Function group_choices 

pub fn group_choices(
    items: &Value,
    data: &Value,
    path: &str,
) -> Result<Vec<GroupChoice>, GroupChoicesError>

The choices of a group source for the root data: one per row of the group in data order whose value gives a non-empty canonical text, each value once, labelled with the text of its label field or else its value. A group missing from the data gives no choices.

items must have passed group_source_failure; a source whose from or label is not of the declared shape gives the message of its declaration failure, located at path.