Add branch test coverage when available. Force track all files to prevent implicit file discovery missing files.

Author: robinetmiller

test/json/test_helper.rb

@@ -1,14 +1,29 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
 require 'coverage'
-Coverage.start
+
+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'