[ruby-core:98767] [Ruby master Feature#16955] A new mode `Warning[:keyword_into_rest_arg] = true` for 2.7

Issue #16955 has been updated by Eregon (Benoit Daloze).


jeremyevans0 (Jeremy Evans) wrote in #note-3:
> Care must be taken when doing so, especially in master when doing so is an API break.

Could you clarify that?
Do you mean calling `foo(1, 2, {option: true})` (seems very unlikely) / `foo(1, 2, h)` (more realistic) would no longer work?

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Feature #16955: A new mode `Warning[:keyword_into_rest_arg] = true` for 2.7
https://bugs.ruby-lang.org/issues/16955#change-86124

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
----------------------------------------
## Problem

Please see #16954.  This ticket is for discussing the second solution.

## Proposal

The problem is caused by automatic conversion from keywords to positional arguments, especially, when the keyword is implicitly absorbed into rest arguments.

```ruby
def app_proxy(*args)
  # args.last is a normal (non-ruby2_keyword-flagged) Hash, { :k => 42 }, converted from the keyword "k: 42"
  ...
end

app_proxy(k: 42)     # this call is troublesome
```

So I propose a new mode `Warning[:keyword_into_rest_arg] = true` to notice such an event.

```ruby
1: Warning[:keyword_into_rest_arg] = true
2:
3: def app_proxy(*args)
4: end
5:
6: app_proxy(k: 42)

# t.rb:6: warning: The keyword argument is absorbed into the rest parameter
# t.rb:3: warning: The called method `app_proxy' is defined here
```

An experimental patch is [here](https://gist.githubusercontent.com/mame/d5fbddc94a4a02b708653d1948836bb9/raw/cb89be7eaa20eb57be8811d3617663d7459e80d7/keyword_into_rest_arg.patch).  (The new mode is enabled by default in the patch just to make the experiment easy.)

## Important note: false positives

Unfortunately, this brings many false positives.  Typically, the following code is completely innocent and will work great in 3.0, but is warned:

```ruby
1: def foo(*args)
2:   opt = args.pop if args.last.is_a?(Hash)
3:   ...
4: end
5:
6: foo(k: 42)

# t.rb:6: warning: The keyword argument is absorbed into the rest parameter
# t.rb:1: warning: The called method `foo' is defined here
```

By an experiment with `make test-all`, [tons of warnings are observed](https://gist.github.com/mame/d5fbddc94a4a02b708653d1948836bb9).  Note that duplicated warings are already filtered out.  (Some tests generates a code dynamically, which makes the duplication filter not work.)

Many warnings occur within standard libraries.  I think that it is possible to fix these warnings, but we need to introduce tons of fixes in stdlib of 2.7.2.  I'm afraid if it is acceptable.

However, @eregon said that it might be still usable with combination of a dedicated warning filter mechanism (such as [deprecation_toolkit](https://github.com/Shopify/deprecation_toolkit)).

## Discussion points

* Is this really helpful?
* We need to add this change into Ruby 2.7.2.  Is it acceptable?  (Matz had already agreed with this change.  But after that, it turned out that it brings so many false positives.)
* Is the name `:keyword_into_rest_arg` okay?
* Should the commandline option `-W:keyword_into_rest_arg` be added?




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