clippy::map_entry false positive with let-else

[0vercl0k]

Summary

The clippy::map_entry seems to have a false-positive when using let-else. I've looked for similar issues in the tracker but couldn't find any so opening this one - apologies if I missed one.

The lint suggests to use or_insert_with which makes the else part return out of the closure instead of out of the function. Because they expect different return types (the closure expects a String, the function expects an Option<&String> in https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b9c4d6dad5586827f5906400a480391), it leads to a compilation error.

Lint Name

clippy::map_entry

Reproducer

I tried this code (also available at https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b9c4d6dad5586827f5906400a480391):

use std::collections::HashMap;

#[derive(Default)]
struct Repro {
    cache: HashMap<u64, String>,
}

impl Repro {
    fn calc(k: u64) -> Option<String> {
        if k == 1337 {
            Some("hello!".to_string())
        } else {
            None
        }
    }

    pub fn lookup(&mut self, k: u64) -> Option<&String> {
        if !self.cache.contains_key(&k) {
            let Some(foo) = Self::calc(k) else {
                return None;
            };

            self.cache.insert(k, foo);
        }

        self.cache.get(&k)
    }
}

fn main() {
    let mut repro = Repro::default();
    println!("{:?}", repro.lookup(1337));
    println!("{:?}", repro.lookup(1338));
}

I saw this happen:

warning: usage of `contains_key` followed by `insert` on a `HashMap`
  --> src/main.rs:18:9
   |
18 | /         if !self.cache.contains_key(&k) {
19 | |             let Some(foo) = Self::calc(k) else {
20 | |                 return None;
21 | |             };
22 | |
23 | |             self.cache.insert(k, foo);
24 | |         }
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_entry
   = note: `#[warn(clippy::map_entry)]` on by default
help: try
   |
18 ~         self.cache.entry(k).or_insert_with(|| {
19 +             let Some(foo) = Self::calc(k) else {
20 +                 return None;
21 +             };
22 + 
23 +             foo
24 +         });
   |

Version

rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: x86_64-pc-windows-msvc
release: 1.76.0
LLVM version: 17.0.6

Additional Labels

No response

[0vercl0k]

Thank you @y21 for fixing this 🙏

Cheers