[ruby-core:107052] [Ruby master Feature#18438] Add `Exception#additional_message` to show additional error information

Issue #18438 has been updated by Eregon (Benoit Daloze).


@nobu How so? Do you have an example?

The responsibilities for `description` are to call `super` and include it in the return value. That's it, nothing else.

The same goes for `additional_message` and is necessary if there are multiple definitions of `additional_message` in the ancestors.
Except it's less clear for `additional_message` because the default `additional_message` returns `""`.
And so how to avoid an extra `\n` before? That'd probably means harder to write a correct `additional_message` => `msg = super; msg.empty? ? extra : "#{msg}\n#{extra}"`.
This is not a problem for `description`, where `super` always returns a non-empty String (unless the original message is empty, but that's then basically a bug of who raised it) => `"#{super}\n#{extra}`.

The class, backtraces and causes are added later by `full_message`, because there is no value to add them before and it's not trivial to add them (the class is always shown on the first line, even for a multi-line message, that's already current behavior).

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Feature #18438: Add `Exception#additional_message` to show additional error information
https://bugs.ruby-lang.org/issues/18438#change-95883

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

I'd like to introduce a method `Exception#additional_message`, and let the Ruby error printer show it after `Exception#message`.

```ruby
class MyError < StandardError
  def message = "my error!"
  def additional_message = "This is\nan additional\nmessage"
end

raise MyError
```

```
$ ./miniruby test.rb
test.rb:6:in `<main>': my error! (MyError)
| This is
| an additional
| message
```

PoC implementation: https://github.com/ruby/ruby/pull/5351

## Motivation

At the present time, did_you_mean and error_highlight overrides `Exception#message` to add their suggestions.

```ruby
begin; 1.time; rescue NoMethodError; pp $!.message; end
#=> "undefined method `time' for 1:Integer\n" +
#   "\n" +
#   "  1.time\n" +
#   "   ^^^^^\n" +
#   "Did you mean?  times"
```

This implementation approach has a practical problem. Because it changes the return value of `Exception#message`, it breaks a test that checks the return value of `Exception#message`.
Though such a test is never recommended, I encountered some actual cases when creating error_highlight. See the change of tests of minitest as a typical example: https://github.com/seattlerb/minitest/pull/880/files

Currently, error_highlight shows hint information only for NoMethodError because it is relatively rare to check the message of `NameError`. Still, it broke some tests unfortunately, though. If possible, I'd like to add suggestions to other kinds of errors, but it will break much more tests.

If `Exception#additional_message` is introduced, and if did_you_mean and error_highlight overrides the method to add their suggestions, this problem will not occur because they no longer changes the result value of `#message` method.

## Cooperation needed

Currently, many Ruby/Rails users montiors their production services by using application monitoring services such as Sentry, DataDog, ScoutAPM, etc. The original motivation of error_highlight is for production (see #17930), so it will lose the significance if such services do not support `Exception#additional_message`. So, I'd like to hear opinions from developers of such services. If they are against this proposal or if we can't get their cooperation, I don't think my proposal should be accepted.

If you are a developer of these services, I would be very grateful if you could comment on this ticket. @ivoanjo

## Bikesheds

* I'm unsure if `Exception#additional_message` is a good name. Please propose alternatives if it is not good.
* Currently, the result of `addtional_message` is printed with no escape. This may be a more compatible solution against https://bugs.ruby-lang.org/issues/18367.
* It may be good to let `Exception#additional_message` accept `highlight` keyword as boolean information whether the output target is a terminal or not. Currently `Exception#full_message` accepts it. I have no plan to use the information in `error_highlight`, though. Not only `highlight` but also any keywords may be forwarded from `full_message(**opt)` to `additional_message(**opt)` for future use case.
* My current PoC adds prefixs "`| `" before each line of `addtional_message`. I'm unsure if this is good or bad. I'm happy to change or remove the prefixes.

---Files--------------------------------
截圖 2021-12-27 15.56.00.png (74.9 KB)


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