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


The logic you've given above is too idealistic unfortunately:

```ruby
[11] pry(main)> x = "Foobar".freeze                                                                                                                                                                                 
=> "Foobar"
[12] pry(main)> x.force_encoding(Encoding::BINARY)                                                                                                                                                                  
FrozenError: can't modify frozen String
from (pry):10:in `force_encoding'
```

The best you can hope for is something like https://github.com/socketry/async-io/blob/master/lib/async/io/buffer.rb#L32-L40

```ruby
def << string
	if string.encoding == BINARY
		super(string)
	else
		super(string.b)
	end
	
	return self
end
```

It's even possible that `string.b` doesn't allocate memory... but I don't actually know.

Maybe a better option is for

```
# Encoding::CompatibilityError: incompatible character encodings: UTF-8 and ASCII-8BIT
u << b
```

to convert the RHS to the LHS encoding - if it can't then it fails. WDYT?

----------------------------------------
Feature #14975: String#append without changing receiver's encoding
https://bugs.ruby-lang.org/issues/14975#change-73468

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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 << "Fbar"
=> "FoobarFbar"

> "Fbar".b << "Fbar"
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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