What does the + operand between types mean?

Question

With this example from core::any

use std::fmt::Debug;
use std::any::Any;

// Logger function for any type that implements Debug.
fn log<T: Any + Debug>(value: &T) {
    let value_any = value as &dyn Any;

    // try to convert our value to a String.  If successful, we want to
    // output the String's length as well as its value.  If not, it's a
    // different type: just print it out unadorned.
    match value_any.downcast_ref::<String>() {
        Some(as_string) => {
            println!("String ({}): {}", as_string.len(), as_string);
        }
        None => {
            println!("{:?}", value);
        }
    }
}

// This function wants to log its parameter out prior to doing work with it.
fn do_work<T: Any + Debug>(value: &T) {
    log(value);
    // ...do some other work
}

fn main() {
    let my_string = "Hello World".to_string();
    do_work(&my_string);

    let my_i8: i8 = 100;
    do_work(&my_i8);
}

This is the first time I’ve seen a + operand between types Any + Debug. I’m assuming its like algebraic types and therefore would be an Any type with Debug type; however, I cannot find any docs under algebraic types in Rust.

What is the + actually doing here, what is it called? Where can I find the docs on this?

Answer 1

T: Any + Debug is a trait bound. The type T must satisfy Any and Debug, therefore the + sign is used here and it is not related to algebraic types. You can read more on traits at the corresponding section in the book.

This section mentions the + sign.