Issue #16993 has been reported by marcandre (Marc-Andre Lafortune).

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Feature #16993: Sets: from hash keys using Hash#key_set
https://bugs.ruby-lang.org/issues/16993

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
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To create a set from hash keys currently implies a temporary array for all keys, rehashing all those keys and rebuilding a hash. Instead, the hash could be copied and its values set to `true`.

```ruby
h = {a: 1}
# Now:
Set.new(h.keys) # => Set[:a]
# After
h.key_set # => Set[:a], efficiently.
```




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