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The Ultimate Guide to Rust Programming

Multiple Bounds

Explore how to use multiple trait bounds on generic parameters in Rust functions. Understand combining traits like Double, Display, and Copy, and when to apply a where clause for clarity and safer code. Learn to resolve common errors involving trait requirements and value ownership.

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I want to write a function, info, that prints out some information about a number. It’s easier if I just show it to you:

Rust 1.40.0
trait Double {
    fn double(self) -> Self;
}
impl Double for i32 {
    fn double(self) -> i32 { self * 2 }
}
fn info(x: i32) {
    println!("Original number: {}", x);
    println!("Doubled number: {}", x.double());
    println!("Quadrupled number: {}", x.double().double());
}
fn main() {
    info(5);
}

Since our info is defined in terms of the double method only, though, it seems like we should be able to parameterize. Try changing the signature from i32 to a type parameter:

Rust 1.40.0
fn info<T: Double>(x: T)

This generates an error message:

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