[ruby-core:101333] [Ruby master Feature#16604] Set default for Encoding.default_external to UTF-8 on Windows

Issue #16604 has been updated by larskanis (Lars Kanis).


This issue can be closed. It's merged in commit:94b6933d1c6f4c8698319fbcac9dcecc9033b4b9

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Feature #16604: Set default for Encoding.default_external to UTF-8 on Windows
https://bugs.ruby-lang.org/issues/16604#change-89030

* Author: larskanis (Lars Kanis)
* Status: Open
* Priority: Normal
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This issue is related to https://bugs.ruby-lang.org/issues/13488 where we already discussed the topic and postponed the change for ruby-3. A patch is here: https://github.com/ruby/ruby/pull/2877

## What should be changed?

Currently `Encoding.default_external` is initialized to the local console encoding of the Windows installation unless changed per option `-E`. This is e.g. cp850 for Western Europe. It should be changed to UTF-8.

The above patch only changes the default for `Encoding.default_external`. It can still be overwritten per command line option `-Elocale` or in ruby code.

## Reasons for the change

Changing to UTF-8 fixes various inconsistencies within ruby and with external tools. A very common case is writing a non-ASCII text to a file. It writes the string content as its binary representation, which is usually UTF-8, since this is the default ruby source encoding. But reading the content back, tags the string with the wrong encoding leading to mojibakes.

```
s = "糜釞
File.write("x", s)   # => 6 bytes
File.read("x") == s  # => true in irb but false in .rb file
```

As noted in the last line, the result in `irb` is different from regular .rb files, since it already sets `Encoding.default_external = "utf-8"` on it's own. This is another inconsistency with the current default.

Another issue is that many non-asian regions have distinct legacy encodings for OEM code page (aka `Encoding.find('locale')` ) and ANSI code page (aka `Encoding.find('filesystem')` ), so that a file written in current default external encoding `Encoding.find('locale')` is not properly interpret in Windows GUI tools like notepad. It is therefore uncommon to store files in OEM-ANSI encoding and doing so is almost certainly wrong.

RubyInstaller ships the MSYS2 environment, which defaults to UTF-8 as well.

Powershell made the switch to UTF-8 (without BOM) in [Powershell-6.0](https://docs.microsoft.com/en-us/powershell/scripting/whats-new/what-s-new-in-powershell-core-60?view=powershell-7#default-encoding-is-utf-8-without-a-bom-except-for-new-modulemanifest) and even more in 6.1.

## Will it work?

Yes. RubyInstaller provided a checkbox for `RUBYOPT=-Eutf-8` since version 2.4. This checkbox was disabled at first, but since RubyInstaller-2.7.0 this checkbox is [enabled per default](https://rubyinstaller.org/2020/01/05/rubyinstaller-2.7.0-1-released.html). So UTF-8 as the default external encoding is the expected encoding for most of the people on Windows, now.

However setting `RUBYOPT` per installer is obtrusive and doesn't work with a 7z archive distribution. I would like to remove this hack starting with ruby-3.0.

## Alternatives

Changing the default of `Encoding.default_external` to UTF-8 is a trade-off. It doesn't fit to every case, but in my experience this is the best overall option. And it's just the default for the default, so that it can be overwritten in many ways.

There are some alternatives to it:

Changing the Windows console to code page 65001:
 * The Windows implementation of 65001 is buggy in the console. I didn't verify it lately but `chcp 65001` didn't work reliable years ago.
 * It is not the default and input methods like IME are incompatible.
 * It sets `locale` to UTF-8, so that the native console encoding isn't easily available.

Setting `Encoding.default_internal` in addition:
 * This triggers transcoding of output strings, which is not enabled on other systems, causing unexpected results and incompatibilities.

Change ruby to use `Encoding.find("filesystem")` as encoding for file operations:
 * That would fix the compatibility with some builtin Windows tools, but doesn't fix encoding issues due to increased use of UTF-8.

## What doesn't change?

Please note that changing `Encoding.default_external` doesn't affect file or IO **output**, unless `Encoding.default_internal` is set as well (which is not the default).

Also "locale" and "filesystem" pseudo encodings don't change. Both can still be used explicit in cases where these encodings are required.

The patch is currently about Windows only, because I would like to focus on that question for now. Possibly it's a subsequent question whether Encoding.default_external should default to UTF-8 on all operating systems or at least in case of `LANG=C` locale (which currently triggers US-ASCII).




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