Simplest answer for me was to declare the mutable borrows in the second function. If foo calls bar, do the mutable borrowing inside bar instead of passing bar a mutable reference, which doesn't work.

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pub fn foo(&mut self, thingid: u32) {

    //stuff

    self.bar(&thingid);

}

pub fn bar(&mut self, thingid: u32) {

    self.things.get_mut(&thingid).unwrap();

    //stuff

}