From: "Eregon (Benoit Daloze)" <noreply@...>
Date: 2021-10-19T17:51:58+00:00
Subject: [ruby-core:105682] [Ruby master Feature#18083] Capture error in ensure block.

Issue #18083 has been updated by Eregon (Benoit Daloze).


ko1 (Koichi Sasada) wrote in #note-18:
> Another proposal is change `$!` semantics to `begin/rescue/ensure' clause local.
> Does it solve the issues?

That seems elegant actually, because 
```ruby
begin
  ...
rescue
  foo($!)
end
```
would behave like:
```ruby
begin
  ...
rescue => exc
  foo(exc)
end
```

And it would avoid escaping the Exception instance to the thread, which mean it can be escaped analyzed.
Also, if the `rescue` doesn't use `=>` or `$!`, there is no need to generate a backtrace which is a huge performance gain!
With current semantics that's difficult to know as any call inside the rescue clause might use `$!`.

I"m not sure regarding compatibility if e.g. `$!` is accessed inside a method called from the rescue clause. But that's probably rare and seems bad code, so I think fine to try/experiment.

----------------------------------------
Feature #18083: Capture error in ensure block.
https://bugs.ruby-lang.org/issues/18083#change-94182

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
As discussed in https://bugs.ruby-lang.org/issues/15567 there are some tricky edge cases.

As a general model, something like the following would be incredibly useful:

``` ruby
begin
 ...
ensure => error
  pp "error occurred" if error
end
```

Currently you can get similar behaviour like this:

``` ruby
begin
  ...
rescue Exception => error
  raise
ensure
  pp "error occurred" if error
end
```

The limitation of this approach is it only works if you don't need any other `rescue` clause. Otherwise, it may not work as expected or require extra care. Also, Rubocop will complain about it.

Using `$!` can be buggy if you call some method from `rescue` or `ensure` clause, since it would be set already. It was discussed extensively in https://bugs.ruby-lang.org/issues/15567 if you want more details.



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