[ruby-core:90774] [Ruby trunk Feature#14975] String#append without changing receiver's encoding

Issue #14975 has been updated by ioquatix (Samuel Williams).

Assignee set to ioquatix (Samuel Williams)
Target version set to 2.7

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Feature #14975: String#append without changing receiver's encoding
https://bugs.ruby-lang.org/issues/14975#change-75948

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Target version: 2.7
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I'm not sure where this fits in, but in order to avoid garbage and superfluous function calls, is it possible that `String#<<`, `String#concat` or the (proposed) `String#append` can avoid changing the encoding of the receiver?

Right now it's very tricky to do this in a way that doesn't require extra allocations. Here is what I do:

```ruby
class Buffer < String
	BINARY = Encoding::BINARY
	
	def initialize
		super
		
		force_encoding(BINARY)
	end
	
	def << string
		if string.encoding == BINARY
			super(string)
		else
			super(string.b) # Requires extra allocation.
		end
		
		return self
	end
	
	alias concat <<
end
```

When the receiver is binary, but contains byte sequences, appending UTF_8 can fail:

```
"Foobar".b << "F淡淡bar"
=> "FoobarF淡淡bar"

> "F淡淡bar".b << "F淡淡bar"
Encoding::CompatibilityError: incompatible character encodings: ASCII-8BIT and UTF-8
```

So, it's not possible to append data, generally, and then call `force_encoding(Encoding::BINARY)`. One must ensure the string is binary before appending it.

It would be nice if there was a solution which didn't require additional allocations/copies/linear scans for what should basically be a `memcpy`.

See also: https://bugs.ruby-lang.org/issues/14033 and https://bugs.ruby-lang.org/issues/13626#note-3

There are two options to fix this:

1/ Don't change receiver encoding in any case.
2/ Apply 1, but only when receiver is using `Encoding::BINARY`




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