[ruby-core:97533] [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 Eregon (Benoit Daloze).


Thanks for explaining your thoughts in details.

> The last reason is the migration path. Currently (some) Rails core contributors use the recent master (which raises errors instead of warnings) to detect/fix keyword argument warnings. Warnings do not give them enough information to fix. The errors from the head do. Once we accept the proposal, even 3.0 would warn, not raise errors.

That's a nice acknowledgment that the delegation warnings in 2.7.0 are imprecise, which I argued for a lot in this thread.
This proposal would provide precise warnings, which I believe would be even more helpful to migrate than the hard errors of 3.0.

This approach would delay the first and only migration of delegation code to around 3.4.
With the current warnings/errors though, we enforce changing delegation code twice:
* once in 2.7 using the temporary workaround of `ruby2_keywords`,
* change again in 3.4 where most likely `*args, **kwargs`-delegation should be used instead.

So in both cases delegation will be fully migrated in 3.4 (i.e., when people drop support for Ruby 2.x).

With this approach we'd only need to change delegation code once in 3.4, and we wouldn't need temporary changes around all methods delegating keyword arguments.

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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-84694

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
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!

PR: https://github.com/ruby/ruby/pull/2853

---

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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