Rust by Example

// This binding *shadows* the previous binding

let shadowed_binding = 2;

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

}

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Declare first

It's possible to declare variable bindings first, and initialize them later. However, this form is seldom used, as it may lead to the use of uninitialized variables.

fn main() {

// Declare a variable binding

let a_binding;

{

let x = 2;

// Initialize the binding

a_binding = x * x;

}

println!("a binding: {}", a_binding);

let another_binding;

// Error! Use of uninitialized binding

println!("another binding: {}", another_binding);

// FIXME ^ Comment out this line

another_binding = 1;

println!("another binding: {}", another_binding);

}

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The compiler forbids use of uninitialized variables, as this would lead to undefined behavior.

Freezing

When data is bound by the same name immutably, it also freezes. Frozen data can't be modified until the immutable binding goes out of scope:

fn main() {

let mut _mutable_integer = 7i32;

{

// Shadowing by immutable `_mutable_integer`

let _mutable_integer = _mutable_integer;

// Error! `_mutable_integer` is frozen in this scope

_mutable_integer = 50;

// FIXME ^ Comment out this line

// `_mutable_integer` goes out of scope

}

// Ok! `_mutable_integer` is not frozen in this scope

_mutable_integer = 3;

}

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