[ruby-core:106159] [Ruby master Feature#6733] New inspect framework

Issue #6733 has been updated by kou (Kouhei Sutou).


This may be out-of-scope but I hope that this mechanism also supports multimedia like Julia: https://docs.julialang.org/en/v1/base/io-network/#Multimedia-I/O

My use case is showing images on irb automatically.

For example with [Red Datasets](https://github.com/red-data-tools/red-datasets) and [Charty](https://github.com/red-data-tools/charty/):

```text
irb(main):001:0> require "datasets"
=> true
irb(main):002:0> require "charty"
=> true
irb(main):003:0> penguins = Datasets::Penguins.new
=> 
#<Datasets::Penguins:0x00007f308dd34d10                                                         
...                                                                                             
irb(main):004:0> Charty.scatter_plot(data: penguins, x: :body_mass_g, y: :flipper_length_mm, color: :species)
=> #<Charty::Plotters::ScatterPlotter:0x0000000000000424> # Show Sixel images here if the terminal supports Sixel
```

ref: [Sixel](https://en.wikipedia.org/wiki/Sixel)



----------------------------------------
Feature #6733: New inspect framework
https://bugs.ruby-lang.org/issues/6733#change-94761

* Author: akr (Akira Tanaka)
* Status: Open
* Priority: Normal
----------------------------------------
After we discussed http://bugs.ruby-lang.org/issues/6291 at a developer meeting,
we re-realized new inspect framework may be useful.

Problem:
* inspect method may generate too long string but sometimes whole string is not required.
  For example, first 70 characters are enough for error messages (backtrace).
* inspect can't know a encoding to be expected.
* inspect generates may short strings and discard them immediately.

If we have a new method, inspect_to(buffer), and
it (or overridden method in subclass) adds the inspected result to buffer,
we can solve above problems.
buffer has a method, <<.
It may be a string, IO or other object.

For too long string, buffer itself can throw (or raise) when buffered output is reached to a specified limit.

For encoding, buffer can record an encoding.
(p method creates a buffer object using $stdout's encoding.)

For small strings, in C level, we can create a rb_buffer_add(VALUE buffer, const char *p, long len) and
it don't need to allocate a String object.

This is big change but we can preserve compatibility by following default inspect_to method:

class Object
  def inspect_to(buffer)
    buffer << self.inspect
  end
end

If legacy class which has inspect but not inspect_to,
Object#inspect_to calls inspect and use the result.




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