Add CI check for RI backward compatibility (ruby#1625)

Author: st0012

.github/workflows/ri-backward-compat.yml

@@ -0,0 +1,69 @@
+name: RI Backward Compatibility
+
+on:
+  push:
+    branches:
+      - '**'
+      - '!dependabot/**'
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  ri-backward-compat:
+    name: RI reads data generated by RDoc ${{ matrix.old_rdoc }}
+    strategy:
+      fail-fast: false
+      matrix:
+        old_rdoc: ['6.5.0', '6.9.0', '7.0.2']
+    runs-on: ubuntu-latest
+    env:
+      RI_DATA_DIR: /tmp/ri_data
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@09a7688d3b55cf0e976497ff046b70949eeaccfd # v1.288.0
+        with:
+          # Must use Ruby 3.x — on Ruby 4.0, Heading is always a Class while old
+          # RDoc versions serialized it as a Struct, so the test would always fail
+          # regardless of whether the current code is correct.
+          ruby-version: '3.4'
+      - name: Generate ri data with old RDoc
+        run: |
+          gem install rdoc -v ${{ matrix.old_rdoc }} --no-document
+          ruby -e '
+            gem "rdoc", "${{ matrix.old_rdoc }}"
+            require "rdoc/rdoc"
+            puts "Generating ri data with RDoc #{RDoc::VERSION}"
+            RDoc::RDoc.new.document(["--ri", "--op", ENV["RI_DATA_DIR"], "--quiet", "lib/"])
+          '
+      - name: Install current RDoc
+        run: |
+          gem build rdoc.gemspec
+          gem install rdoc-*.gem --no-document
+      - name: Verify current ri can read old data
+        run: |
+          ruby -e '
+            require "rdoc"
+            puts "Reading ri data with RDoc #{RDoc::VERSION}"
+
+            store = RDoc::Store.new(RDoc::Options.new, path: ENV["RI_DATA_DIR"], type: :extra)
+            store.load_cache
+
+            modules = store.module_names
+            errors = []
+
+            modules.each do |mod_name|
+              store.load_class(mod_name)
+            rescue => e
+              errors << [mod_name, e]
+              warn "FAIL: #{mod_name} - #{e.class}: #{e.message}"
+            end
+
+            if errors.empty?
+              puts "All #{modules.size} modules loaded successfully"
+            else
+              abort "#{errors.size} of #{modules.size} modules failed to load"
+            end
+          '