[ruby-core:116988] [Ruby master Feature#20275] Avoid extra backtrace entries for rescue and ensure

28 Feb 2024

      Issue #20275 has been updated by mame (Yusuke Endoh).

I am in favor of this change, but I do not plan to write a patch and estimate its impact. Someone who wants this, please do it.
I recommend doing this earlier. Since it is incompatible, it may be too late closer to the upcoming release.

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Feature #20275: Avoid extra backtrace entries for rescue and ensure
https://bugs.ruby-lang.org/issues/20275#change-107037

* Author: Eregon (Benoit Daloze)
* Status: Open
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
From https://bugs.ruby-lang.org/issues/19117#note-48:
```ruby
def raise_nested_exceptions
  raise "First error"
rescue
  begin
    raise "Second error"
  rescue
    raise "Third error"
  end
end

raise_nested_exceptions
```

Current:
```
-:7:in 'Object#raise_nested_exceptions': Third error (RuntimeError)
	from -:4:in 'Object#raise_nested_exceptions'
	from -:1:in 'Object#raise_nested_exceptions'
	from -:11:in '<main>'
-:5:in 'Object#raise_nested_exceptions': Second error (RuntimeError)
	from -:1:in 'Object#raise_nested_exceptions'
	from -:11:in '<main>'
-:2:in 'Object#raise_nested_exceptions': First error (RuntimeError)
	from -:11:in '<main>'
```

The above repeated backtrace entries for `Object#raise_nested_exceptions` are because of CRuby implementation details.
It seems best to hide them because they do not help understanding the program, and semantically there is no (method or proc) call on line 1 or 4.

Proposed:
```
-:7:in 'Object#raise_nested_exceptions': Third error (RuntimeError)
	from -:11:in '<main>'
-:5:in 'Object#raise_nested_exceptions': Second error (RuntimeError)
	from -:11:in '<main>'
-:2:in 'Object#raise_nested_exceptions': First error (RuntimeError)
	from -:11:in '<main>'
```

We can see TruffleRuby and JRuby already behave this way, most likely because they do not use an extra frame for `rescue`.
It is definitely useful to have the same backtrace on different Ruby implementations as some tests rely on this.

TruffleRuby 23.1.2:
```
-:7:in `raise_nested_exceptions': Third error (RuntimeError)
	from -:11:in `<main>'
-:5:in `raise_nested_exceptions': Second error (RuntimeError)
	from -:11:in `<main>'
-:2:in `raise_nested_exceptions': First error (RuntimeError)
	from -:11:in `<main>'
```
JRuby 9.4.5.0:
```
RuntimeError: Third error
  raise_nested_exceptions at -:7
                   <main> at -:11
RuntimeError: Second error
  raise_nested_exceptions at -:5
                   <main> at -:11
RuntimeError: First error
  raise_nested_exceptions at -:2
                   <main> at -:11
```

-- 
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