[ruby-core:106028] [Ruby master Bug#18296] Custom exception formatting should override `Exception#full_message`.

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


While initially thinking about this issue I kind of forgot `full_message` includes the backtrace (the name doesn't make that super obvious).

One thing to address is how `did_you_mean/error_highlight` can insert additional output after the original message, but before the backtrace (for `order: :top` the default).
`super(highlight: highlight, **options) + "\nextra"` wouldn't work as that would put it after the end of the backtrace.
Also the backtrace can be before the message with `order: :bottom`.

I think one possibility is:
```ruby
module DidYouMean
  module Formatter
    def full_message(highlight: Exception.to_tty?, order: :top, backtrace: true, did_you_mean: true, **options)
      super if backtrace == :only || !did_you_mean
      message = super(highlight: highlight, **options, backtrace: false)
      backtrace = super(highlight: highlight, **options, backtrace: :only)
      if order == :top
        "#{message}\nextra stuff\n#{backtrace}"
      else
        "#{backtrace}#{message}\nextra stuff\n"
      end
    end
  end
end

Exception.prepend DidYouMean::Formatter
```

If we want to avoid the `if order == :top`, we'd need a new method that's basically the same as `full_message` but always without the backtrace,
and that method would be called from `Exception#full_message` (needs at least the `highlight:` keyword, best to forward all keywords).
(then we don't need the `backtrace:` kwarg for `full_message` of course)
Like:
```ruby
module DidYouMean
  module Formatter
    def message_with_extras(highlight: Exception.to_tty?, did_you_mean: true, **options)
      super unless did_you_mean
      message = super(highlight: highlight, **options)
      "#{message}\nextra stuff\n"
    end
  end
end

Exception.prepend DidYouMean::Formatter
# And Exception#full_message would call #message_with_extras` and not #message.
# The default Exception#message_with_extras just calls #message.
```

I'm not sure which is better, WDYT?

Do we want to also have "message with extras" for causes' messages in the `full_message` output?
If so the #message_with_extras approach seems better, because that's quite difficult to make it work with `full_message`.

Of course `#message_with_extras` is just a name, maybe we can find a better one.

(We could also add kwargs to Exception#message but that would be incompatible with all definitions of `message` which don't accept any argument, hence doesn't seem possible.)

----------------------------------------
Bug #18296: Custom exception formatting should override `Exception#full_message`.
https://bugs.ruby-lang.org/issues/18296#change-94615

* 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_highlight` 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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