Why does the compiler need an implementation of a trait to call a default free function?

Question

When calling a default implementation on a trait which does not take self, why does it neeed an implementing type to be annotated?

A minimal, reproducible ex

Answer 1

Provided methods in Rust are not like static methods in Java. Even a function with no arguments and a default implementation can be overridden by implementors. Consider adding another type that implements Builder but overrides the new function:

struct Noisy {}

impl builder::Builder for Noisy {
    fn new() -> builder::Simple {
        println!("Ahahahah I'm creating a Simple!!!11!");
        builder::Simple
    }
}

fn main() {
    // wait, should this call builder::Simple::new() or Noisy::new()?
    let _ = builder::Builder::new();
}

If you want the effect of a Java static function that always has the same behavior, you should use a free function, as prog-fh's answer also suggests.