Hashmap multiple mutable borrow issue after reference drop

Question

I am trying to pass around a HashMap which stores values through a set of nested enums/structs. The problem of multiple mutability happens during iteration, even all references should be dropped.

The general idea is to have a vector of values, iterate through them and simplify them, keeping track of them within the HashMap. There are two stages of simplification.

The general flow looks something like

run(Vec<ComplexVal>) 
-for each val-> 
val.fix_complex(holder) 
-for each `smp` SimpleVal in val->
basicval = Simplifier::step(smp, holder)
holder.insert("name", basicval)

But the problem is that the holder is borrowed mutably in each stage, and there isn't supposed to be any reference from the ComplexVal to the holder and since the borrowchecker doesn't like multiple borrows, it fails.

Full playground snippet: here

It happens in this snippet:

    pub fn run(&mut self, mut vals: Vec<ComplexVal>) {
        let mut holder = Holder{hold:HashMap::new()};
        // .. setup holder code omitted
        let len = vals.len();
        for _ in 0..len {
            let mut val = vals.remove(0); // remove from vec, should drop after running
            println!("Running {:?}", val);
            match val {
                ComplexVal::Cmplx1(mut c) => {
                    c.fix_complex(&mut holder)
                },
                //... more cases of different types of values omitted for simplicity
            }
            // val *should* be dropped here, and therefore the mutable borrow of holder?
        }

        println!("Holder: {:?}", holder);
    }
}

The only thing I can think of is that it somehow is related to the BasicVal::Ref(&BasicVal) value when created.

I need to return a reference of type &BasicVal so I can't use a regular fn() -> &BasicVal as the reference would be dangling, so I pass a ret value which is to be modified and used as the storage for the return value.

I have also tried just returning the enum BasicVal::Ref(&BasicVal), but run into the same mutability issues.

The example below is a much more simple version which (sort of) demonstrates the same error, just thought I'd include this context in case someone has another idea on how to implement this which wouldn't have these issues

Code (edited)

Updated playground link

Edit: I made a mistake in not needing the lifetimes of both holder and ret to explicitly be the same, so I have made an updated example for it

use std::borrow::BorrowMut;
///////////////////////////////
use std::cell::{RefCell, RefMut};
use std::collections::HashMap;

#[derive(Debug)]
enum BasicVal<'a> {
    Ref(&'a BasicVal<'a>),
    Val1(BasicStruct),
}

#[derive(Debug)]
struct Holder<'b> {
    hold: HashMap<String, RefCell<BasicVal<'b>>>,
}

#[derive(Debug)]
struct BasicStruct {
    val: i32,
}

impl<'a> BasicVal<'a> {
    pub fn empty() -> Self { BasicVal::Val1(BasicStruct { val: 0 }) }
}

// must match sig of modify_val_ref
fn modify_val<'f>(holder: &'f mut Holder<'f>, mut ret: RefMut<BasicVal<'f>>) {
    *ret = BasicVal::Val1(BasicStruct { val: 5 });
}

// must match sig of modify_val
fn modify_val_ref<'f>(holder: &'f mut Holder<'f>, mut ret: RefMut<BasicVal<'f>>) {
    ret = holder.hold.get("reference_val").unwrap().borrow_mut();
}


fn do_modify<'f>(holder: &'f mut Holder<'f>) {
    let mut v = RefCell::new(BasicVal::empty());
    println!("Original {:?}", v);

    modify_val(holder, v.borrow_mut());
    holder.hold.insert("Data".to_string(), v);

    println!("Modified {:?}", holder.hold.get("Data"));
}

pub fn test_dropborrow() {
    let mut holder = Holder { hold: HashMap::new() };
    holder.hold.insert(
        "reference_val".to_string(),
        RefCell::new(BasicVal::Val1(BasicStruct { val: 8 })),
    );
    do_modify(&mut holder);
}

pub fn main() {
    test_dropborrow();
}

Edit: Using just the holder for a temp return value gives me a multiple mutable borrow issue, so that workaround doesn't work. I have also tried it with a RefCell with the same issue.

fn modify_val<'f>(holder: &'f mut Holder<'f>) {
    holder.hold.insert("$return".to_string(), BasicVal::Val1(BasicStruct{val: 5}));
}

fn do_modify<'f>(holder: &'f mut Holder<'f>) {

    modify_val(holder);
    let mut v = holder.hold.remove("$return").unwrap();
    holder.hold.insert("Data".to_string(), v);

    println!("Modified {:?}", v);
}

Error:

935 | fn do_modify<'f>(holder: &'f mut Holder<'f>) {
    |              -- lifetime `'f` defined here
936 | 
937 |     modify_val(holder);
    |     ------------------
    |     |          |
    |     |          first mutable borrow occurs here
    |     argument requires that `*holder` is borrowed for `'f`
938 |     let mut v = holder.hold.remove("$return").unwrap();
    |                 ^^^^^^^^^^^ second mutable borrow occurs here

Any help is greatly appreciated!!!