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Chapter 6: Variables, Mutability, and Shadowing

Prerequisites

Ch 5: Cargo

You will understand

  • let vs let mut — immutable by default
  • Shadowing is rebinding, not mutation
  • Why Rust defaults to immutability

Reading time

20 min
+ 15 min exercises
Binding Cards

`let` vs `let mut`

let x = 5 binding promised stability let mut x = 5 reassignment authority is explicit
Scope Diagram

Shadowing Creates New Bindings

x = 5 x = 6 x = 12 exit inner scope → outer binding becomes visible again
Comparison

Shadowing vs Mutation Are Different Mechanisms

Shadowing pipeline Mutation attempt port = "8080" port = 8080 port = ValidPort same concept, refined representation mut port = "8080" port = 8080 type mismatch if binding type must stay `&str`

Watch the difference between shadowing and mutation directly in the stack frame.

Interactive simulation (requires JavaScript): shadowing creates a brand-new binding (allowing a type change from &str to u16) while the old one becomes unreachable; mutation with mut overwrites the same stack slot in place.

## Step 1 - The Problem