
let mut s = String::new();
#}

Listing 8-11: Creating a new, empty String

This line creates a new empty string called s that we can then load data into. Often, we’ll have some initial data that we want to start the string with. For that, we use the to_string method, which is available on any type that implements the Display trait, which string literals do. Listing 8-12 shows two examples:

# #![allow(unused_variables)]
#fn main() {
let data = "initial contents";

let s = data.to_string();

// the method also works on a literal directly:
let s = "initial contents".to_string();
#}

Listing 8-12: Using the to_string method to create a String from a string literal

This code creates a string containing initial contents.

We can also use the function String::from to create a String from a string literal. The code in Listing 8-13 is equivalent to the code from Listing 8-12 that uses to_string:

# #![allow(unused_variables)]
#fn main() {
let s = String::from("initial contents");
#}

Listing 8-13: Using the String::from function to create a String from a string literal

Because strings are used for so many things, we can use many different generic APIs for strings, providing us with a lot of options. Some of them can seem redundant, but they all have their place! In this case, String::from and to_string do the same thing, so which you choose is a matter of style.

Remember that strings are UTF-8 encoded, so we can include any properly encoded data in them, as shown in Listing 8-14:

# #![allow(unused_variables)]
#fn main() {
let hello = String::from("السلام عليكم");
let hello = String::from("Dobrý den");
let hello = String::from("Hello");
let hello = String::from("שָׁלוֹם");
let hello = String::from("नमस्ते");
let hello = String::from("こんにちは");
let hello = String::from("안녕하세요");
let hello = String::from("你好");
let hello = String::from("Olá");
let hello = String::from("Здравствуйте");
let hello = String::from("Hola");
#}

Listing 8-14: Storing greetings in different languages in strings

All of these are valid String values.

A String can grow in size and its contents can change, just like the contents of a Vec<T>, by pushing more data into it. In addition, we can conveniently use the + operator or the format! macro to concatenate String values together.

We can grow a String by using the push_str method to append a string slice, as shown in Listing 8-15:

# #![allow(unused_variables)]
#fn main() {
let mut s = String::from("foo");
s.push_str("bar");
#}

Listing 8-15: Appending a string slice to a String using the push_str method

After these two lines, s will contain foobar. The push_str method takes a string slice because we don’t necessarily want to take ownership of the parameter. For example, the code in Listing 8-16 shows that it would be unfortunate if we weren’t able to use s2 after appending its contents to s1:

# #![allow(unused_variables)]
#fn main() {
let mut s1 = String::from("foo");
let s2 = "bar";
s1.push_str(&s2);
println!("s2 is {}", s2);
#}

Listing 8-16: Using a string slice after appending its contents to a String

If the push_str method took ownership of s2, we wouldn’t be able to print out its value on the last line. However, this code works as we’d expect!

The push method takes a single character as a parameter and adds it to the String. Listing 8-17 shows code that adds the letter l character to a String using the push method:

# #![allow(unused_variables)]
#fn main() {
let mut s = String::from("lo");
s.push('l');
#}

Listing 8-17: Adding one character to a String value using push

As a result of this code, s will contain lol.

Often, we’ll want to combine two existing strings. One way is to use the + operator, as shown in Listing 8-18:

# #![allow(unused_variables)]
#fn main() {
let s1 = String::from("Hello, ");
let s2 = String::from("world!");
let s3 = s1 + &s2; // Note that s1 has been moved here and can no longer be used
#}

Listing 8-18: Using the + operator to combine two String values into a new String value

The string s3 will contain Hello, world! as a result of this code. The reason s1 is no longer valid after the addition and the reason we used a reference to s2 has to do with the signature of the method that gets called when we use the + operator. The + operator uses the add method, whose signature looks something like this:

fn add(self, s: &str) -> String {

This isn’t the exact signature that’s in the standard library: in the standard library, add is defined using generics. Here, we’re looking at the signature of add with concrete types substituted for the generic ones, which is what happens when we call this method with String values. We’ll discuss generics in Chapter 10. This signature gives us the clues we need to understand the tricky bits of the + operator.

First, s2 has an &, meaning that we’re adding a reference of the second string to the first string because of the s parameter in the add function: we can only add a &str to a String; we can’t add two String values together. But wait - the type of &s2 is &String, not &str, as specified in the second parameter to add. So why does Listing 8-18 compile?

The reason we’re able to use &s2 in the call to add is that the compiler can coerce the &String argument into a &str. When we call the add method, Rust uses a deref coercion, which here turns &s2 into &s2[..]. We’ll discuss deref coercion in more depth in Chapter 15. Because add does not take ownership of the s parameter, s2 will still be a valid String after this operation.

Second, we can see in the signature that add takes ownership of self, because self does not have an &. This means s1 in Listing 8-18 will be moved into the add call and no longer be valid after that. So although let s3 = s1 + &s2; looks like it will copy both strings and create a new one, this statement actually takes ownership of s1, appends a copy of the contents of s2, and then returns ownership of the result. In other words, it looks like it’s making a lot of copies but isn’t: the implementation is more efficient than copying.

If we need to concatenate multiple strings, the behavior of + gets unwieldy:

# #![allow(unused_variables)]
#fn main() {
let s1 = String::from("tic");
let s2 = String::from("tac");
let s3 = String::from("toe");

let s = s1 + "-" + &s2 + "-" + &s3;
#}

At this point, s will be tic-tac-toe. With all of the + and " characters, it’s difficult to see what’s going on. For more complicated string combining, we can use the format! macro:

# #![allow(unused_variables)]
#fn main() {
let s1 = String::from("tic");
let s2 = String::from("tac");
let s3 = String::from("toe");

let s = format!("{}-{}-{}", s1, s2, s3);
#}

This code also sets s to tic-tac-toe. The format! macro works in the same way as println!, but instead of printing the output to the screen, it returns a String with the contents. The version of the code using format! is much easier to read and also doesn’t take ownership of any of its parameters.

In many other programming languages, accessing individual characters in a string by referencing them by index is a valid and common operation. However, if we try to access parts of a String using indexing syntax in Rust, we’ll get an error. Consider the invalid code in Listing 8-19:

let s1 = String::from("hello");
let h = s1[0];

Listing 8-19: Attempting to use indexing syntax with a String

This code will result in the following error:

error[E0277]: the trait bound `std::string::String: std::ops::Index<{integer}>` is not satisfied
 -->
  |
3 |     let h = s1[0];
  |             ^^^^^ the type `std::string::String` cannot be indexed by `{integer}`
  |
  = help: the trait `std::ops::Index<{integer}>` is not implemented for `std::string::String`

The error and the note tell the story: Rust strings don’t support indexing. But why not? To answer that question, we need to discuss how Rust stores strings in memory.

A String is a wrapper over a Vec<u8>. Let’s look at some of our properly encoded UTF-8 example strings from Listing 8-14. First, this one:

# #![allow(unused_variables)]
#fn main() {
let len = String::from("Hola").len();
#}

In this case, len will be four, which means the Vec storing the string “Hola” is four bytes long. Each of these letters takes one byte when encoded in UTF-8. But what about the following line?

# #![allow(unused_variables)]
#fn main() {
let len = String::from("Здравствуйте").len();
#}

Note that this string begins with the capital Cyrillic letter Ze, not the Arabic number 3. Asked how long the string is, you might say 12. However, Rust’s answer is 24: that’s the number of bytes it takes to encode “Здравствуйте” in UTF-8, because each Unicode scalar value takes two bytes of storage. Therefore, an index into the string’s bytes will not always correlate to a valid Unicode scalar value. To demonstrate, consider this invalid Rust code:

let hello = "Здравствуйте";
let answer = &hello[0];

What should the value of answer be? Should it be З, the first letter? When encoded in UTF-8, the first byte of З is 208, and the second is 151, so answer should in fact be 208, but 208 is not a valid character on its own. Returning 208 is likely not what a user would want if they asked for the first letter of this string; however, that’s the only data that Rust has at byte index 0. Returning the byte value is probably not what users want, even if the string contains only Latin letters: if &"hello"[0] was valid code that returned the byte value, it would return 104, not h. To avoid returning an unexpected value and causing bugs that might not be discovered immediately, Rust doesn’t compile this code at all and prevents misunderstandings earlier in the development process.

Another point about UTF-8 is that there are actually three relevant ways to look at strings from Rust’s perspective: as bytes, scalar values, and grapheme clusters (the closest thing to what we would call letters).

If we look at the Hindi word “नमस्ते” written in the Devanagari script, it is ultimately stored as a Vec of u8 values that looks like this:

[224, 164, 168, 224, 164, 174, 224, 164, 184, 224, 165, 141, 224, 164, 164,
224, 165, 135]

That’s 18 bytes and is how computers ultimately store this data. If we look at them as Unicode scalar values, which are what Rust’s char type is, those bytes look like this:

['न', 'म', 'स', '्', 'त', 'े']

There are six char values here, but the fourth and sixth are not letters: they’re diacritics that don’t make sense on their own. Finally, if we look at them as grapheme clusters, we’d get what a person would call the four letters that make up the Hindi word:

["न", "म", "स्", "ते"]

Rust provides different ways of interpreting the raw string data that computers store so that each program can choose the interpretation it needs, no matter what human language the data is in.

A final reason Rust doesn’t allow us to index into a String to get a character is that indexing operations are expected to always take constant time (O(1)). But it isn’t possible to guarantee that performance with a String, because Rust would have to walk through the contents from the beginning to the index to determine how many valid characters there were.

Indexing into a string is often a bad idea because it’s not clear what the return type of the string indexing operation should be: a byte value, a character, a grapheme cluster, or a string slice. Therefore, Rust asks you to be more specific if you really need to use indices to create string slices. To be more specific in your indexing and indicate that you want a string slice, rather than indexing using [] with a single number, you can use [] with a range to create a string slice containing particular bytes:

# #![allow(unused_variables)]
#fn main() {
let hello = "Здравствуйте";

let s = &hello[0..4];
#}

Here, s will be a &str that contains the first four bytes of the string. Earlier, we mentioned that each of these characters was two bytes, which means s will be Зд.

What would happen if we used &hello[0..1]? The answer: Rust will panic at runtime in the same way that accessing an invalid index in a vector does:

thread 'main' panicked at 'byte index 1 is not a char boundary; it is inside 'З' (bytes 0..2) of `Здравствуйте`', src/libcore/str/mod.rs:2188:4

You should use ranges to create string slices with caution, because it can crash your program.

Fortunately, we can access elements in a string in other ways.

If we need to perform operations on individual Unicode scalar values, the best way to do so is to use the chars method. Calling chars on “नमस्ते” separates out and returns six values of type char, and we can iterate over the result in order to access each element:

# #![allow(unused_variables)]
#fn main() {
for c in "नमस्ते".chars() {
    println!("{}", c);
}
#}

This code will print the following:

न
म
स
्
त
े

The bytes method returns each raw byte, which might be appropriate for your domain:

# #![allow(unused_variables)]
#fn main() {
for b in "नमस्ते".bytes() {
    println!("{}", b);
}
#}

This code will print the 18 bytes that make up this String, starting with:

224
164
168
224
// ... etc

But be sure to remember that valid Unicode scalar values may be made up of more than one byte.

Getting grapheme clusters from strings is complex, so this functionality is not provided by the standard library. Crates are available on crates.io if this is the functionality you need.

To summarize, strings are complicated. Different programming languages make different choices about how to present this complexity to the programmer. Rust has chosen to make the correct handling of String data the default behavior for all Rust programs, which means programmers have to put more thought into handling UTF-8 data upfront. This trade-off exposes more of the complexity of strings than other programming languages do but prevents you from having to handle errors involving non-ASCII characters later in your development life cycle.

Let’s switch to something a bit less complex: hash maps!