Display a line in application code instead of gems

Currently, error_highlight spots the deepest Ruby frame. This may show code in a gem, which may not be very useful for application writers.

Consider:

# gem code
class SomeGem
  def foo(x)
    raise ArgumentError, "wrong input" if x == nil
  end
end

# app code
SomeGem.new.foo(nil)

Here is the current behavior:

$ ruby test.rb
test.rb:4:in `foo': wrong input (ArgumentError)

    raise ArgumentError, "wrong input" if x == nil
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        from test.rb:9:in `<main>'

However, application writers may want to see the line of SomeGem.new.foo(nil).

How might this be resolved?

1. Display code snippets for all frames, not just for the deepest one. Python does this, but I am not very keen on it because it makes the error trace very messy.
2. Add an argument Kernel#raise to tell error_highlight which frame should be displayed, for example, raise ArgumentError, "wrong input", error_highlight_skip_frames: 1.

For 2, there are two ways how to show the error.

2-1. Keep the error trace as is and change only the snippet. This might be confusing because the snippet line does not exist in test.rb:4.

$ ruby test.rb
test.rb:4:in `foo': wrong input (ArgumentError)

SomeGem.new.foo(nil)
                ^^^
        from test.rb:9:in `<main>'

2-2. Skip the error trace. This looks good to me, but I think this is beyond the responsibilities of error_highlight.

$ ruby test.rb
test.rb:9:in `<main>': wrong input (ArgumentError)

SomeGem.new.foo(nil)
                ^^^

I wonder if we should put something like raise skip_frames: n in the Ruby core side? I need to organize my thoughts.

Start by examining error_highlight's current deepest-frame selection and the Ruby Kernel#raise behavior discussed in the issue. Compare the proposed all-frames, skip-frame, snippet-only, and trace-skipping approaches, but note that the issue does not name files or tests and does not establish an agreed completion condition.