struct Hoge {
    pub v: i32,
}

struct Fuga {
    pub v: i32,
}

impl std::convert::From<Hoge> for Fuga {
    fn from(hoge: Hoge) -> Self {
        Fuga {v: hoge.v}
    }
}

impl std::convert::From<Hoge> for Result<Fuga, String> {
    fn from(hoge: Hoge) -> Self {
        Ok(Fuga {v: hoge.v})
    }
}

fn main() {
    // To Fuga
    {
        let h = Hoge {v: 10};
        let f: Fuga = h.into();
    }

    // To Result of Fuga
    {
        let h = Hoge {v: 10};
        let f: Result<Fuga, String>  = h.into();
    }

    //
    {
        let h = Hoge {v: 10};
        // これをしたい
        //let f: Fuga  = h.into()?;

        // なるほどこうは書ける。でも長い
        let f: Fuga = std::convert::Into::<Result<Fuga, String>>::into(h).unwrap();
    }
}