only perform coverage checks on ruby 3.3

This allows us to check coverage on lines that only run on modern
rubies, rather than needing to :nocov: them so older rubies can pass the
test suite. It also makes it easy to keep track of and change which
ruby's coverage is being _used_ in CI, since it's just being specified
in an environment variable on the matrix.

(Also mark the legacy builds as NO_COVERAGE: true, for clarity)

Author: nevinera

.github/workflows/ci.yml

@@ -71,7 +71,9 @@ jobs:
               DIFF_LCS_VERSION: "1.4.3"
       fail-fast: false
     continue-on-error: ${{ matrix.allow_failure || endsWith(matrix.ruby, 'head') }}
-    env: ${{ matrix.env }}
+    env:
+      COVERAGE_ONLY_ON: "3.3"
+      ${{ matrix.env }}
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
@@ -129,6 +131,7 @@ jobs:
             options: "--add-host rubygems.org:151.101.129.227 --add-host api.rubygems.org:151.101.129.227"
     env:
       LEGACY_CI: true
+      NO_COVERAGE: true
       JRUBY_OPTS: ${{ matrix.container.jruby_opts || '--dev' }}
     steps:
       - uses: actions/checkout@v3

script/rspec_with_simplecov

@@ -24,7 +24,11 @@ begin
   # Simplecov emits some ruby warnings when loaded, so silence them.
   old_verbose, $VERBOSE = $VERBOSE, false
 
-  unless ENV['NO_COVERAGE'] || RUBY_VERSION.to_f < 2.1
+  skip_because_no_coverage = ENV['NO_COVERAGE']
+  skip_because_ruby_too_low = RUBY_VERSION.to_f < 2.1
+  skip_because_wrong_version = ENV['COVERAGE_ONLY_ON'] && ENV['COVERAGE_ONLY_ON'].to_f != RUBY_VERSION.to_f
+
+  unless skip_because_no_coverage || skip_because_ruby_too_low || skip_because_wrong_version
     require 'simplecov'
 
     SimpleCov.start do