1.92 regression: Behavior change in PartialEq impl of BTreeSet?

Test failure found in the beta crater run:

[INFO] [stdout] thread 'atom::tests::test_possible_states_eq' (22) panicked at src/value.rs:63:18:
[INFO] [stdout] can't compare values String("a") and Number(0)

https://crater-reports.s3.amazonaws.com/beta-1.92-4/beta-2025-11-10/gh/ttaubert.tlc/log.txt

Minimized reproducer:

use std::cmp::Ordering;
use std::collections::BTreeSet;

#[derive(Debug)]
pub(crate) enum Value {
    One,
    Two(i32),
}

impl Ord for Value {
    fn cmp(&self, other: &Value) -> Ordering {
        match (self, other) {
            (Value::One, Value::One) => Ordering::Equal,
            (Value::Two(x), Value::Two(y)) => x.cmp(y),
            _ => panic!("can't compare values {:?} and {:?}", self, other),
        }
    }
}

impl PartialOrd for Value {
    fn partial_cmp(&self, other: &Value) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl PartialEq for Value {
    fn eq(&self, other: &Value) -> bool {
        match (self, other) {
            (Value::One, Value::One) => true,
            (Value::Two(x), Value::Two(y)) => x == y,
            _ => panic!("can't compare values {:?} and {:?}", self, other),
        }
    }
}

impl Eq for Value {}

fn main() {
    let lhs = BTreeSet::from([Value::One]);
    let rhs = BTreeSet::from([Value::Two(1), Value::Two(2)]);
    let _ = lhs == rhs;
}

This code runs without errors on stable, but panics on beta.

It's reasonable to say that this code should have panicked in the first place, but still, it's strange that the behavior changed. So, maybe this needs some investigation on what happened?

@yotamofek

Bisection results:

searched toolchains 064cc81354a940e297a1be4dfa9e26759c8431be through f435972085b697a1ece8ee6a1ac76efff8d1df7b


********************************************************************************
Regression in 21a13b8864a7dd614e9a96afd57b58c7fcf0bd6b
********************************************************************************

Attempting to search unrolled perf builds
Found commits ["1d14c616", "ba8f00d9", "196d798a", "69e8b5e1", "3ac23352"]
installing 1d14c61677c25ed1ce05d1460b27ffdd93e49fce
uninstalling 1d14c61677c25ed1ce05d1460b27ffdd93e49fce
installing ba8f00d9c8f62c468d9e3afc4f48dff026e746da
uninstalling ba8f00d9c8f62c468d9e3afc4f48dff026e746da
installing 196d798afa1c6d8ef56fd0c50da732bfaafcebd6
uninstalling 196d798afa1c6d8ef56fd0c50da732bfaafcebd6
installing 69e8b5e177df0c8d4da415cd457f6f129a3da461
uninstalling 69e8b5e177df0c8d4da415cd457f6f129a3da461
installing 3ac23352136cc99fd425c8ca6c40235197979e56
uninstalling 3ac23352136cc99fd425c8ca6c40235197979e56
ERROR: none of the toolchains satisfied the predicate

Seems like it regressed in #147101. cc @yotamofek

My guess is that, comparing two BTreeSets with different sizes used to not look at the set contents if the number of elements are unequal. In beta, it now looks at the actual set contents without checking the number of elements first. So, this is likely a performance regression, even for correctly-written code.

@jdonszelmann

cc @jdonszelmann also as reviewer

Auto merge of #149125 - zachs18:btreemap-eq-perf, r=workingjubilee

Auto merge of rust-lang#149125 - zachs18:btreemap-eq-perf, r=workingj…

Auto merge of #149125 - zachs18:btreemap-eq-perf, r=workingjubilee

Auto merge of rust-lang#149125 - zachs18:btreemap-eq-perf, r=workingj…

Auto merge of #149125 - zachs18:btreemap-eq-perf, r=workingjubilee

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1.92 regression: Behavior change in `PartialEq` impl of `BTreeSet`? #149122

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1.92 regression: Behavior change in PartialEq impl of BTreeSet? #149122

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theemathas commented on Nov 20, 2025

theemathas commented on Nov 20, 2025

workingjubilee commented on Nov 20, 2025

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1.92 regression: Behavior change in `PartialEq` impl of `BTreeSet`? #149122