Iterator performance regression 1.81 -> 1.82

Code

I tried this code:

use std::time::Instant;

fn main() {
    let start = Instant::now();

    (0..=1000)
    .flat_map(|a| {
        (0..=1000).flat_map(move |b| {
            (0..=1000)
                .filter(move |&c| a * a + b * b == c * c && a + b + c == 1000)
                .map(move |c| (a, b, c))
        })
    })
    .for_each(|(a, b, c)| {
        println!("a: {} b: {} c: {}", a, b, c);
    });

    let duration = start.elapsed();
    println!("{} seconds", duration.as_secs_f64());
}

I expected to see this happen: code behaves the same across compiler versions

Instead, this happened: code runs 80% slower when compiled with Rust 1.82+

Version it worked on

It most recently worked on: Rust 1.81

Version with regression

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: x86_64-pc-windows-msvc
release: 1.82.0
LLVM version: 19.1.1

More details in this post

https://internals.rust-lang.org/t/iterator-performance-regression-1-81-1-82/22473

Start by reproducing the supplied iterator example with Rust 1.81 and 1.82 on the reported Windows target, then read the linked Internals post for investigation context. Compare compiler behavior across those versions and narrow down the source of the regression; done means the example no longer suffers the reported slowdown in the affected release.