为什么此Rust 2018代码使用`cargo build`编译但不使用rustc编译？

Question

When compiling this snippet below using cargo build, the borrow checker seems fine but when using rustc I get the error

error[E0502]: cannot borrow `char_counts` as mutable because it is also borrowed as immutable
  --> src/lib.rs:14:17
   |
10 |         let count = char_counts.get(&char);
   |                     ----------- immutable borrow occurs here
...
14 |                 char_counts.insert(char, rem);
   |                 ^^^^^^^^^^^ mutable borrow occurs here
...
19 |     }
   |     - immutable borrow ends here

Any ideas why this happens?

use std::collections::HashMap;

pub fn anagram(word: &str, another_word: &str) -> i32 {
    let mut char_counts = HashMap::new();
    for char in word.chars() {
        let count = char_counts.entry(char).or_insert(0);
        *count += 1;
    }
    for char in another_word.chars() {
        let count = char_counts.get(&char);
        if let Some(val) = count {
            let rem = val - 1;
            if rem > 0 {
                char_counts.insert(char, rem);
            } else {
                char_counts.remove(&char);
            }
        }
    }
    println!("{:?}", char_counts);
    return char_counts.keys().len() as i32;
}

cargo --version and rustc --version commands both output 1.33

Answer 1

This function will compile fine if you have non-lexical lifetimes enabled and won't compile without them. The 2018 edition enables them by default. Perhaps you have edition = "2018" in your Cargo.toml , but are not passing it as parameter when using rustc directly?