Rust by Example

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Mutability

Variable bindings are immutable by default, but this can be overridden using the mut modifier.

fn main() {

let _immutable_binding = 1;

let mut mutable_binding = 1;

println!("Before mutation: {}", mutable_binding);

// Ok

mutable_binding += 1;

println!("After mutation: {}", mutable_binding);

// Error!

_immutable_binding += 1;

// FIXME ^ Comment out this line

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

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The compiler will throw a detailed diagnostic about mutability errors.

Scope and Shadowing

Variable bindings have a scope, and are constrained to live in a block. A block is a collection of statements enclosed by braces {}.

fn main() {

// This binding lives in the main function

let long_lived_binding = 1;

// This is a block, and has a smaller scope than the main function

{

// This binding only exists in this block

let short_lived_binding = 2;

println!("inner short: {}", short_lived_binding);

}

// End of the block

// Error! `short_lived_binding` doesn't exist in this scope

println!("outer short: {}", short_lived_binding);

// FIXME ^ Comment out this line

println!("outer long: {}", long_lived_binding);

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

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Also, variable shadowing is allowed.

fn main() {

let shadowed_binding = 1;

{

println!("before being shadowed: {}", shadowed_binding);

// This binding *shadows* the outer one

let shadowed_binding = "abc";

println!("shadowed in inner block: {}", shadowed_binding);

}

println!("outside inner block: {}", shadowed_binding);