Function resolve_option_general 

pub fn resolve_option_general<T, ISF, DF>(
    input: Option<Option<&T>>,
    is_sentinel: ISF,
    def: DF,
) -> Option<T>
where
    T: Clone,
    DF: FnOnce() -> Option<T>,
    ISF: FnOnce(&T) -> bool,

Resolves an Option<Option<&T>> (in a builder) into an Option<T>, more generally

Like resolve_option, but:

• Doesn’t rely on T being Default + PartialEq to determine whether it’s the sentinel value; instead, takes is_sentinel.

• Takes Option<Option<&T>> which is more general, but less like the usual call sites.

Behavior

• 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(v)) where is_sentinel(v) returns true, 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

⚠ Stability Warning ⚠

We may significantly change this so that it is an method in an extension trait.

Example

use tor_config::resolve_option_general;

// Use 0 as a sentinel meaning "explicitly clear" in this example
let is_sentinel = |v: &i32| *v == 0;

// No value provided: use default
assert_eq!(
    resolve_option_general(None, is_sentinel, || Some(10)),
    Some(10),
);

// Explicitly None
assert_eq!(
    resolve_option_general(Some(None), is_sentinel, || Some(10)),
    None,
);

// Sentinel value (0) -> return None
assert_eq!(
    resolve_option_general(Some(Some(&0)), is_sentinel, || Some(10)),
    None,
);

// Set to actual value -> return that value
assert_eq!(
    resolve_option_general(Some(Some(&5)), is_sentinel, || Some(10)),
    Some(5),
);