Some questions about generics

[Luosuu]

https://google.github.io/comprehensive-rust/generics/impl-trait.html

Does this mean the return type of get_x function is impl Display? I am a bit confused.

BTW, I think the code in this page can be more understandable like

use std::fmt::Display;

fn get_x(name: impl Display) -> impl Display {
    format!("Hello {name}")
}

fn main() {
    let x = get_x("foo");
    let y = get_x(123);
    println!("{x}");
    println!("{y}")
}

This can better show the idea of

impl Trait allows you to work with types which you cannot name.

[mgeisler]

Hi @Luosuu,

Yeah, the instructor should highlight that get_x can be called on both integers and strings! Let us add that to the speaker notes...

As for the return type, you're right that it's impl Display. The return type is chosen by the person who writes get_x. The caller can only rely on Display being implemented, nothing more.

This is different from the generic in the argument position: the type of name is chosen by the caller of get_x. The person who write the function cannot know anything about the type, except that it implements Display. This allows name to be used with format! as shown in the example.

Does that help a bit? Generics is a complex topic and I need to expand the slides with significant amounts of speaker notes to fully explain this 😄

[Luosuu]

Thank you very much!