“cannot borrow as mutable because it is also borrowed as immutable” with match [duplicate]

问题

I'm attempting to count the character frequency in a string and store the count of each character in a BTreeMap. However, I'm getting a warning and would like to get rid of it.

This is what I've tried:

use std::collections::BTreeMap;

fn letter_frequency(input: &str) -> BTreeMap<char, i32> {
    let mut tree: BTreeMap<char, i32> = BTreeMap::new();
    for item in &input.chars().collect::<Vec<char>>() {
        match tree.get(item) {
            Some(count) => tree.insert(*item, *count + 1),
            None => tree.insert(*item, 1)
        };
    }
    tree
}

This is the warning:

warning: cannot borrow `tree` as mutable because it is also borrowed as immutable
 --> src/lib.rs:7:28
  |
6 |         match tree.get(item) {
  |               ---- immutable borrow occurs here
7 |             Some(count) => tree.insert(*item, *count + 1),
  |                            ^^^^               ------ immutable borrow later used here
  |                            |
  |                            mutable borrow occurs here
  |
  = note: #[warn(mutable_borrow_reservation_conflict)] on by default
  = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
  = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

How do I correctly use match with a BTreeMap to avoid the error ?

回答1:

Like Svetlin mentions in a comment, the entry API is your friend. Below I've also removed one unneeded collect.

fn letter_frequency(input: &str) -> BTreeMap<char, i32> {
    let mut tree = BTreeMap::new();
    for item in input.chars() {
        let count = tree.entry(item).or_insert(0);
        *count += 1;
    }
    tree
}

The temporary variable count is not really needed: *tree.entry(item).or_insert(0) += 1; works just fine but might look a bit crowded at first.

来源：https://stackoverflow.com/questions/59347094/cannot-borrow-as-mutable-because-it-is-also-borrowed-as-immutable-with-match

标签

rust