[ruby-core:115860] [Ruby master Misc#20038] Strings with mixed escapes not detected around interpolation

Issue #20038 has been updated by kddnewton (Kevin Newton).


Thank you for your response. I understand that interpolation is sugar for concatenation. However these strings will always cause an error, but they won't cause an error until runtime. This information is statically known, so I am proposing we detect it in the parser for a better developer experience. We know that it is impossible for this code to run.

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Misc #20038: Strings with mixed escapes not detected around interpolation
https://bugs.ruby-lang.org/issues/20038#change-105818

* Author: kddnewton (Kevin Newton)
* Status: Closed
* Priority: Normal
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I'm not 100% my understanding is correct, but here's what I think is happening:

When a string is being parsed, it starts out by assuming it's the same encoding as the encoding of the file. If an \x escape sequence is found that has a value >= 0x80, it locks in that encoding. Then if a \u escape sequence is found, it will raise an error if the value is >= 0x80 and say that there are mixed escape sequences. This works for:

```ruby
# encoding: ascii
"\xFF \u{80}"
```

The locked in encoding is recalculated on every token, however, so if you do something like:

```ruby
# encoding: ascii
foo = 1
"\xFF #{foo} \u{80}"
```

then the first string token (everything up to the interpolation) will be ASCII-8BIT, and everything after the interpolation will be UTF-8. This will not be marked as an error, and will instead be compiled as normal. However, as soon as it is executed, it will raise `Encoding::CompatibilityError`.

Since this is statically detectable in the parser, it seems incorrect to compile it only for it to blow up later. This could be surprising for people since if it's inside an infrequently-called method, it could end up making its way into production.



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