Rust: return &mut reference from downcast_ref of HashMap mutable value

Question

As the title says, we have an application which runs wasm plugins. Each plugin can register their own State in the StateRegistry . Then whenever the plugins are executed, they will modify their respective State .

The code below illustrates this:

use std::{collections::HashMap, any::Any};

type ContractId = String;
type GenericContractState = Box<dyn Any>;

// This will be WASM code
mod foo_contract {
    use super::StateRegistry;

    pub struct State {
    }

    pub fn update(states: &mut StateRegistry) {
        let state = states.states.get_mut(&"foo_contract".to_string()).unwrap();
        let state = state.downcast_mut::<State>().unwrap();
        // I'd prefer something like:
        // let state = state.lookup::<State>().unwrap()
    }
}

pub struct StateRegistry {
    pub states: HashMap<ContractId, GenericContractState>,
}

impl StateRegistry {
    fn new() -> Self {
        Self { states: HashMap::new() }
    }

    fn register(&mut self, contract_id: ContractId, state: GenericContractState) {
        self.states.insert(contract_id, state);
    }

    /*
    fn lookup<'a, S>(&'a mut self, contract_id: &ContractId) -> Option<StateRefWrapper<'a, S>> {
        match self.states.get_mut(contract_id) {
            Some(state) => {
                let ptr = state.downcast_mut::<S>();
                match ptr {
                    Some(ptr) => Some(StateRefWrapper { _mut: state, ptr }),
                    None => None,
                }
            }
            None => None,
        }
    }
    */
}

/*
struct StateRefWrapper<'a, S> {
    _mut: &'a mut Box<dyn Any>,
    ptr: &'a mut S,
}
*/

fn main() {
    let mut states = StateRegistry::new();
    let foo_state = Box::new(foo_contract::State {});
    states.register("foo_contract".to_string(), foo_state);

    foo_contract::update(&mut states);
}

The part I want to improve is that currently the plugin developer has to use this code to lookup their State in the registry:

        let state = states.states.get_mut(&"foo_contract".to_string()).unwrap();
        let state = state.downcast_mut::<State>().unwrap();

I want to create a respective method in StateRegistry that is usable like this:

        let state = state.lookup::<State>().unwrap()

My attempt (commented above) was something like this:

impl StateRegistry {
    // ...

    fn lookup<'a, S>(&'a mut self, contract_id: &ContractId) -> Option<StateRefWrapper<'a, S>> {
        match self.states.get_mut(contract_id) {
            Some(state) => {
                let ptr = state.downcast_mut::<S>();
                match ptr {
                    Some(ptr) => Some(StateRefWrapper { _mut: state, ptr }),
                    None => None,
                }
            }
            None => None,
        }
    }
}

struct StateRefWrapper<'a, S> {
    _mut: &'a mut Box<dyn Any>,
    ptr: &'a mut S,
}

How can I get this lookup() function working?

Thanks

Answer 1

You can't construct your StateRefWrapper in the way you want to, because it would contain two mutable borrows of the same value. You can, however, return just a &mut S which seems sufficient:

fn lookup<'a, S: 'static>(&'a mut self, contract_id: &ContractId) -> Option<&'a mut S> {
    self.states
        .get_mut(contract_id)
        .and_then(|state| state.downcast_mut())
}

And use it like this:

pub fn update(states: &mut StateRegistry) {
    let state = states.lookup::<State>(&"foo_contract".to_string()).unwrap();
    // state.modify();
}