Unicode string manipulation (str type)

Basic Usage

Rust's string type is one of the core primitive types of the language. While represented by the name str, the name str is not actually a valid type in Rust. Each string must also be decorated with a pointer. String is used for an owned string, so there is only one commonly-used str type in Rust: &str.

&str is the borrowed string type. This type of string can only be created from other strings, unless it is a static string (see below). As the word "borrowed" implies, this type of string is owned elsewhere, and this string cannot be moved out of.

As an example, here's some code that uses a string.

fn main() {
    let borrowed_string = "This string is borrowed with the 'static lifetime";
}

From the example above, you can see that Rust's string literals have the 'static lifetime. This is akin to C's concept of a static string.

String literals are allocated statically in the rodata of the executable/library. The string then has the type &'static str meaning that the string is valid for the 'static lifetime, otherwise known as the lifetime of the entire program. As can be inferred from the type, these static strings are not mutable.

Mutability

Many languages have immutable strings by default, and Rust has a particular flavor on this idea. As with the rest of Rust types, strings are immutable by default. If a string is declared as mut, however, it may be mutated. This works the same way as the rest of Rust's type system in the sense that if there's a mutable reference to a string, there may only be one mutable reference to that string. With these guarantees, strings can easily transition between being mutable/immutable with the same benefits of having mutable strings in other languages.

Representation

Rust's string type, str, is a sequence of unicode codepoints encoded as a stream of UTF-8 bytes. All safely-created strings are guaranteed to be validly encoded UTF-8 sequences. Additionally, strings are not null-terminated and can contain null codepoints.

The actual representation of strings have direct mappings to vectors: &str is the same as &[u8].