`@[local ext]` should not generate a global `ext_iff` lemma

[YaelDillies]

Prerequisites

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• [x] Check that your issue is not already filed: https://github.com/leanprover/lean4/issues
• [x] Reduce the issue to a minimal, self-contained, reproducible test case. Avoid dependencies to Mathlib or Batteries.
• [x] Test your test case against the latest nightly release, for example on https://live.lean-lang.org/#project=lean-nightly (You can also use the settings there to switch to “Lean nightly”)

Description

Tagging a structure Foo with @[local ext] generates global Foo.ext and Foo.ext_iff. It seems better to generate them as private lemmas instead.

Context

Zulip

Steps to Reproduce

In file MWE:

@[local ext]
structure Foo where
  toNat := Nat

In another file,

import MWE

#check Foo.ext
#check Foo.ext_iff

Expected behavior: The #checks fail

Actual behavior: They succeed

Versions

4.14.0-rc2 Ubuntu

Impact

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[adomani]

3643 seems related and closed.