Borrow checker error in fixed code for map_entry lint

[mwylde]

Summary

clippy --fix, while trying to address a clippy::map_entry lint, introduces a "cannot borrow *self as immutable because it is also borrowed as mutable" compiler error.

Lint Name

map_entry

Reproducer

Here is a minimal reproduction:

use std::collections::HashMap;

struct A {
    m: HashMap<String, String>,
}

impl A {
    fn make(&self) -> String {
        "v".to_string()
    }

    fn f(&mut self, k: String) {
        if !self.m.contains_key(&k) {
            self.m.insert(k, self.make());
        }
    }
}

fn main() {
    let mut a = A { m: HashMap::new() };
    a.f("k".to_string());
}

clippy --fix replaces f with

    fn f(&mut self, k: String) {
        self.m.entry(k).or_insert_with(|| self.make());
    }

which causes a compilation error

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
  --> src/main.rs:13:40
   |
13 |         self.m.entry(k).or_insert_with(|| self.make());
   |         ------          -------------- ^^ ---- second borrow occurs due to use of `*self` in closure
   |         |               |              |
   |         |               |              immutable borrow occurs here
   |         |               mutable borrow later used by call
   |         mutable borrow occurs here

Version

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: aarch64-apple-darwin
release: 1.84.0
LLVM version: 19.1.5

Additional Labels

No response