Variable Assigment and Mutability
The basic syntax in Rust to assign data to a variable is:
let x = 5;
x acts as a container, storing the value 5. println!("{}", x + 1) would display 6.
In Rust variables are by default immutable, meaning they can't be changed once assigned. This is a safety feature of the language. For instance, the following code would not compile:
let x = 5;
x += 1;
println!("x is {}", x);
error[E0384]: cannot assign twice to immutable variable `x`
--> src/main.rs:3:5
|
2 | let x = 5;
| - first assignment to `x`
3 | x += 1;
| ^^^^^^ cannot assign twice to immutable variable
|
help: consider making this binding mutable
|
2 | let mut x = 5;
| +++
For more information about this error, try `rustc --explain E0384`.
error: could not compile `playground` (bin "playground") due to 1 previous error
In order to allow Rust to mutate the value stored in a variable, you must use the mut keyword:
let mut x = 5;
x += 1;
println!("x is {}", x);
This would compile, and the output would be x is 6.
