`@[csimp]` can be used to smuggle axioms and `unsafe` into a proof

Prerequisites

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Description

@[csimp] is sold as the "safe" version of @[implemented_by], however it is just as unsafe due to axioms used in its proof not being propagated through native_decide when the lemma is used to justify a definition replacement.

Context

#lean4 > Axioms used by csimp are not reported @ 💬

Steps to Reproduce

def one := 1
def bad_one := 2

axiom cheating : False

@[csimp] theorem oops : one = bad_one := cheating.elim -- or `sorry`

theorem ohno : one = 2 := by native_decide

#print axioms ohno

Expected behavior: 'ohno' depends on axioms: [Lean.ofReduceBool, cheating] (or [Lean.ofReduceBool, sorryAx])
Actual behavior: 'ohno' depends on axioms: [Lean.ofReduceBool]

Versions

4.17.0

Additional Information

The docstring for Lean.reduceBool warns me that implemented_by and extern can cause soundness issues; but it does not warn me that csimp can do the same.

Impact

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