[ruby-core:109648] [Ruby master Bug#18972] String#byteslice should return BINARY (aka ASCII-8BIT) Strings

Issue #18972 has been updated by byroot (Jean Boussier).

Status changed from Open to Rejected

Ok, I suppose your point of view make sense, and either way the backward compatibility concern is just too big.

Closing.

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Bug #18972: String#byteslice should return BINARY (aka ASCII-8BIT) Strings
https://bugs.ruby-lang.org/issues/18972#change-98871

* Author: byroot (Jean Boussier)
* Status: Rejected
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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While working on implementing https://bugs.ruby-lang.org/issues/13626, I noticed `byteslice` assign the receiver encoding to the returned String.

I believe this is incorrect, as since you are doing a byte based operation, you do expect a binary string in return, otherwise if you'd call it on an UTF-8 string, you'd likely get a string with invalid encoding.

I read the original feature request and there's no mention of what the returned encoding should be: https://bugs.ruby-lang.org/issues/4447


### Current behavior

```ruby
>> "f辿e".byteslice(1).valid_encoding?
=> false
>> "f辿e".byteslice(1).encoding
=> #<Encoding:UTF-8>
```

### Expected behavior

```ruby
>> "f辿e".byteslice(1).valid_encoding?
=> true
>> "f辿e".byteslice(1).encoding
=> #<Encoding:ASCII-8BIT>
```

### Backward compatibility concerns

I'm honestly not quite sure what the backward incompatibility impact may be.

From my point of view if you are calling `byteslice` it's to use it with other binary string, but it's indeed
possible that there is existing code mixing UTF-8 and BINARY that somewhat work and would be broken by this change.

Especially since binary strings can silently be promoted from BINARY to UTF-8:

```ruby
buffer = "".b 
buffer << "f辿e" # buffer was promoted to Encoding::UTF-8 silently
buffer << "f辿e".byteslice(1)
```

The above currently "works", but would raise `Encoding::CompatibilityError` with this change.





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