Function plan_function 

pub fn plan_function(
    text: &str,
    requested: Span,
    requirements: &[Requirement],
    locals: &[LocalBinding],
    expr_types: &[(Span, TyId)],
    tys: &Types,
) -> Result<FunctionPlan, FunctionRefusal>

#2100 slice 2: the extract-function plan for requested, or why there is none. Extracted functions stay capability-free (a fn has no given clause), and a selection containing ? is refused: lifted into a new fn, the ? would return from that fn instead of the one it was written in.