[ruby-core:106009] [Ruby master Bug#18296] Custom exception formatting should override `Exception#full_message`.

Issue #18296 has been updated by Eregon (Benoit Daloze).


I think this is a clean and simple proposal/fix, which enables users to choose whether they want that extra information on a per-call to #full_message-basis.

As a side effect it also enables DidYouMean and ErrorHighlight to e.g. look at the value of the `highlight:` keyword argument value and decide whether to use ANSI escape sequences.

cc @mame @yuki24 I think this is the best way to integrate ErrorHighlight and DidYouMean cleanly.
Overriding `message` was always a shortcut/hack (because it doesn't let the user choose what they want, only globally via `--disable` which is not good enough), and I believe this is the way to fix it.

----------------------------------------
Bug #18296: Custom exception formatting should override `Exception#full_message`.
https://bugs.ruby-lang.org/issues/18296#change-94562

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
After discussing with @eregon, we came to the conclusion that the current implementation of `did_you_mean` and `error_highlighter` could avoid many issues by using `Exception#full_message`.

We propose to introduce a more nuanced interface:

```ruby
class Exception
  def full_message(highlight: bool, order: [:top or :bottom], **options)
    # ...
  end
end

module DidYouMean
  class Formatter
    def full_message(highlight:, did_you_mean: true, **options)
      buffer = super(highlight: highlight, **options).dup
      buffer << "extra stuff"
    end
  end
end

module ErrorHighlight
  class Formatter
    def full_message(highlight:, error_highlight: true, **options)
      # same as above
    end
  end
end
```



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