Rust Brain Teasers/

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Puzzle 8: Explanation

Puzzle 8: Explanation

Let’s find out how type conversion works in Rust.

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fn double_it(n: u64, _: i32) -> u64 {
    n * 2
}
fn main() {
    let one : i32 = 1;
    let n = double_it(one as _, 3);
    println!("{}", n);
}

Explanation

There are two surprises in the code above:

  1. We can name a function parameter _, which would require users of that function to send a variable to that parameter location. The variable won’t be used and will get optimized away during release builds.

  2. The one as _ syntax compiles and works. The i32 converts to a u64 without specifying a type.

Rust doesn’t support duck typing, which is an automatic conversion between types if any similar type is available. Rust is generally very strict about type conversions.

Rust’s underscore (or placeholder) symbol has different meanings in different contexts:

  • When used as a variable name prefix (for example, _ignore_me : i32), the underscore indicates to Rust that the variable is deliberately unused. Rust will suppress unused variable warnings.

  • When we use it as an entire variable name, we tell Rust that we don’t intend to use the variable at all. When used in a match statement (for ...