`is_bi_equal` use of BinderInfo-unaware hashes causes many collisions (a.k.a. timeout when saving oleans)

Prerequisites

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  • Reduced the issue to a self-contained, reproducible test case.

Description

@digama0's testcase from https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/.22saving.20olean.22.3F, reproduced below, fails to produce oleans with lean --make -T100000 test.lean:

import algebra.field.basic
import algebra.order.ring

set_option old_structure_cmd true

class linear_ordered_semifield (α : Type*)
  extends linear_ordered_semiring α, semifield α

class linear_ordered_field (α : Type*) extends linear_ordered_comm_ring α, field α

I investigated the problem a bit and found out that serializing those objects involves a lot of comparisons which increment the heartbeat counter. In particular, a lot of time is spent comparing deep expressions for BinderInfo-aware equality; the expression hash used in the comparison does not include BinderInfo data, and the above example seems to involve expressions which all have the same hash despite being different for the purposes of is_bi_equal.

Since there are only two comparison modes, a possible fix would be to have two hashes, one for each choice of CompareBinderInfo. I can't tell if this is worth the extra time and memory.

(Note: The above example may hit #748 on a debug build. Those issues may or may not be related.)

Steps to Reproduce

1. Run lean --make -T100000 test.lean on the above file

Expected behavior: Valid oleans should be output

Actual behavior: Empty oleans

Reproduces how often: 100%

Versions

Lean (version 3.45.0, commit 22b09be35ef6, Release)

Reproduce the failure with the supplied test.lean command and inspect src/util/object_serializer.h, src/kernel/expr_eq_fn.cpp, and the BinderInfo-aware comparison and hashing paths referenced in the report. Done means the testcase produces valid, non-empty oleans without the timeout or excessive comparisons described.