[CI] Test compiler backwards compatibility

Author: straight-shoota

.github/workflows/forward-compatibility.yml

@@ -1,4 +1,4 @@
-name: Forward Compatibility
+name: Compiler Compatibility
 
 on:
   push:
@@ -62,3 +62,59 @@ jobs:
 
       - name: Test
         run: ${{ matrix.flags }} bin/ci build
+
+  # Test against *each* supported Crystal version for backward compatibility.
+  #
+  # This workflow runs on a nightly schedule on `master`.
+  # It can also be dispatched manually when necessary.
+  #
+  # We run a basic version of this test on every commit which covers only the earliest and latest supported versions in `linux.yml#x86_64-gnu-test`.
+  x86_64-gnu-test-backward_compatibility:
+    env:
+      ARCH: x86_64
+      ARCH_CMD: linux64
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        crystal_bootstrap_version: [1.7.3, 1.8.2, 1.9.2, 1.10.1, 1.11.2, 1.12.2, 1.13.3, 1.14.1, 1.15.1, 1.16.2]
+        flags: [""]
+        include:
+          # libffi is only available starting from the 1.2.2 build images
+          - crystal_bootstrap_version: 1.1.1
+            flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.2.2
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.3.2
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.4.1
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.5.1
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.6.2
+            flags: "USE_PCRE1=true"
+    steps:
+      - name: Download Crystal source
+        uses: actions/checkout@v4
+
+      - name: Prepare System
+        run: bin/ci prepare_system
+
+      - name: Prepare Build
+        run: bin/ci prepare_build
+
+      - name: Build current compiler
+        run: ${{ matrix.flags }} bin/ci with_build_env 'make crystal'
+        env:
+          DOCKER_TEST_PREFIX: crystallang/crystal:1.16.2
+
+      - name: Checkout Crystal stdlib ${{ matrix.crystal_bootstrap_version }}
+        uses: actions/[email protected]
+        with:
+          ref: "${{ matrix.crystal_bootstrap_version }}"
+
+      - name: Run specs
+        run: bin/ci with_build_env 'make std_spec compiler_spec samples'
+
+      - name: Build compiler
+        run: bin/ci with_build_env 'make crystal'