[ruby-core:98123] [Ruby master Feature#15921] R-assign (rightward-assignment) operator

Issue #15921 has been updated by duerst (Martin Dst).


For me, the main use case is at the end of method chains. Instead of e.g.
```
Word = Struct.new(:text, :count)
words = $stdin.read
        .scan(/[-\w']+/)
        .group_by(&:downcase)
        .collect { |key, value| Word.new(key, value.count) }
        .sort_by { |w| [-w.text.length, w.text] }
```
we can now write
```
$stdin.read
      .scan(/[-\w']+/)
      .group_by(&:downcase)
      .collect { |key, value| Word.new(key, value.count) }
      .sort_by { |w| [-w.text.length, w.text] } => words
```
where the order of the code pieces aligns with the order of what's happening, and there's no need to go back with your eyes.

(This is an example of a problem given to students, they have to count words in a text and output them in a specific order. It's from a C programming class where I show them after the submission deadline that it's much easier and shorter in Ruby.)

I'm not sure if having the assignment on the following line is possible (see below), but I think it should be. I'll submit a feature request if it is not yet possible.
```
$stdin.read
      .scan(/[-\w']+/)
      .group_by(&:downcase)
      .collect { |key, value| Word.new(key, value.count) }
      .sort_by { |w| [-w.text.length, w.text] }
=> words
```


----------------------------------------
Feature #15921: R-assign (rightward-assignment) operator
https://bugs.ruby-lang.org/issues/15921#change-85365

* Author: nobu (Nobuyoshi Nakada)
* Status: Closed
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
From https://bugs.ruby-lang.org/issues/15799#change-78465, proposal of the rightward-assignment operator by `=>`.

```
$ ./ruby -v -e '(1..).lazy.map {|x| x*2} => x' -e 'p x.first(10)'
ruby 2.7.0dev (2019-06-12T06:32:32Z feature/rassgn-assoc c928f06b79) [x86_64-darwin18]
last_commit=Rightward-assign by ASSOC
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
```

https://github.com/nobu/ruby/tree/feature/rassgn-assoc



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