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Advanced 14 min readModule: Module 2: Variables, Mutability & Shadowing

Immutability by Default & Variable Shadowing

Understand why variables in Rust are immutable by default and how variable shadowing enables safe transformations.

What You Will Learn in This Lesson

  • Why Rust defaults to immutable variables (let x = 5)
  • Explicit mutability with 'let mut'
  • Reusing variable names and changing types with Variable Shadowing

Introduction & Core Concept

In Rust, variables are immutable by default. This is one of many nudges Rust gives you to write code in a way that takes advantage of the safety and easy concurrency that Rust offers.
WHY DOES THIS MATTER IN THE REAL WORLD?

Default immutability prevents accidental mutations across complex concurrent threads.

Variable Shadowing & Mutability

rust
rust
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fn main() {
    // Mutable variable
    let mut score = 100;
    score += 50;
 
    // Variable Shadowing (transforms type safely)
    let spaces = "   ";
    let spaces = spaces.len(); // spaces is now usize: 3
    println!("Score: {}, Spaces count: {}", score, spaces);
}

Line-by-Line Technical Breakdown

1Constants (const MAX: u32 = 100) must be annotated with explicit types and evaluated at compile time.

Try It Yourself (Interactive Editor)

Modify the code in real-time and click Run to test live browser output and console logs.

Intelligent Code Runner & Live Sandbox[RUST]
RUST SOURCE EDITOR
Interactive Live Code

Industry Best Practices & Professional Standards

  • Prefer variable shadowing over making variables mutable whenever transforming data.

Lesson Summary & Core Takeaways

  • Immutability and shadowing ensure deterministic state transitions.