Function fn_

pub fn fn_(
    visibility: Option<Visibility>,
    fn_name: Name,
    type_params: Option<GenericParamList>,
    where_clause: Option<WhereClause>,
    params: ParamList,
    body: BlockExpr,
    ret_type: Option<RetType>,
    is_async: bool,
    is_const: bool,
    is_unsafe: bool,
    is_gen: bool,
) -> Fn