Rust error: cannot assign twice to immutable variable

Created Apr 21, 2022 at 10:32

Modified Apr 21, 2022 at 10:34

Variables Immutable

You can make them mutable by adding mut in front of the variable name. For example,

fn main() {
  let x = 2;
  println!("value x: {}", x);

  x = 4;
  println!("value x: {}", x);
}

error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:5:3
  |
2 |   let x = 2;
  |       -
  |       |
  |       first assignment to `x`
  |       help: consider making this binding mutable: `mut x`
...
5 |   x = 4;
  |   ^^^^^ cannot assign twice to immutable variable

For more information about this error, try `rustc --explain E0384`.
error: could not compile `hello-rust` due to previous error

Save and run the program using cargo run. You should receive an error message.

Variables Mutable

When a variable is immutable, once a value is bound to a name, you can’t change that value. For example,

fn main() {
  let mut x = 2;
  println!("value x: {}", x);

  x = 4;
  println!("value x: {}", x);
}

value x: 2
value x: 4

Mismatched Types

For example,

fn main() {
  let mut s = "abcd";

  // mutate a variable’s type
  s = s.len()
}

error[E0308]: mismatched types
 --> src/main.rs:3:7
  |
2 |   let mut s = "abcd";
  |               ------ expected due to this value
3 |   s = s.len()
  |       ^^^^^^^ expected `&str`, found `usize`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `hello-rust` due to previous error