[ruby-core:91937] [Ruby trunk Feature#15724] Optionally suppress output from IRB for assignment expressions

Issue #15724 has been reported by onlynone (Steven Willis).

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Feature #15724: Optionally suppress output from IRB for assignment expressions
https://bugs.ruby-lang.org/issues/15724

* Author: onlynone (Steven Willis)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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Most REPLs I've used don't print out the output of assignment expressions like `a = 1`.  Being able to suppress this output is very useful when you have a string which is essentially a blob, an object with a large string representation, or even an array of several objects like this. This happens to me often with rails' ActiveRecords where a query might return hundreds of objects that I just want to assign to a variable and don't want to inspect yet. It's even worse with rails because queries are run lazily, just because I did `a = User.where(name='Sally')` doesn't mean I actually wanted to execute that query against the db yet. I might want to do `a = a.where('age > 35')`. But irb doing the echo forces the evaluation.

I wrote this patch to irb: https://github.com/ruby/irb/pull/12 which would suppress irb's echo for assignment expressions when a new config setting: `context.echo_on_assignment` is false. It uses the new `RubyVM::AbstractSyntaxTree` to determine whether an expression is an assignment expression, so it only works on ruby >= 2.6.0. I'd very much welcome some feedback and any ideas on how to get it to work for earlier ruby versions.

I had also written a first-pass proof-of-concept that used the `parser` gem and would work for ruby >= 2.0.0 (https://github.com/ruby/irb/pull/11). This version obviously can't be merged because of the external dependency, but perhaps it can be treated like readline: use it if it's already installed, but don't depend on it?



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