https://gitter.im/ruRust/easy?at=5a7466a8e217167e2c32caea

playground.rs

use std::borrow::Cow;
struct A<'a> {
    geometry_column: &'a str,
}

impl<'a> A<'a> {
    fn geometry_column_mercator(&self, flag: bool) -> Cow<str> {
        if flag {
            self.geometry_column.into()
        } else {
            String::from("Hello world!").into()
        }
    }
}

fn main() {
    let a = A {
        geometry_column: "Foo",
    };
    println!("{:?}", a.geometry_column_mercator(true))
}