Branch coverage & Explicit File Inclusion

Rakefile

@@ -6,6 +6,14 @@ include RbConfig
 require 'rake/testtask'
 class UndocumentedTestTask < Rake::TestTask
   def desc(*) end
+
+  def ruby(...)
+    env_was = ENV["JSON_COVERAGE"]
+    ENV["JSON_COVERAGE"] = "1"
+    ret = super
+    ENV["JSON_COVERAGE"] = env_was
+    ret
+  end
 end
 
 PKG_VERSION       = File.foreach(File.join(__dir__, "lib/json/version.rb")) do |line|

test/json/test_helper.rb

@@ -1,14 +1,27 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
-require 'coverage'
-Coverage.start
+if ENV["JSON_COVERAGE"]
+  # This test helper is loaded inside Ruby's own test suite, so we try to not mess it up.
+  require 'coverage'
+
+  branches_supported = Coverage.respond_to?(:supported?) && Coverage.supported?(:branches)
+
+  # Coverage module must be started before SimpleCov to work around the cyclic require order.
+  # Track both branches and lines, or else SimpleCov misleadingly reports 0/0 = 100% for non-branching files.
+  Coverage.start(lines:    true,
+                 branches: branches_supported)
 
-begin
   require 'simplecov'
-rescue LoadError
-  # Don't fail Ruby's test suite
-else
-  SimpleCov.start
+  SimpleCov.start do
+    # Enabling both coverage types to let SimpleCov know to output them together in reports
+    enable_coverage :line
+    enable_coverage :branch if branches_supported
+
+    # Can't always trust SimpleCov to find files implicitly
+    track_files 'lib/**/*.rb'
+
+    add_filter 'lib/json/truffle_ruby' unless RUBY_ENGINE == 'truffleruby'
+  end
 end
 
 require 'json'