Fix avoid linking libpcre when unused

[kojix2]

As reported by galapagdos on the Crystal forum, crystal always links libpcre/libpcre2 even if you don not use regular expressions, and calls the function pcre2_config_8.

crystal build hello_world.cr
ldd hello_world # libpcre2 is linked even if regular expressions are not used

This occurs because begin is used in class_getter in the Regex::PCRE module (src/regex/pcre.cr).

I searched the Crystal code with grep command and could not find any other uses of begin in class_getters.
supplement

grep class_getter | grep begin

src/regex/pcre2.cr:  class_getter version_number : {Int32, Int32} = begin
src/regex/pcre.cr:  class_getter version_number : {Int32, Int32} = begin

According to crystal tool expand, the macro is expanded as below.

Before:

module Regex::PCRE2
  @@version_number : ::Tuple(Int32, Int32) = begin
    version = self.version
    dot = if __temp_98 = version.index('.')
            __temp_98
          else
            raise(RuntimeError.new("Invalid libpcre2 version"))
          end
    space = if __temp_105 = version.index(' ', dot)
              __temp_105
            else
              raise(RuntimeError.new("Invalid libpcre2 version"))
            end
    {(version.byte_slice(0, dot)).to_i, (version.byte_slice(dot + 1, (space - dot) - 1)).to_i(false)}
  end

  def self.version_number : ::Tuple(Int32, Int32)
    @@version_number
  end
end

After:

module Regex::PCRE2
  @@version_number : ::Tuple(Int32, Int32) | ::Nil

  def self.version_number : ::Tuple(Int32, Int32)
    if (value = @@version_number).nil?
      @@version_number = (begin
        version = self.version
        dot = (version.index('.')) || (raise(RuntimeError.new("Invalid libpcre2 version")))
        space = (version.index(' ', dot)) || (raise(RuntimeError.new("Invalid libpcre2 version")))
        {(version.byte_slice(0, dot)).to_i, (version.byte_slice(dot + 1, (space - dot) - 1)).to_i(strict: false)}
      end)
    else
      value
    end
  end
end

Could you please review if it's ok not to use begin here?
Thank you.

Resolves #5379

[crysbot]

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

https://forum.crystal-lang.org/t/issue-with-running-crystal-executable-on-another-macos-machine/7080/10

[ysbaddaden]

Good catch!

One difference is that instead of being allocated by the runtime when the application is starting (before concurrency & parallelism have started) it will now be lazily generated on demand. It's thus possible that it will be called from 2 threads at the same time. In this specific case, the worst that can happen is that they both generate & store the version number.

Another difference is that instead of being a Tuple(Int32, Int32) it's now a nilable type, and it must be checked every time.

Both aren't an issue for this specific case.