Helper macro for `NotImplementedError`

[HertzDevil]

The standard library and the compiler together raise NotImplementedError in 141 places, and their message is almost always the fully qualified type name followed by the method name. That calls for a refactor:

macro not_implemented!
  ::raise(::NotImplementedError.new(\{{ "#{@type}##{@def.name}" }}))
end

Then we can write things such as:

class Process::Status
  def exit_signal : Signal
    {% if flag?(:unix) %}
      Signal.from_value(signal_code)
    {% else %}
      not_implemented!
    {% end %}
  end
end

# on Windows:
$?.exit_signal # Unhandled exception: Not Implemented: Process::Status#exit_signal (NotImplementedError)

A more robust macro should at least also check for class methods and top-level defs when producing that exception message, such as WinError.value and the string_build_via_utf16 spec helper. (Yet other things, like file-private defs, #11764, and methods on Class itself are also possible.)

The above defines not_implemented! as a public, top-level macro, since third-party libraries may write platform-specific checks similar to the ones we have in the standard library. If we keep this internally it would be Crystal.not_implemented! instead.

[yxhuvud]

Related in many ways: #4749

[straight-shoota]

A more robust macro should at least also check for class methods and top-level defs when producing that exception message

I figure the compiler should probably provide a method to get the fully qualified name of a method. Basically exposing something like Def#short_reference, optionally with arguments maybe.

[crysbot]

This issue has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/rfc-annotating-methods-that-raise-notimplementederror/7692/1

[hugopl]

I already use a macro like that in projects of mine.