Variables and Mutablity

20 April, 2020   ·   Rust

In Rust, Variables are Immutable by default.

Demo Program


// This program won't compile because
// the value of `x` has been changed after
// the first declaration(without using `mut`)

fn main() {
  let x = 5;
  println!("Value of X is {}", x);

  x = 15;
  println!("Value of X is {}", x);

Output of cargo run:

$ cargo run
Compiling variables v0.1.0 (/home/crow/Dev/rust/sources/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
2 |     let x = 5;
  |         -
  |         |
  |         first assignment to `x`
  |         help: make this binding mutable: `mut x`
3 |     println!("Value of X is {}", x);
4 |     x = 15;
  |     ^^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0384`.
error: could not compile `variables`.

To learn more, run the command again with --verbose.


fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);

Output of cargo run

$ cargo run
Compiling variables v0.1.0 (/home/crow/Dev/rust/sources/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/variables`
Value of X is 5
Value of X is 6

Mutating a Value

In order to change the value of a variable after it’s been declared, one needs to use mut keyword along with let. For example: let mut foo = 'bar'.


const MAX_POINTS: u32 = 100_000;

In Rust, immutable variables may look like a constant, but in real, they are not. There is a little difference between variables and constants. They are:

  1. Declared using const keywords, instead of let.
  2. Data Type must be annotated while declaring
  3. Constants are always immutable.
  4. Unlike variables, constants can be declared in any scope.
  5. Constants may only be set to a constant expression, not the result of a function call or any other value that could only be computed at runtime.

Variable Shadowing

Shadowing is the process of redeclearing a already decleared variable. Shadowing does not assign a new value to the previous variable, but creates a new variable of the same name and assigns a value to it. So, shadowed variable can have different data types.

// Shadowing example
fn main() {
    let x = 5;
    let x = x + 1;
    let x = x * 2;

    println!("The value of x is: {}", x);

Shadoing is different from making a variable mut, as a mutated variable can be reassigned a new variable of same data-type. But shadowed variables can be assigned a new value of different data-type than original variable.

Following block is allowed in Rust, because the spaces variable is being shadowed. So, the first instance of spaces is dropped from memory as soon as the second instance is decleared.

let spaces = "   ";         // type: string
let spaces = spaces.len();  // type: unsigned 32bit integer

But this block is not allowed, as the variable is being mutated. Hence, the data-type of reassigned value must match the data-type of previous value.

let mut spaces = "   "; // mutated
spaces = spaces.len();  // error[E0308]: mismatched types
Guessing Game↠ ↞Data Types