I've a shallow struct nesting with an inner and an outer. I implemented put-methods that shall modify the inner via the outer struct. Both methods take a mutable &self as first parameter. The outer does not compile and I got stuck with error[E0596]: cannot borrow **ias mutable, as it is behind a& reference

use std::collections::BTreeSet;

#[derive(Eq, Ord, PartialEq, PartialOrd)]
struct Inner { values: Vec<String> }
impl Inner {
    fn new() -> Self { Self { values: Vec::new() } }
    fn put(&mut self, val: &str) -> usize {
        self.values.push(val.to_string());
        self.values.len()
    }
}

struct Outer { container: BTreeSet<Inner> }
impl Outer {
    fn new() -> Self { Self { container: BTreeSet::new() } }
    fn put(&mut self, val: &str) -> usize {
        let i = &mut self.container.first().unwrap();
        i.put(&val.to_string())
    }
}

The retrieval of i is as explicit as this post suggests but that does not help.

Usually the compiler gives helpful hints but this time I am lost. Both methods get the underlying struct mutable. i is mutable and enclosed by an owned o so what is wrong here?

I can trade this error in for error[[E0308](https://doc.rust-lang.org/error_codes/E0308.html)]: mismatched types by making i's retrieval more explicit with let i: &mut Inner = self.container.first().unwrap(); but that doesn't help. I assume the expression &mut self.container.first().unwrap(); does not return a mutable owned Inner but that's what should happen.

What am I missing?

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