No warning on duplicate function outside trait implementation

[arnabanimesh]

struct Person {
    name: String,
    age: u8,
}

trait HasVoiceBox {
    fn speak(&self);
    fn can_speak(&self) -> bool;
}

impl HasVoiceBox for Person {
    fn speak(&self) {
        println!("Hello, my name is {}.", self.name);
    }
    // This does not run
    fn can_speak(&self) -> bool {
        !matches!(self.age, 0..=2)
    }
}

impl Person {
    // This runs
    fn can_speak(&self) -> bool {
        matches!(self.age, 0..=2)
    }
}

fn main() {
    let person = Person {
        name: String::from("Bob"),
        age: 3,
    };
    /* Returns false, there should be an error or warning of some sort
    on the functions that won't run and unused traits*/
    println!("Can {} speak? {}", person.name, person.can_speak());
}

[jonas-schievink]

New lints generally have to go through the RFC process, so closing in favor of that (or alternatively discussion on https://internals.rust-lang.org/).