Rust works fine because types are not required to have a default, if you want your type to have a default you implement Rust's Default trait. Stuff which only makes sense if there's a default just depends on that trait and so won't be available, for example you can't core::mem::take your custom Goose type which has no default because core::mem::take<T: Default>. In Rust if we say there's a variable of type Goose and don't initialize it, it's not initialized, and if the compiler can't see why it's initialized before it's used the program is rejected as nonsense because Rust is a safe language and that's an unsafe outcome.

I don't write Kotlin, so I can't speak to the details there.

C++ like Rust does not require that types have a default. In C++ the way you provide a "default" is usually via a zero argument constructor, since the compiler can just call that wherever you asked for an instance of that type and there's no requirement to write such a constructor, or indeed to provide any public constructor at all. So "just use the default" could not work in C++ as it exists today yes.

The other reason C++ can't do anything like this is that it makes a newer C++ with this behaviour behave differently despite no syntactical change. Rust is OK with that, because it has the Edition system to differentiate Rust 2015 code which means one thing from Rust 2024 code which means something else despite having the same text, but in C++ they do not have anything like that, it's not rare for somebody's C++ 17 code to get compiled in C++ 23 and people expect that to work (it doesn't always work but that's what they expect).