Why define a struct with single private field of unit type?

问题

I do not know why struct ParseBoolError has the _priv field, as shown below:

pub struct ParseBoolError {
    _priv: (),
}

https://doc.rust-lang.org/src/core/str/mod.rs.html#150

I think that the _priv field is not used.

回答1:

You can't create an instance of a struct if it has private fields. This is just a trick to prevent ParseBoolError from being constructed in user code.

One reason to do this is for forwards compatibility. If users could create it with:

let error = ParseBoolError {};

then a future version of ParseBoolError couldn't add fields without breaking that code.

来源：https://stackoverflow.com/questions/63338208/why-define-a-struct-with-single-private-field-of-unit-type