Extract version number from the source

"requiring version.rb" strategy has some issues.

- cannot work when cross-compiling
- often introduces wrong namespace
- must know the superclasses
- costs at each runtime than at build-time

etc.

lib/tracer/tracer.gemspec

@@ -1,13 +1,15 @@
-begin
-  require_relative "lib/tracer/version"
-rescue LoadError
-  # for Ruby core repository
-  require_relative "version"
+# frozen_string_literal: true
+
+name = File.basename(__FILE__, ".gemspec")
+version = ["lib", Array.new(name.count("-")+1, "..").join("/")].find do |dir|
+  break File.foreach(File.join(__dir__, dir, "#{name.tr('-', '/')}.rb")) do |line|
+    /^\s*VERSION\s*=\s*"(.*)"/ =~ line and break $1
+  end rescue nil
 end
 
 Gem::Specification.new do |spec|
-  spec.name          = "tracer"
-  spec.version       = Tracer::VERSION
+  spec.name          = name
+  spec.version       = version
   spec.authors       = ["Keiju ISHITSUKA"]
   spec.email         = ["keiju@ruby-lang.org"]
 

lib/tracer/version.rb

@@ -1,5 +0,0 @@
-# frozen_string_literal: true
-
-class Tracer
-  VERSION = "0.1.0"
-end