[tialaramex]

Rust will cheerfully:

    assert_eq!("ὀδυσσεύς", "ὈΔΥΣΣΕΎΣ".to_lowercase());

[Notice that this is in fact entirely impossible with the naive strategy since Greek cares about position of symbols]

Some of the latter examples aren't cases where a programming language or library should just "do the right thing" but cases of ambiguity where you need locale information to decide what's appropriate, which isn't "just as wrong as the C++ version" it's a whole other problem. It isn't wrong to capitalise A-acute as a capital A-acute, it's just not always appropriate depending on the locale.

[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.