Issue #15854 has been updated by igaiga (Kuniaki IGARASHI).


Thanks to comments.

> :ivar_assign (close to original proposal)
> :instance_variable_set (same as official method name, maybe too long but "ivar" may not be friendly either)

Looks good. I feel they are good to read, so better than my suggested :iasgn.

> Cloud you describe a use case? ... but I don't think it applies to :casgn.

I've tried to describe the use cases, but I can't write use cases for class variables.

I thought following patterns that how to torigger their assign timings.

- global variable: Kernel#trace_var
- class variable: none
- instance variable: suggested by this issue
- local variable: none? (but we don't need)

So I thought class variable assignment trigger is useful for us. But cases to use class variables are not so often, so the trigger of :class_variable_set would be not so useful.


----------------------------------------
Feature #15854: Tracing instance variable assignment
https://bugs.ruby-lang.org/issues/15854#change-78103

* Author: igaiga (Kuniaki IGARASHI)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I suggest a feature "tracing instance variable assignment". It's useful for debugging.

Use case:

In Rails, we use instance variables in views and controllers. When we got a bug caused by instance variable unintentional values, if we traced instance variable assignment timing, it would be good informations.

And in Rails views, there are no source codes of self class. That's built dynamically.

Current behavior (Ruby2.6):

In Ruby 2.6, only if there is a source code file to assign instance variable, we can trace instance variable assignment by following code (check_instance_variable_assignment.rb). But it's difficult if the assignment codes are defined dynamically. For example, in Rails view.

(And in another story, global variables assignment are traced by Kernel#trace_var.)

check_instance_variable_assignment.rb
```ruby
def trace_start
  TracePoint.trace(:line) do |tp|
    target_class_name = "Foo"
    target_instance_variable_name = "@bar"

    line = File.open(tp.path, "r"){|f| f.readlines[tp.lineno - 1] }
    node = RubyVM::AbstractSyntaxTree.parse(line).children.last

    # check instance variable assignment
    next unless node.type == :IASGN

    # check class name
    target_class = Kernel.const_get(target_class_name)
    next unless tp.self.is_a?(target_class)

    # check variable name
    instance_variable_name = node.children.first
    next unless instance_variable_name == target_instance_variable_name.to_sym

    puts "#{target_class_name} #{target_instance_variable_name} is assigned in #{tp.path}:#{tp.lineno} #{tp.defined_class} #{tp.method_id}"
  end
end

class Foo
  def bar
    @bar = "text"
  end
end

trace_start
Foo.new.bar
#=> Foo @bar is assigned in check_instance_variable_assignment.rb:25 Foo bar
```

Suggesting feature example:

Add new arguments for TracePoint.new method like :line and :call to trace instance variables assignment.

- :iasgn (IASGN name from RubyVM::AbstractSyntaxTree::Node)
- :casgn (CVASGN (or CASGN?) name from RubyVM::AbstractSyntaxTree::Node. I think class variables tracing is useful too.)

And get informations

- class name (It might be get by trace_point.self)
- variable name ("@foo", "@@foo")

A sample code to use the feature:

tp_iasgn.rb

```ruby
TracePoint.trace(:iasgn) do |tp|
  target_class_name = "Foo"
  target_instance_variable_name = "@bar"

  # check class name
  target_class = Kernel.const_get(target_class_name)
  next unless tp.self.is_a?(target_class)

  # check variable name
  next unless target_instance_variable_name == tp.variable_name 

  puts "#{target_class_name} #{target_instance_variable_name} is assigned in #{tp.path}:#{tp.lineno} #{tp.method_id} #{tp.defined_class}"
  puts caller # even in dynamic code case, we can get caller informations.
end
```




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