[ruby-core:116219] [Ruby master Bug#19857] Eval coverage is reset after each `eval`.

Issue #19857 has been updated by mame (Yusuke Endoh).

Status changed from Open to Rejected

Sorry, but I am not keen on this proposal. Let me reject this once.

When I started developing the covearge library, I decided that its target is "source files that actually exist".
The measured coverage is visually observed by a human, which requires source code, and Ruby interpreter did not keep the eval'ed source code (at that time). I think that the design decision to target only source files was natural.

Because the Rails view code (.erb) exists as a file, I thought that it is the target of covearge library. So I accepted #19008.

Beyond that, measuring coverage on dynamically generated code conflicts with the coverage library's initial design decision.

To be honest, I don't want to further complicate the coverage library. Frankly, its API is already very complicated. I no longer want to expand it beyond the initial design decision.

---

One constructive note.
When I first started implementing the coverage library, I had no choice but to implement it as a built-in, but now we have TracePoint, which is well-designed and general-purpose than the coverage library.
Also, Prism API is being developed, and it may be possible (although I think TracePoint needs to be extended) to radically improve the coverage measurement on a column-by-column basis rather than line-by-line basis.
I don't have time right now, but I would like to rebuild the library as a pure external gem in future, i.e. `coverage2` or something, with dynamically generated code, etc. in mind from the beginning.

----------------------------------------
Bug #19857: Eval coverage is reset after each `eval`.
https://bugs.ruby-lang.org/issues/19857#change-106238

* Author: ioquatix (Samuel Williams)
* Status: Rejected
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
----------------------------------------
It seems like `eval` based coverage is reset every time eval is invoked.

```ruby
#!/usr/bin/env ruby

require 'coverage'

def measure(flag)
  c = Class.new
  c.class_eval(<<~RUBY, "foo.rb", 1)
    def foo(flag)
      if flag
        puts "foo"
      else
        puts "bar"
      end
    end
  RUBY
  
  return c.new.foo(flag)
end

Coverage.start(lines: true, eval: true)

# Depending on the order of these two operations, different coverage is calculated, because the evaluation of the code is considered different, even if the content/path is the same.
measure(false)
measure(true)

p Coverage.result
```

Further investigation is required.



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