Editing Immutable object (section)

=== Rust ===
Rust's [[Rust (programming language)#Ownership|ownership]] system allows developers to declare immutable variables, and pass immutable references. By default, all variables and references are immutable. Mutable variables and references are explicitly created with the <code>mut</code> keyword.

[https://doc.rust-lang.org/reference/items/constant-items.html Constant items] in Rust are always immutable. 

<syntaxhighlight lang="rust">
// constant items are always immutable
const ALWAYS_IMMUTABLE: bool = true;

struct Object {
    x: usize,
    y: usize,
}

fn main() {
    // explicitly declare a mutable variable
    let mut mutable_obj = Object { x: 1, y: 2 };
    mutable_obj.x = 3; // okay

    let mutable_ref = &mut mutable_obj;
    mutable_ref.x = 1; // okay

    let immutable_ref = &mutable_obj;
    immutable_ref.x = 3; // error E0594

    // by default, variables are immutable
    let immutable_obj = Object { x: 4, y: 5 };
    immutable_obj.x = 6; // error E0596

    let mutable_ref2 = 
        &mut immutable_obj; // error E0596

    let immutable_ref2 = &immutable_obj;
    immutable_ref2.x = 6; // error E0594
    
}
</syntaxhighlight>