Variables shadowing

Comprehension

In Rust, if a variable (let or let mut) named a is declared again later, this behavior is called shadowing.

const variables cannot be shadowed
Shadowing can be scoped to a block ({})
Shadowing lets you declare a new type while reusing the same variable name

Origin

https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#shadowing

fn main() {
    let x = 5;

    let x = x + 1;

    {
        let x = x * 2;
        println!("The value of x in the inner scope is: {}", x);
    }

    println!("The value of x is: {}", x);
}

This program first binds x to a value of 5. Then it shadows x by repeating let x =, taking the original value and adding 1 so the value of x is then 6. Then, within an inner scope, the third let statement also shadows x, multiplying the previous value by 2 to give x a value of 12. When that scope is over, the inner shadowing ends and x returns to being 6. When we run this program, it will output the following:


$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/variables`
The value of x in the inner scope is: 12
The value of x is: 6

Shadowing is different from marking a variable as mut, because we’ll get a compile-time error if we accidentally try to reassign to this variable without using the let keyword. By using let, we can perform a few transformations on a value but have the variable be immutable after those transformations have been completed.

The other difference between mut and shadowing is that because we’re effectively creating a new variable when we use the let keyword again, we can change the type of the value but reuse the same name. For example, say our program asks a user to show how many spaces they want between some text by inputting space characters, but we really want to store that input as a number:


    let spaces = "   ";
    let spaces = spaces.len();

This construct is allowed because the first spaces variable is a string type and the second spaces variable, which is a brand-new variable that happens to have the same name as the first one, is a number type. Shadowing thus spares us from having to come up with different names, such as spaces_str and spaces_num; instead, we can reuse the simpler spaces name. However, if we try to use mut for this, as shown here, we’ll get a compile-time error:


    let mut spaces = "   ";
    spaces = spaces.len();

The error says we’re not allowed to mutate a variable’s type:


$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
error[E0308]: mismatched types
 --> src/main.rs:3:14
  |
3 |     spaces = spaces.len();
  |              ^^^^^^^^^^^^ expected `&str`, found `usize`

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

Rust - Basic - 01 - Variables shadowing

Variables shadowing

Comprehension

Origin