Built-in attributes

Source

Initialize a new workspace with cargo init --lib.

#![allow(unused)]
fn main() {
#[derive(Clone)]
#[repr(C)]
pub struct Person {
    name: String,
    age: u32,
}

pub fn attribute_person() -> Person {
    let person = Person {
        name: "Rustacean".to_string(),
        age: 22,
    };
    person
}
}

Attributes are metadata either attached to the containing item (inner attribute) or attached to the following item (outer attribute). There are many types of built-in attributes but from code generation point of view, we only care about the ones directly influencing the generated code, such as derive, repr or inline, where derive generates additional code, repr affects the memory layout and inline is passed as a hint to the LLVM backend.

attribute_person

#![allow(unused)]
fn main() {
#[derive(Clone)]
#[repr(C)]
pub struct Person {
    name: String,
    age: u32,
}

pub fn attribute_person() -> Person {
    let person = Person {
        name: "Rustacean".to_string(),
        age: 22,
    };
    person
}
}

$ cargo expand
...
#[repr(C)]
pub struct Person {
    name: String,
    age: u32,
}
#[automatically_derived]
impl ::core::clone::Clone for Person {
    #[inline]
    fn clone(&self) -> Person {
        Person {
            name: ::core::clone::Clone::clone(&self.name),
            age: ::core::clone::Clone::clone(&self.age),
        }
    }
}
pub fn attribute_person() -> Person {
    let person = Person {
        name: "Rustacean".to_string(),
        age: 22,
    };
    person
}