variables-mutability

Basic Concepts

• In rust variables are immutable by default
• When a variable is immutable, once a value is bound to a name, you can’t change that value.

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         -
  |         |
  |         first assignment to `x`
  |         help: consider making this binding mutable: `mut x`
3 |     println!("The value of x is: {x}");
4 |     x = 6;
  |     ^^^^^ cannot assign twice to immutable variable

For more information about this error, try `rustc --explain E0384`.
error: could not compile `variables` (bin "variables") due to 1 previous error

• To make variable mutable, use

let mut var = 6;

Constants

• Like immutable variables, constants are values that are bound to a name and are not allowed to change, but there are a few differences between constants and variables.
• you aren’t allowed to use mut with constants. Constants aren’t just immutable by default—they’re always immutable
• The last difference is that constants may be set only to a constant expression, not the result of a value that could only be computed at runtime.
• Note that there is a differnece between compile time and runtime. So compiler compiles the expression

const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;

at compile time, meanwhile runtime means when the programming is running of sorts.

Shadowing

fn main() {
    let x = 5;

    let x = x + 1;

    {
        let x = x * 2;
        println!("The value of x in the inner scope is: {x}");
    }

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

what it basically means is declaring a variable with the same name as a previous variable.

Also something to note Most executable formats have special sections where you can store data that the code can reference >in one way or another. static explicitly puts the data in those special sections. const on the other hand just inlines the result of the expression where the constant is used.

Data Types