Variables and Mutability
In Rust, variables are immutable by default. This is one of Rust's key features that helps you write safer, more predictable code.
Key Concept: Immutability means once a value is bound to a variable, you cannot change that value unless you explicitly make it mutable.
Declaring Variables
Use the let keyword to declare variables:
The code above creates an immutable variable x with the value 5. If you try to change it, Rust will give you a compile-time error.
Mutable Variables
To make a variable mutable, use the mut keyword:
Best Practice: Only use
mut when you actually need to change a variable. Immutability by default helps prevent bugs.Constants
Constants are similar to immutable variables, but with key differences:
Constants vs Variables:
- Constants use
constinstead oflet - You must annotate the type (e.g.,
u32) - Constants can be declared in any scope, including global
- Constants must be set to a constant expression, not computed at runtime
- Naming convention: SCREAMING_SNAKE_CASE
Shadowing
Rust allows you to declare a new variable with the same name as a previous variable. This is called shadowing:
Shadowing vs Mutability:
Shadowing is different from marking a variable as mut. With shadowing, you're creating a new variable, which means you can even change its type:
Practice Exercise
Try to predict what this code will output:
Answer: x = 20, y = 30