Rust Scope
Scope
Now that you understand how functions work, it is important to learn how variables act inside and outside of functions.
Scope refers to where a variable is allowed to be used.
A variable only lives inside the block where it was created. A block is anything inside curly braces { }.
Variable Inside a Function
A variable created inside a function only exists inside that function:
Example
fn myFunction() {
let message = "Hello!";
println!("{}", message); // You can access the message
variable here
}
myFunction();
println!("{}", message); // Error - you cannot access the message variable
outside of the function
Try it Yourself »
Note: The variable message only exists inside the function.
Trying to use it outside the function will cause an error.
Variable Inside a Block
You can also create blocks inside other code, like in if statements or loops.
Variables created in these blocks are only valid inside them.
Example
let score = 80;
if score > 50 {
let result = "Pass";
println!("Result: {}", result);
}
println!("Result: {}", result);
// Error: result is out of scope here
Try it Yourself »
Variables in the Same Scope
In Rust, you can declare a new variable with the same name in the same scope using let. This is called shadowing:
The second x replaces the first one. The value 5 is no longer accessible after the second declaration.
This is different from languages that disallow reusing variable names. In Rust, it's a feature used to transform or update values safely.
You can also reuse a variable name inside a new block:
Example
let x = 5;
{
let x = 10;
println!("Inside block: {}", x);
}
println!("Outside block: {}", x);
Try it Yourself »
Here, the two x variables are in different scopes. The inner x only exists inside the block. Outside the block, the original value remains.
Note: Even though shadowing is allowed, using the same name too often can make your code harder to read. Use clear names when possible.
Why Scope Matters
Understanding scope helps you:
- Know where a variable can be used
- Prevent naming conflicts
- Avoid errors when working with functions, loops, and conditionals
