Variables and mutability

In plain English

A variable is declared with let, and in Rust a variable is immutable by default, which means once you give it a value you cannot change it. Adding mut makes a binding mutable so you can assign a new value later. Shadowing is declaring a variable again with the same name using a new let, which lets you reuse a name and even switch its type without ever mutating the original binding. Constants are declared with const, always need an explicit type and a value known at compile time, and are never mutable. The primitive types are signed and unsigned integers of several widths, floats, booleans and characters, and overflowing an integer in a debug build causes a panic, a controlled crash that stops the program instead of silently wrapping the number.

Step by step

1. 1Declare a value with let and try to change it to feel the immutability rule.
2. 2Add mut to a binding you need to reassign.
3. 3Shadow a variable with a second let to reuse the name or change its type.
4. 4Define a const with an explicit type and a compile-time value.

fn main() {
    let title = "Variables and mutability";
    println!("{}", title);

    let mut minutes = 12;
    minutes = 14;
    println!("{} minutes", minutes);

    let rating = 4;
    let rating = rating + 1;
    let rating = rating as f64 + 0.5;
    println!("rating {}", rating);

    let price: f64 = 19.99;
    let published: bool = true;
    let grade: char = 'A';

    const MAX_LESSONS: u32 = 100;
    println!("{} {} {} {}", price, published, grade, MAX_LESSONS);
}

Common mistakes

• Trying to reassign a plain let binding fails to compile, because let bindings are immutable by default.
• Assuming const and let behave the same is wrong: const needs an explicit type and a compile-time value and cannot use mut.
• Using a too-small integer type and then overflowing it panics in a debug build instead of silently wrapping.
• Confusing shadowing with mutation is a slip: shadowing creates a new binding, so the original value still exists until it goes out of scope.