extern crate serde; // 1.0.78
#[macro_use] extern crate serde_derive; // 1.0.78
extern crate serde_json; // 1.0.27

#[derive(Deserialize, Debug)]
struct Foo {
    #[serde(flatten)]
    bar: StringIsh,
}

#[derive(Deserialize, Debug)]
#[serde(untagged)]
enum StringIsh {
    String(String),
    Int(u64),
}

fn main() {
    println!(
        "{:#?}",
        serde_json::from_str::<Vec<Foo>>(r#"[{"bar": "42"}, {"bar": 42}]"#)
    );
}