I can't figure out this lifetime problem: cannot infer an appropriate lifetime for autoref due to conflicting requirements

[ChechyLevas]

You suggestion works (after removing some other lifetime specifiers that shouldn't be there). Thank you.
However I still don't understand your explanation; this must be how @ZiCog feels about monads

@alice

This use of the 'b lifetime tells the compiler that the references stored in the returned vector point into the self argument.

Are you saying that for some implementation of EconomicAgent, the returned vector should be owned and contained by the implementor? (That is my interpretation of the word "point" when applied to references). My intention here is that the returned vector is a subset of the input vector.

@alice

that they are in fact mutable borrows of different things, hence the error.

This part really confused me. maker and partially_settled are different types so they are trivially different things, no? What have I misunderstood?

@alice

This means that the mutable borrow of maker lasts until partially_settled goes out of scope. However, mutable borrows must be exclusive

This is how I have finished off that bit of code, which seems to compile without complaints.

    let partially_settled:Vec<&ProposedTrade> =
        proposed_trades_map.into_iter().flat_map(|(maker_id, pts)|{
            let maker = agent_col.agents.get_mut(&maker_id).unwrap();
            maker.settle_maker_side(pts)
        }).collect();

    let partially_settled_map = 
        partially_settled.iter()
        .map(|pt|(pt.taker_id, *pt))
        .into_group_map();

    for (taker_id, pts) in partially_settled_map.into_iter() {
        let taker = agent_col.agents.get_mut(&taker_id).unwrap();
        taker.settle_taker_side(pts);
    }

Have I not taken another mutable borrow of the contents of agents (which is what maker was earlier) whilst partially_settled is still in scope?