A Rust Struct Groups Named Fields into One Type

A struct defines a type whose fields have names and types. An instance is a value of that type, with a value supplied for every field (I’ll explain with an example, this statement might have confused you a little). We access a field with a dot followed by its name in source.

struct Book {
    title: String,
    pages: u32,
}
 
fn main() {
    let book = Book {
        title: String::from("The Hobbit"),
        pages: 310,
    };
 
    println!("{} has {} pages", book.title, book.pages);
}

So here the struct Book defines the type, Book { ... } creates one instance, this is very similar to other languages like C and Go where they have structs. The field names tell us what the two values represent. book.pages reads the page count without requiring us to remember a tuple position

Try removing pages: 310 from the struct literal and run cargo check. Rust tells you that the instance is missing a field. Put it back, then give pages a string instead of a u32. The compiler reports the type mismatch. A struct definition fixes the names and types that each instance must provide.

We already learned that Rust bindings are immutable by default. That rule also applies here: write let mut book if you later need to assign to book.pages. There is no separate mut marker on an individual field in this struct definition in source.