Why does Rust require ownership annotations instead of inferring it?

Question

How come Rust does not fully infer ownership of its variables? Why are annotations needed?

Answer 1

If that were even possible I believe it would be a terrible user experience because:

• if the compiler cannot deduce ownership of an object, the error can barely be understood (like with trial-and-error approach in C++ templates link);
• the ownership policy doesn't seem to be easy to grasp (that's one opinion though) and trying to understand which semantic has been chosen by a compiler may lead to unexpected behaviors (reference a Javascript weird type conversions);
• more bugs during refactoring can be introduced (implied by the point above);
• full program inference would definitely take a huge amount of time, if it is even a solvable problem.

However, if you struggle with a lack of polymorphism, it is usually possible to parametrize a method with an ownership kind, which might be considered a somewhat explicit alternative to inference, e.g.:

fn print_str(s: impl AsRef<str>) {
    println!("{}", s.as_ref());
}

fn main() {
    print_str("borrowed");
    print_str("owned".to_owned());
}