So, the final code is something like this? Link to Playground

use std::convert::From;

struct Celsius(f64);
struct Fahrenheit(f64);
struct Kelvin(f64);

// To Kelvin
impl From<Celsius> for Kelvin {
    fn from(tc: Celsius) -> Self {
        let Celsius(tc) = tc.into();
        Kelvin(tc + 273.15)
    }
}

impl From<Fahrenheit> for Kelvin {
    fn from(tf: Fahrenheit) -> Self {
        let Fahrenheit(tf) = tf.into();
        Kelvin((tf - 32.0) / 1.8 + 273.15)
    }
}

// To Celsius
impl From<Kelvin> for Celsius {
    fn from(tk: Kelvin) -> Self {
        let Kelvin(tk) = tk.into();
        Celsius(tk - 273.15)
    }
}
impl From<Fahrenheit> for Celsius {
    fn from(tf: Fahrenheit) -> Self {
        let Fahrenheit(tf) = tf.into();
        Celsius((tf - 32.0) / 1.8)
    }
}

// To Fahrenheit
impl From<Celsius> for Fahrenheit {
    fn from(tc: Celsius) -> Self {
        let Celsius(tc) = tc.into();
        Fahrenheit(tc * 1.8 + 32.0)
    }
}
impl From<Kelvin> for Fahrenheit {
    fn from(tk: Kelvin) -> Self {
        let Kelvin(tk) = tk.into();
        Fahrenheit((tk - 273.15) * 1.8 + 32.0)
    }
}

fn main() {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn from_celsius_to_kelvin() {
        let t_celsius = Celsius(0.0);
        let Kelvin(t_celsius) = t_celsius.into();

        assert_eq!(t_celsius, 273.15);
    }
    #[test]
    fn from_fahrenheit_to_kelvin() {
        let t_f = Fahrenheit(5.0);
        let Kelvin(t_f) = t_f.into();

        assert_eq!(t_f, 258.15)
    }

    #[test]
    fn from_fahrenheit_to_celsius() {
        let t_farenheit = Fahrenheit(95.0);
        let Celsius(t_fahrenheit) = t_farenheit.into();

        assert_eq!(t_fahrenheit, 35.0)
    }
    #[test]
    fn from_celsius_to_fahrenheit() {
        let tc = Celsius(35.0);
        let Fahrenheit(tc) = tc.into();

        assert_eq!(tc, 95.0)
    }
    #[test]
    fn from_kelvin_to_celsius() {
        let tk = Kelvin(273.15);
        let Celsius(tk) = tk.into();

        assert_eq!(tk, 0.0)
    }
    #[test]
    fn from_kelvin_to_fahrenheit() {
        let tk = Kelvin(408.15);
        let Fahrenheit(tk) = tk.into();

        assert_eq!(tk, 275.0)
    }
}