Function tor_config::resolve_option

pub fn resolve_option<T, DF>(input: &Option<Option<T>>, def: DF) -> Option<T> where
    T: Clone + Default + PartialEq,
    DF: FnOnce() -> Option<T>, 

Resolves an Option<Option<T>> (in a builder) into an Option<T>

• If the input is None, this indicates that the user did not specify a value, and we therefore use def to obtain the default value.

• If the input is Some(None), or Some(Some(Default::default())), the user has explicitly specified that this config item should be null/none/nothing, so we return None.

• Otherwise the user provided an actual value, and we return Some of it.

See https://gitlab.torproject.org/tpo/core/arti/-/issues/488

For consistency with other APIs in Arti, when using this, do not pass setter(strip_option) to derive_builder.

⚠ Stability Warning ⚠

We hope to significantly change this so that it is an method in an extension trait. We may also make it able to support settings where the special “no such thing” value is not T::Default.