[ruby-core:90915] [Ruby trunk Bug#15497] Encoding of error messages should not depend on the locale encoding

Issue #15497 has been updated by duerst (Martin D端rst).


I agree that the locale encoding should only be taken into account when the message is actually output, not as long as it is passed around inside Ruby.

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Bug #15497: Encoding of error messages should not depend on the locale encoding
https://bugs.ruby-lang.org/issues/15497#change-76106

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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This seems to happen mostly for internal errors, as `raise` in Ruby code of course just uses the passed String's encoding for the message.

Example:
```ruby
name = "辿t辿"
p name.encoding

begin
  Module.new.const_set(name, 1)
rescue => e
  p e
  p e.message.encoding
end
```

When run, it gives:
```
$ LANG=en_US.UTF-8 ruby c.rb
#<Encoding:UTF-8>
#<NameError: wrong constant name 辿t辿>
#<Encoding:UTF-8>

$ LANG=C ruby c.rb   
#<Encoding:UTF-8>
#<NameError: wrong constant name "\u00E9t\u00E9">
#<Encoding:US-ASCII>
```

Depending on the locale encoding, the encoding of the message changes!
This seems very unexpected, is inconvenient for testing (e.g., https://github.com/ruby/spec/commit/a6101a6e and any test checking exception messages with non-US-ASCII characters),
and does not represent what is in the source code (here it's clearly a valid UTF-8 String).

I think for such a case, the encoding of the constant name should be used, i.e., UTF-8.
Another way to see it is the message should be built like `"wrong constant name ".force_encoding('us-ascii') + constant_name`.
Indeed, if we do build the message manually like that it works as expected:

```
name = "辿t辿"
begin
  raise "wrong constant name ".force_encoding('US-ASCII') + name
rescue => e
  p e
  p e.message.encoding
end
```
gives
```
$ LANG=en_US.UTF-8 ruby c.rb
#<Encoding:UTF-8>
#<RuntimeError: wrong constant name 辿t辿>
#<Encoding:UTF-8>

$ LANG=C ruby c.rb          
#<Encoding:UTF-8>
#<RuntimeError: wrong constant name \u00E9t\u00E9>
#<Encoding:UTF-8>
```

Note that the message still looks different, but that's the effect of `Kernel#p`, because it does not know how to display UTF-8 characters in a US-ASCII terminal.
Nevertheless, both messages have the same bytes and encoding, which fixes all 3 problems mentioned above.

Setting `Encoding.default_internal` can workaround this but it's a bad workaround as this cannot work reliably in a multithreaded Ruby application,
affects many more things than just error messages, and the default behavior should be error messages with a deterministic encoding, just like `raise` in Ruby code.



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