[ruby-core:113950] [Ruby master Feature#19737] Add `IO::Buffer#cat` for concat `IO::Buffer` instances

Issue #19737 has been updated by unasuke (Yusuke Nakamura).


My use case is this: concat given two buffers, then modify (copy, slice, xor) it.
(More concretely, I want to apply/remove header protection in the QUIC protocol.)

I understood that using copy method is a better way than concat buffers.

But, described style is a bit redundant for me (especially size calc).

```ruby
a = IO::Buffer.for(unknown_length_data)
b = IO::Buffer.for(from_somewhere)
c = IO::Buffer.new(a.size + b.size)
c.copy(a, 0)
c.copy(b, a.size)
pp c
# =>
# #<IO::Buffer 0x0000600003f021a0+2 INTERNAL>
# 0x00000000  41 42                                           AB
```

Is it API design? If so, I accept this style.

----------------------------------------
Feature #19737: Add `IO::Buffer#cat` for concat `IO::Buffer` instances
https://bugs.ruby-lang.org/issues/19737#change-103603

* Author: unasuke (Yusuke Nakamura)
* Status: Open
* Priority: Normal
----------------------------------------
## motivation
In my use case, I want to concat two IO::Buffer instances. But current implementation doesn't have that way.
Then I created a patch. Opend here: TBD

## concern
I have two concerns about it.

### 1. Should we provide `IO::Buffer#+` as an alias?
In String instance, `"a" + "b"` returns `"ab",`. It feels intuitive.

So, should we provide the same way as `IO::Buffer.for("a") + IO::Buffer.for("b")`?
If `+` is provided, I naturally assume that `*` is also provided as an operator.
Should we also provide an `IO::Buffer#*` method for symmetry with the String class?

I thought the behavior of the "*" method is not obvious to me... (Is it right to just return joined buffers?)

### 2. Should it accept multiple IO::Buffer instances?
In the `cat` command, it accepts multiple inputs like this.

```
$ cat a.txt b.txt c.txt
a
b
c
```

Should `IO::Buffer#cat` accept multiple inputs too?



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