[ruby-core:105778] [Ruby master Bug#18170] Exception#inspect should not include newlines

Issue #18170 has been updated by mame (Yusuke Endoh).


We discussed this ticket at the dev-meeting, and matz basically accepted as above.

The approved spec is very hacky. Only when the message includes new lines, `Exception#inspect` applies `inspect` to the message. Otherwise, it embeds the message as is.

```
p StandardError.new("foo\nbar") #=> #<StandardError:"foo\nbar">
p StandardError.new("foo bar")  #=> #<StandardError: foo bar>
p StandardError.new("foo\\bar") #=> #<StandardError: foo\bar>
```

Note that there is no space between the colon and the double quote in the first line. This is a mark showing whether `inspect` is used.

```
p StandardError.new("foo\nbar")   #=> #<StandardError:"foo\nbar">
p StandardError.new('"foo\nbar"') #=> #<StandardError: "foo\nbar">
```

Parsing the result of `#inspect` is never recommended, though. I'll update my PR later to follow the approved spec.

----------------------------------------
Bug #18170: Exception#inspect should not include newlines
https://bugs.ruby-lang.org/issues/18170#change-94292

* Author: mame (Yusuke Endoh)
* Status: Assigned
* Priority: Normal
* Assignee: mame (Yusuke Endoh)
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Is this intentional?

```
p StandardError.new("foo\nbar")
#=>
# #<StandardError: foo
# bar>
```

I expect `#inspect` returns a one-line string. How about returning `#<StandardError: "foo\nbar">` or something?

Recently, multi-line error messages have been increasing by the introduction of did_you_mean and error_highlight. Printing an object that contains such an exception leads to a tricky output:

```
class Foo
  def initialize
    @exception = begin; exampl; rescue Exception; $!; end
  end

  def example
  end
end

p Foo.new
#=>
# #<Foo:0x00007f15aeb4ba48 @exception=#<NameError: undefined local variable or method `exampl' for #<Foo:0x00007f15aeb4ba48 ...>
#
#     @exception = begin; exampl; rescue Exception; $!; end
#                         ^^^^^^
# Did you mean?  example>>
```

This issue was originally found by @ioquatix 



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