1 files changed, 25 insertions, 0 deletions
diff --git a/test/json/test_helper.rb b/test/json/test_helper.rb
index d849e28b9b..4c5a91a192 100644
--- a/test/json/test_helper.rb
+++ b/test/json/test_helper.rb
@@ -1,5 +1,30 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
+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)
+
+  require 'simplecov'
+  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'
+    add_filter 'test/'
+  end
+end
+
 require 'json'
 require 'test/unit'