Empty array instead of nil for source_extract

[itsWill]

Summary

The source_extract method will return nil when it can't find the file name in
the backtrace, methods that consume this method expect an array and the nil ends
up causing type errors down the road like it happened here: #36341. This patch
introduces a to_a call that converts the nil into an empty array this makes it so
client classes can correctly handle a empty list of source code lines and not
trigger a type error.

Other Information

Note this is a bit defensive and preventative imo, this occured for the edge case described here: #36341 (comment) and potentially other unknown code paths could still cause this behaviour to occur, but I don't know of any.

At any rate I do like having the method return a [] instead of a nil and thing that is a better interface for this class in general since the method will be returning consistent types.

discussed this with @rafaelfranca and @gmcgibbon

[gmcgibbon]

I'm good to merge this, but I want a second opinion as this could be considered overly defensive programming.

[kaspth]

I've been looking into the surrounding code and I don't source_extract is right. It uses formatted_code_for which could return a Hash if output was :html. However it never seems to be anything but the default :console from source_extract (because source_extract is only called from annotated_source_code.

My take is that we should simplify formatted_code_for to this while we're here:

def formatted_code_for(source_code, line_counter, indent)
  indent_template = "%#{indent}s: %s"

  source_code.map.with_index(line_counter) do |line, counter|
    indent_template % [counter, line]
  end
end

Otherwise calling to_a on the Hash would also break the code. And if not, the Hash would this when concat(annotated_source_code) is hit:

>> [].concat({})
Traceback (most recent call last):
        5: from /Users/kaspth/.rbenv/versions/2.6.3/bin/irb:23:in `<main>'
        4: from /Users/kaspth/.rbenv/versions/2.6.3/bin/irb:23:in `load'
        3: from /Users/kaspth/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        2: from (irb):2
        1: from (irb):2:in `concat'
TypeError (no implicit conversion of Hash into Array)

Finally source_extract should have it's output argument removed. In fact, maybe we should just inline source_extract into annotated_source_code. I think we need to dig deeper here than just going for to_a and calling it a day — I'm very happy you're looking into this! 😊

[itsWill]

@kaspth good catch 👍

I've removed the output parameter and cleaned up the code some, so we return [] instead of nil so we no longer need the to_a.

This is a much cleaner solution ❤️

[kaspth]

❤️💪