Issue #16989 has been updated by Eregon (Benoit Daloze).


IMHO there is no need for a literal Set notation (which would anyway restrict the type of keys, etc).

> Some of the upcoming feature requests would be easier (or only possible) to implement were Sets builtin.

@marcandre Which feature requests except having a literal notation (#16994) require Set to be builtin? I think only #16993.
I think we could make good progress here by addressing the other feature requests first.

Also moving Set to core makes might make it more complicated to backport to older Rubies.
(Unfortunately Set is only a default gem in 3.0, but it's probably easy enough to require some Set additions with a different require name)

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Feature #16989: Sets: need 
https://bugs.ruby-lang.org/issues/16989#change-87357

* Author: marcandre (Marc-Andre Lafortune)
* Status: Assigned
* Priority: Normal
* Assignee: knu (Akinori MUSHA)
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I am opening a series of feature requests on `Set`, all of them based on this usecase.

The main usecase I have in mind is my recent experience with `RuboCop`. I noticed a big number of frozen arrays being used only to later call `include?` on them. This is `O(n)` instead of `O(1)`.

Trying to convert them to `Set`s causes major compatibility issues, as well as very frustrating situations and some cases that would make them much less efficient.

Because of these incompatibilities, `RuboCop` is in the process of using a custom class based on `Array` with optimized `include?` and `===`. `RuboCop` runs multiple checks on Ruby code. Those checks are called cops. `RuboCop` performance is (IMO) pretty bad and some cops  currently are in `O(n^2)` where n is the size of the code being inspected. Even given these extremely inefficient cops, optimizing the 100+ such arrays (most of which are quite small btw) gave a 5% speed boost.

RuboCop PRs for reference: https://github.com/rubocop-hq/rubocop-ast/pull/29
https://github.com/rubocop-hq/rubocop/pull/8133

My experience tells me that there are many other opportunities to use `Set`s that are missed because `Set`s are not builtin, not known enough and have no shorthand notation.

In this issue I'd like to concentrate the discussion on the following request: `Set`s should be core objects, in the same way that `Complex` were not and are now. Some of the upcoming feature requests would be easier (or only possible) to implement were `Set`s builtin.



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