[ruby-core:96554] [Ruby master Feature#16463] Fixing *args-delegation in Ruby 2.7: ruby2_keywords semantics by default in 2.7.1

Issue #16463 has been updated by rafaelfranca (Rafael Fran軋).


This solution would be my first choice if for some reason Rails needs to ask users to change their code to use `ruby2_keywords`.

In our current problem, users define Mailers and those mailers have actions, that are public methods in the mailer class. The way Rails dispatch the actions uses a few levels of delegations and all those delegations can pass keyword arguments. If it was not possible to remove the warnings without asking users to mark their actions using `ruby2_keywords` I was going to use the `method_added` hook on the mailer base class to automatically call `ruby2_keywords` in all public methods of a mailer.

I'm not sure yet if that solution is needed at all, but I think that by marking all methods with `ruby2_keywords` in 2.7 we are delaying the warnings and all code changes to Ruby 3. Maybe that is a good thing and more inline with the stability of Ruby releases but, as a library maintainer, I don't mind a rocky path in order to arrive to a good destination. And, I prefer to do that sooner than later.

We are being pushing this style of changes in Rails for a while. If the Rails 4 to Rails 5 upgrade path taught me something is that the more you delay those breaking change, the more people will delay fixing them. It took us more than 1 year to upgrade the Shopify application from Rails 4.2 to Rails 5.0 because we deprecated behavior in Rails 4.0 but only removed it in Rails 5.0. Instead of fixing the deprecated behavior through the 3 years between Rails 4.0 and 5.0 we left everything to be done only when it was needed and that had a huge price.

I think by delaying the warnings we will fall in the same trap we did with Rails 5.0. People will only fix the warnings after Ruby 3 is released. If we think that Ruby 3 will have more changes that will require user to change their code, we are just piling up more work to the users at the same time, what will delay their upgrades and give the impression that upgrading to Ruby 3 is too hard. If we split the path to upgrade to Ruby 3 in many bumps in our road, it will be still bumpy, but it will at least be passable.

I'm being very vocal against this change not because it is annoying, but because it is without precedents, at least while I'm in the community, and incoherent with other decisions in the same release. I don't remember any Ruby upgrade since Ruby 1.9 that caused so much trouble to application and library developers, and I didn't expected this kind of decision from the Ruby Core team, given their track record of stability. But, I'm used to this kind of decision, since I'm taking decisions like this in the Rails Core team for years, and I'm happy we are choosing to go by this path now.

In my opinion, if a bumpy road to a amazing destination is what we want, we should be coherent. In the same release we pushed the community to change a lot of code to add `ruby2_keywords` for the sake of consistency we also [reverted a performance related change because a few gems would need to be updated](https://bugs.ruby-lang.org/issues/16150). Not to say about the frozen_string_literal comments that we are pushing down to the community for a while and now that we were arriving closer to the date where finally we could drop that comment we just made it obligatory for maybe more 10 years.

In summary, I prefer if we make all changes necessary to the keyword arguments split sooner than later but that is coming from someone with a different opinion about stability when compared with the Ruby Core.

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Feature #16463: Fixing *args-delegation in Ruby 2.7: ruby2_keywords semantics by default in 2.7.1
https://bugs.ruby-lang.org/issues/16463#change-83486

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Ruby 2.7.0 is out.
It aims to warn for every keyword argument change that will happen in Ruby 3.0.
Most warnings are useful: adding `**`, etc is needed to not break code when migrating to 3.0.

Ruby 2.7 also aims at remaining compatible with 2.6.
However there is a big breaking change here: __`*args`-delegation broke in Ruby 2.7 for keyword arguments__.
The workaround is adding `ruby2_keywords` to the method/block receiving the keywords arguments to delegate later on.

But is it needed or useful at all to require everyone to add `ruby2_keywords` in many places of their codebase?
And for rubyists to get major headaches as to why `*args`-delegation broke and instead has strange semantics in Ruby 2.7?
Was it useful to break delegation in Ruby 2.7?

I think not, and here I propose a solution to keep delegation in 2.7 compatible with 2.6 (just use `*args` as before).

---

First I'll introduce some context.
The end goal is to have [separation of positional and keyword arguments](https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/).
However, this will not happen in 3.0, because as long as `ruby2_keyword` exist, the separation will only be partial.
For example, `foo(*args)` should only pass positional arguments, never keyword arguments, but this can only be guaranteed once `ruby2_keyword` is removed.

The plan to get there, as far as I heard and imagine it is:
* In Ruby release 3.warn (around Ruby 2.7 EOL, maybe 3.3?), warn for every usage of `ruby2_keywords`, mentioning it should be replaced by `*args, **kwargs`-delegation (or `...`, but that's severely restricted currently: #16378). `*args, **kwargs`-delegation is only correct in Ruby 3.0+ so at that point Ruby 2.x support needs to be dropped, or a version check be used.
* In Ruby release 3.clean (that is 3.(warn+1), maybe 3.4?), remove `ruby2_keywords`. At that point, the separation of positional and keyword arguments is finally achieved. `foo(*args)` will always mean "pass only positional arguments". Everytime keyword arguments are passed it will be explicit (`foo(**kwargs)` or `foo(key: value)`), no more magic and a clean separation.

So no matter what, to get the clean separation we'll have to wait many (5?) years for Ruby 3.clean, and delegation code will need to change in 3.warn.

But right now, we broke delegation in 2.7 and require to add `ruby2_keywords` (which means __changing twice delegation code__ in this period) for seemingly little to no benefit.

---

My proposition is to simply use ruby2_keywords semantics for all methods and blocks in Ruby 2.7 (and until version 3.warn). This would be compatible with Ruby 2.6 and before.
This means, no explicit `ruby2_keywords` anywhere, no need to change anything for delegation to work in Ruby 2.7, 3.0, ... until Ruby 3.clean.

Importantly, it means __only change delegation code once__ and __Ruby 2.0 until Ruby.warn keep `*args`-delegation compatible and working__.

The semantics of that are (same as if `ruby2_keywords` was applied to all methods):
* When passing keyword arguments syntactically (using either `foo(**kwargs)` or `foo(key: value)`) to a method not accepting keyword arguments (e.g., `def m(*args)`), flag the keyword arguments Hash as "keyword arguments".
* Whenever calling a method with a `*rest` argument and no keyword arguments (e.g., `foo(*args)`), if the last argument is flagged as "keyword arguments", pass them as keyword arguments.
  If the called method doesn't accept keyword arguments, pass the Hash as positional argument and keep the "keyword arguments" flag.

That way, code like this just keeps working:
```ruby
def target(*args, **kwargs)
  [args, kwargs]
end

def delegate(*args, &block)
  target(*args, &block)
end

target(1, b: 2) # => [[1], {b: 2}] in Ruby 2 & 3
delegate(1, b: 2) # => [[1], {b: 2}] in Ruby 2 & 3, no warning in 2.7 because {b: 2} is passed as keyword arguments to target
```

And also if `args` is stored somewhere or delegated multiple levels down.

Do we lose anything by not marking delegation methods with `ruby2_keywords`?
I think we lose nothing, and we gain a lot (compatibility and avoiding needless ugly changes).
In Ruby 3.warn we can easily warn for every case that passes keyword arguments using `foo(*args)` and even have a debug mode telling where the Hash was flagged as a "keyword Hash".

Thoughts?
Should we fix delegation in Ruby 2.7 .. Ruby 3.warn so it works again and not needlessly break Ruby code? I believe YES!

---

P.S.: I actually proposed this idea on the ruby-core Slack on 13th December, but got just one response from @jeremyevans0:

> Me: If we applied `ruby2_keywords` automatically on all methods, would `*args`-delegation just keep working in 2.7 and later? I think the fundamental issue with kwargs changes is that we break *args by changing its meaning, in a way it no longer works to delegate "all arguments except block". Probably almost every method that takes `(*args)` and then call some methods with `*args` intents to pass positional and kwargs as-is, no matter the Ruby version. If we could save this pattern we'd make the transition much nicer.
> Jeremy: I worked on a branch with `ruby2_keywords`  behavior by default (for all methods taking `*args`, not just those that delegate `*args` inside the method: https://github.com/jeremyevans/ruby/tree/ruby2_keywords-by-default . I don't recommend that approach, as it is much more likely to result in a keyword-flag hashed being created to a method where the hash should be treated as positional.
> Me: Does it matter if the Hash is flagged and passed to a method not taking kwargs? It would still be the same behavior, no?
> Jeremy: You can end up with the hash being passed as keywords when you expect it to be passed as non-keywords.  It's not safe in general unless you know the method will be used for argument delegation.

Jeremy's concern is sometimes you might want `foo(*args)`, with `args[-1]` a Hash with a "keyword arguments" flag, to pass as positional to `def foo(*args, **kwargs)`.
However, that seems extremely unlikely to me, and not worth breaking delegation in Ruby 2.7.
To have the "keyword arguments" flag, the Hash must have been passed originally as keyword arguments. It sounds unlikely you would then want to pass it as positional to a method taking keyword arguments.
If you do want that, it's always possible to do `foo(*args, **{})`, which also works in Ruby 2.6 (and before).



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