rust - Implementing a trait on HashMap that uses methods with bounds -

i have following trait want implement on hashmaps:

trait idassigner<k, v> {     // if key has been seen, returns (id_for_key, false). otherwise     // assigns new id_for_key (starting 0 , increasing) , returns     // (id_for_key, true).     fn assign_id(mut self, key: k) -> (v, bool); }  impl<k, v> idassigner<k, v> hashmap<k, v> v: add<v> {     fn assign_id(mut self, key: k) -> (v, bool) {         if self.contains_key(&key) {             (self[&key], false)         } else {             let id = self.len() v;             self[&key] = id;             (id, true)         }     } } 

the compiler gives various errors methods want call in implementation on hashmap don't exist. suspect need add same bounds impl statement exist on methods. how fix code?

playground

error: no method named contains_key found type

as can see in documentation of contains_key, impl block in contains_key defined, has bounds where k: eq + hash, s: hashstate. adding bound k clause fix other no method errors , cannot index value of typestd::collections::hash::map::hashmap` error.

after get

error: non-scalar cast: usize v

which corresponds line:

let id = self.len() v; 

what trying here cast usize whatever type v instanciated to. can't work in general, v might not compatible usize (e.g. vec or other struct). since want hashmap key id, can remove v generics , directly set value parameter of hashmap:

trait idassigner<k> {     fn assign_id(mut self, key: k) -> (usize, bool); }  impl<k> idassigner<k> hashmap<k, usize>     k: eq + hash,