Fix handling of double dashes -- in crystal eval command

[kojix2]

This is the main part of the pull request #15474

---

The purpose of this pull request is to pass arguments after a double dash to the program when using the eval command.

crystal eval 'puts ARGV' -- meow neigh

Expected behavior

["meow", "neigh"]

Actual behavior

syntax error in eval:1
Error: unexpected token: "meow"

Workaround

By using two double dashes, you can pass arguments to the program as expected.

crystal eval 'puts ARGV' -- -- meow neigh

The reason for this behavior is that the OptionParser stops at -- by default, and removes --.
You need to use unknown_args to distinguish between arguments that were before -- and arguments that were after --.

This issue was reported to the forum.

[straight-shoota]

Ah now the code fails because program_source is a closure.
In other situations like that we introduce a separate local variable (e.g. opt_program_source) as the closured var and then assign it to the actual variable after the block.

[kojix2]

I tried the following code, but it did not work. It is nighttime in Japan. Good night.

# Implementation of the `crystal eval` command

class Crystal::Command
  private def eval
    compiler = new_compiler
    program_source = ""
    opt_program_source = nil
    program_args = [] of String

    parse_with_crystal_opts do |opts|
      opts.banner = "Usage: crystal eval [options] [source]\n\nOptions:"
      setup_simple_compiler_options compiler, opts

      opts.unknown_args do |before_dash, after_dash|
        opt_program_source = before_dash.join " "
        program_args = after_dash
      end
    end
  
    if opt_program_source.nil?
      program_source = STDIN.gets_to_end
    else
      program_source = opt_program_source
    end
    sources = [Compiler::Source.new("eval", program_source)]

    output_filename = Crystal.temp_executable "eval"

    compiler.compile sources, output_filename
    execute output_filename, program_args, compiler
  end
end

In src/compiler/crystal/command/eval.cr:25:45

 25 | sources = [Compiler::Source.new("eval", program_source)]
                                              ^-------------
Error: expected argument #2 to 'Crystal::Compiler::Source.new' to be String, not (String | Nil)

Overloads are:
 - Crystal::Compiler::Source.new(filename : String, code : String)
make: *** [Makefile:247: .build/crystal] Error 1

[straight-shoota]

I pushed a commit with the change I had in mind.