Why does invoking a method on a mutable link include "borrowing"?

Question

I am learning Rust and I am trying to process this code when compiling:

use std::vec::Vec;
use std::collections::BTreeMap;

struct Occ {
    docnum: u64,
    weight: f32,
}

struct PostWriter<'a> {
    bytes: Vec<u8>,
    occurrences: BTreeMap<&'a [u8], Vec<Occ>>,
}

impl<'a> PostWriter<'a> {
    fn new() -> PostWriter<'a> {
        PostWriter {
            bytes: Vec::new(),
            occurrences: BTreeMap::new(),
        }
    }

    fn add_occurrence(&'a mut self, term: &[u8], occ: Occ) {
        let occurrences = &mut self.occurrences;
        match occurrences.get_mut(term) {
            Some(x) => x.push(occ),
            None => {
                // Add the term bytes to the big vector of all terms
                let termstart = self.bytes.len();
                self.bytes.extend(term);
                // Create a new occurrences vector
                let occs = vec![occ];
                // Take the appended term as a slice to use as a key
                // ERROR: cannot borrow `*occurrences` as mutable more than once at a time
                occurrences.insert(&self.bytes[termstart..], occs);
            }
        }
    }
}

fn main() {}

I get an error message:

error[E0499]: cannot borrow `*occurrences` as mutable more than once at a time
  --> src/main.rs:34:17
   |
24 |         match occurrences.get_mut(term) {
   |               ----------- first mutable borrow occurs here
...
34 |                 occurrences.insert(&self.bytes[termstart..], occs);
   |                 ^^^^^^^^^^^ second mutable borrow occurs here
35 |             }
36 |         }
   |         - first borrow ends here

I don’t understand ... I'm just calling a method on a mutable link, why does this line include borrowing?

Answer 1

I'm just calling a method on a mutable link, why is this line related to borrowing?

, , . , . , , -, . , ! ... bang !

, " , ". entry :

use std::collections::BTreeMap;

fn main() {
    let mut map = BTreeMap::new();

    {
        let nicknames = map.entry("joe").or_insert(Vec::new());
        nicknames.push("shmoe");

        // Using scoping to indicate that we are done with borrowing `nicknames`
        // If we didn't, then we couldn't borrow map as
        // immutable because we could still change it via `nicknames`
    }

    println!("{:?}", map)
}

Answer 2

,

, - . BTreeMap insert fn insert(&mut self..

, .insert, , BTreeMap .