Mutability

important

Every variable in Rust is immutable by default.

To see this in action, go and run the module3 code:

let integer_example = 99;
integer_example = 12;

And oops! There's a problem:

error[E0384]: cannot assign twice to immutable variable integer_example

So how would you go about declaring a variable that you could change? That's easy, introducing the mut keyword.

important

The mut keywords explicitally declares a variable as something you want to mutate later. If you declare a variable as mutable but never actually change it, the compiler will give you a warning and recommend you remove the mut keyword. Neat!

let mut str_example = "This string is now mutable";
str_example = "And can be edited";
println!("{}", str_example);