Oh crap. I'm using 's Rayon crate to modify each entry in a BTreeMap in parallel using a mutable iterator. Pretty easy, no problems there. But that operation can take a while, and now I want to periodically flush the contents of that BTreeMap to disk while it's working - so we don't lose much progress if the program is interrupted halfway. But that flush operation needs to be able to read the entire BTreeMap, which Rust's type system helpfully prevents.

And down the rabbit hole I go...

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