[ruby-core:98759] [Ruby master Feature#16954] A new mode `Warning[:deprecated] = :error` for 2.7

Issue #16954 has been updated by decuplet (Nikita Shilnikov).


For the record, I used Jeremy's gem to deal with all 2.7 warnings, it saved me a lot of time. My typical setup in tests looks like
```ruby
Warning.ignore(/some-gem-issuing-warnings-i-don-t-care-about/)
Warning.ignore(/another-gem/)
Warning.process { |w| raise RuntimeError, w } unless ENV['NO_RAISE_ON_WARNING']
```

----------------------------------------
Feature #16954: A new mode `Warning[:deprecated] = :error` for 2.7
https://bugs.ruby-lang.org/issues/16954#change-86116

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

(This is [what I already wrote in rubyonrails discourse](https://discuss.rubyonrails.org/t/new-2-7-3-0-keyword-argument-pain-point/74980/43) but I repeat this to make this discussion easy.)

The plan of 3.0 keyword change (#14183) will prohibit the automatic conversion from the last Hash positional argument to keyword arguments, but will keep one from keywords to a positional argument.  This asymmetry made it difficult to fix code of argument forwarding.  @kamipo and @matsuda, Rails developers who did much work to support the 3.0 keyword change, found the following (simplified) case where a warning is emitted within Rails code although Rails is innocent.

Rails code:
```ruby
def target(**opt)       # warning: The called method `foo' is defined here
end

ruby2_keywords def lib_proxy(*args)
  target(*args)         # warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
end
```

User application code (or gem code other than Rails):
```ruby
def app_proxy(*args) # actually, ruby2_keywords is required here
  lib_proxy(*args)
end

app_proxy(k: 42)
```

The user code attempts to pass keywords to Rails' `target` method via argument forwarding methods `app_proxy` and `lib_proxy`. Rails appropriately annotates `lib_proxy` with ruby2_keywords, but user code does not yet for `app_proxy`. Calling `app_proxy` converts the keywords to a normal Hash, so the final `target` method accepts them as a positional Hash, which leads to the warning. Unfortunately, the warning points only Rails code. So, the user sends a bug report to Rails and there is no good way for Rails developer to diagnose the issue.

@kamipo said that actual cases include rails/rails [#39562](https://github.com/rails/rails/issues/39562) and [#39227](https://github.com/rails/rails/issues/39227).

## Proposal

We can think of two solutions for this issue:

* Make the warning print a full backtrace (including `app_proxy` in the above case) so that they can debug the issue.
* Warn an event of automatic conversion from keywords to a positional one,  This will point the call of `app_proxy`, though it brings false positives.

Focus on the first solution in this ticket. (I'll create another ticket for the second one.)


There are two (orthogonal) approaches to make the first solution:

1. `Warning[:deprecated] = :error`, to make the warning into an error which produces a full backtrace (and stops the execution).
2. `Warning[:deprecated] = :debug`, to make the warning print a full backtrace (and continues the execution).

The former one is like `Thread.abort_on_exception = true`, and the latter is like `Thread.report_on_exception = true`.

A patch is not availrable yet.  (I'm asking @nobu to create a patch.)

## Discussion points

* We need to add this change into Ruby 2.7.2.  Is it acceptable?  (Matz has already agreed with this change, but we need to get approval from @nagachika, the 2.7 branch maintainer.)
* Which approach is preferrable?  Both?
* Are the names `:error` and `:debug` okay?  Or `:abort` and `:report` like `Thread.*_on_exception`?  Or other names?
* Should the commandline option `-W:deprecated` support the new modes?  Like `-W:deprecated=error`?



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