[account42]

Is this

    assert_eq!("\u1F41δυσσεύς", "ὈΔΥΣΣΕΎΣ".to_lowercase());

or

    assert_eq!("\u03BF\u0314δυσσεύς", "ὈΔΥΣΣΕΎΣ".to_lowercase());

For display it doesn't matter but most other applications really want some kind of normalizatin which does much much more so having a convenient to_lowercase() doesn't buy you as much as you think and can be actively misleading.