Lesson 03 of 20

6 mins read

Variables, Mutability & Shadowing

In Plain English

In Rust, variables are immutable by default. To allow reassignment, you must explicitly declare the variable with the mut keyword. Shadowing allows re-declaring a variable with let to transform types cleanly.

The Mental Model

"In Rust, a variable without mut is like stone carving—once engraved, it cannot be altered. Adding mut is like using an erasable whiteboard."

Deep Dive: How It Works

Immutable by Default: Prevents unintended state mutations across large concurrent codebases.

mut Keyword: let mut count = 0; explicitly marks intent to mutate.

Shadowing: let x = 5; let x = x + 1; creates a new variable reusing the same identifier, allowing type transformation.

Syntax Blueprint

let x = 5; (Immutable) | let mut y = 10; y = 20; (Mutable)

Use let for read-only values, mut for values that change, and shadowing for type changes.

Core Rules to Remember

Safety by Default: Immutability prevents race conditions and accidental variable reassignment.

mut Keyword: Required on any variable binding that requires modification.

Shadowing: Re-declaring with let lets you change type (e.g. string to parsed integer) without new names.

Common Beginner Traps & How to Fix Them

Mistake:Reassigning an immutable variable.

Why it happens: let x = 5; x = 6; causes compiler error E0384: cannot assign twice to immutable variable.

How to fix: Declare with let mut x = 5; if reassignment is intentional.

Pro Tip: Shadowing is cleaner than inventing throwaway names like input_str and input_int—just use let input = input.trim().parse()?;.

Live Interactive Example

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Mutability and Shadowing in Action

Rust 1.78 / rustc

Output Console

Click "Run Code" to view the rendered output.

How it works: Demonstrates mutable state updates and shadowing string to integer length.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Update mutable counter

Declare let mut energy = 100;
Subtract 30 from energy (energy -= 30);
Print "Remaining Energy: 70".

Sandbox Output

Click "Run & Check" to test your solution.

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