Function is_len 

pub fn is_len(callee: Option<DefId>) -> bool

Whether callee is a len query method. Matched by DefId via crate::def_id::len_fns, which resolves every ::len fn_def in the std crates and the local crate — so the std-challenge suites’ re-implemented len methods are modelled too, without substring-matching a def_path_str.