[ruby-core:110639] [Ruby master Bug#19108] Format routines like pack blindly treat a string as ASCII-encoded

["chrisseaton (Chris Seaton)" <noreply@ruby-lang.org>]

Issue #19108 has been updated by chrisseaton (Chris Seaton).


Possibly we should raise an exception if the string is not `ascii_only?`

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Bug #19108: Format routines like pack blindly treat a string as ASCII-encoded
https://bugs.ruby-lang.org/issues/19108#change-99977

* Author: chrisseaton (Chris Seaton)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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Format routines like pack and unpack blindly treat a string as ASCII-encoded, even if they aren't ASCII or ASCII-compatible.

I tried to construct code that was misleading using ASCII-incompatible-encodings but couldn't do it in practice (no ASCII-incompatible encodings have a pack directive ASCII byte that is encoded as a printable character.)

But I could demonstrate at least some strange behaviour:

```
p ['foo'].pack('u').encoding # => #<Encoding:US-ASCII>
p ['foo'].pack('u'.encode('UTF-32BE')).encoding # => #<Encoding:ASCII-8BIT>
```

This is because the NUL characters in the second one (which aren't really NUL characters - they're part of the directive characters) explicitly trigger the encoding to change to binary.

There is a warning, but the warning is only for unexpected directives. How about disallowing or warning for non-ascii compatible format strings?



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