Add coverage on all files

test/json/test_helper.rb

@@ -1,14 +1,5 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
-begin
-  require 'simplecov'
-rescue LoadError
-  # Don't fail Ruby's test suite
-else
-  SimpleCov.start
-end
-
-require 'json'
 require 'test/unit'
 
 if ENV["JSON_COMPACT"]
@@ -35,3 +26,46 @@
   require "core_assertions"
   Test::Unit::TestCase.include Test::Unit::CoreAssertions
 end
+
+# The built-in Coverage module (and therefore SimpleCov) need to be activated prior to any require statements.
+# But! SimpleCov requires 'json' before activating Coverage measurement, meaning it misses several files.
+#
+# The solution is to defer any JSON requires from SimpleCov, which works out because we require it ourselves before
+# SimpleCov actually uses it for anything.
+module JSONTestPatch
+  def require(name)
+    if name == 'json'
+      caller_path = caller_locations.first.path
+
+      return false if caller_path.match? %r(/simplecov/)
+    end
+
+    super(name)
+  end
+end
+Kernel.prepend JSONTestPatch
+
+begin
+  require 'simplecov'
+rescue LoadError
+  # Don't fail Ruby's test suite
+else
+  # Override default at_exit or else it will fire when the Rake task process ends early
+  SimpleCov.external_at_exit = true
+  Test::Unit.at_exit do
+    SimpleCov.at_exit_behavior
+  end
+
+  SimpleCov.start do
+    # Force SimpleCov to include all files in its report, avoiding accidental require-order misses
+    track_files 'lib/**/*.rb'
+
+    add_filter 'lib/json/truffle_ruby' unless RUBY_ENGINE == 'truffleruby'
+
+    enable_coverage :branch
+    primary_coverage :branch
+  end
+end
+
+# Require must be after SimpleCov is started for it to see the code
+require 'json'