`unnecessary_lazy_evaluations` fix causes a panic when used with `Range::contains()`

[rezzubs]

Summary

I encountered this error during Advent of Code where Clippy suggests to change an Option::then to then_some. I used std::ops::Range::contains() to make sure the closure passed to then is safe to call. It can panic when the number is not within the range and should not be evaluated eagerly.

I'm not sure if a lint like this could possibly take outside context into account but it's a false positive nonetheless.

Lint Name

unnecessary_lazy_evaluations

Reproducer

The version with then

use std::ops::Range;

fn main() {
    let map = Mapping {
        range: 10..20,
        offset: 10,
    };
    
    println!("{:?}", map.get(10));
    println!("{:?}", map.get(9));
}

struct Mapping {
    range: Range<u64>,
    offset: u64,
}

impl Mapping {
    fn get(&self, seed: u64) -> Option<u64> {
        // This can panic when replaced with then_some().
        self.range.contains(&seed).then(|| {
            seed - self.offset
        })
    }
}

works as expected.

The version with then_some

use std::ops::Range;

fn main() {
    let map = Mapping {
        range: 10..20,
        offset: 10,
    };
    
    println!("{:?}", map.get(10));
    println!("{:?}", map.get(9)); // panics here
}

struct Mapping {
    range: Range<u64>,
    offset: u64,
}

impl Mapping {
    fn get(&self, seed: u64) -> Option<u64> {
        self.range.contains(&seed).then_some({
            seed - self.offset
        })
    }
}

Version

rustc 1.74.0 (79e9716c9 2023-11-13)
binary: rustc
commit-hash: 79e9716c980570bfd1f666e3b16ac583f0168962
commit-date: 2023-11-13
host: x86_64-unknown-linux-gnu
release: 1.74.0
LLVM version: 17.0.4

Additional Labels

No response

[y21]

#11002 should have fixed this. Can you try updating to the latest nightly and see if that fixes it?

[y21]

Closing as fixed, the reproducer in the description no longer emits a warning. The fix should be in all channels now (stable, beta, nightly).