[ruby-core:99435] [Ruby master Feature#14844] Future of RubyVM::AST?

[eregontp@gmail.com]

Issue #14844 has been updated by Eregon (Benoit Daloze).


[RBS](https://github.com/ruby/rbs) is using `RubyVM::AbstractSyntaxTree` for `rbs prototype`.
RBS is an official project under the Ruby organization, so I believe it should be able to fully work on other Ruby implementations too ([issue](https://github.com/ruby/rbs/issues/348) about this in RBS).
That is impossible as long as `AbstractSyntaxTree` is under `RubyVM`, or as long as RBS uses `RubyVM::AbstractSyntaxTree` with no fallback.

So, I think it is time to move `AbstractSyntaxTree` outside of RubyVM.
Otherwise, other Ruby implementations will have no choice but to define `RubyVM` too, which everyone seems to agree is unwanted as RubyVM is (at least currently) meant CRuby-only.

I see three ways forward:
* Move `AbstractSyntaxTree` under `ExperimentalFeatures` (#15752), so it is still experimental but at least other Ruby implementations can implement it too.
* Make `AbstractSyntaxTree` stable, and move it under a stable namespace (maybe just `::AbstractSyntaxTree`?)
* Change the meaning of RubyVM so it is not CRuby-specific but also exists on other Ruby implementations. Many people don't know that `RubyVM` means experimental, so `ExperimentalFeatures` seems much clearer.

I prefer the first option, but any of the 3 unblocks the situation (which is also explained in #15752).

Can we pick one?
@matz can you decide?

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Feature #14844: Future of RubyVM::AST? 
https://bugs.ruby-lang.org/issues/14844#change-86890

* Author: rmosolgo (Robert Mosolgo)
* Status: Open
* Priority: Normal
* Assignee: yui-knk (Kaneko Yuichiro)
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Hi! Thanks for all your great work on the Ruby language. 

I saw the new RubyVM::AST module in 2.6.0-preview2 and I quickly went to try it out. 

I'd love to have a well-documented, user-friendly way to parse and manipulate Ruby code using the Ruby standard library, so I'm pretty excited to try it out. (I've been trying to learn Ripper recently, too: https://ripper-preview.herokuapp.com/, https://rmosolgo.github.io/ripper_events/ .)

Based on my exploration, I opened a small PR on GitHub with some documentation: https://github.com/ruby/ruby/pull/1888

I'm curious though, are there future plans for this module? For example, we might: 

- Add more details about each node (for example, we could expose the names of identifiers and operators through the node classes)
- Document each node type 

I see there is a lot more information in the C structures that we could expose, and I'm interested to help out if it's valuable. What do you think? 



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