Add C++20 windows test but only run it on main ci, not on pr ci

Author: Wumpf

.github/workflows/cpp_matrix_full.json

@@ -5,25 +5,36 @@
             "runs_on": "ubuntu-latest-16-cores",
             "cache_key": "build-linux",
             "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=17 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
-            "additional_commands": "pixi run -e cpp cpp-docs"
+            "additional_commands": "pixi run -e cpp cpp-docs",
+            "pr_ci": true
         },
         {
-            "name": "Linux x64, C++20",
+            "name": "Linux x64, C++20 (skipped on PRs)",
             "runs_on": "ubuntu-latest-16-cores",
             "cache_key": "build-linux",
-            "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=20 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp"
+            "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=20 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
+            "pr_ci": false
         },
         {
-            "name": "Windows x64",
+            "name": "Windows x64, C++17",
             "runs_on": "windows-latest-8-cores",
             "cache_key": "build-windows",
-            "extra_env_vars": ""
+            "extra_env_vars": "RERUN_SET_CXX_VERSION=17",
+            "pr_ci": true
+        },
+        {
+            "name": "Windows x64, C++20 (skipped on PRs)",
+            "runs_on": "windows-latest-8-cores",
+            "cache_key": "build-windows",
+            "extra_env_vars": "RERUN_SET_CXX_VERSION=20",
+            "pr_ci": false
         },
         {
             "name": "Mac aarch64",
             "runs_on": "macos-15-large",
             "cache_key": "build-macos-arm64",
-            "extra_env_vars": "RERUN_USE_ASAN=1 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp"
+            "extra_env_vars": "RERUN_USE_ASAN=1 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
+            "pr_ci": true
         }
     ]
 }

.github/workflows/reusable_checks_cpp.yml

@@ -62,17 +62,23 @@ jobs:
       matrix: ${{ fromJson(needs.matrix_prep.outputs.MATRIX) }}
     runs-on: ${{ matrix.runs_on }}
     steps:
+      # Skipping the entire step would apparently require a separate job, not doing that here.
+      # Instead we keep checking for the `matrix.pr_ci` flag.
+      # See https://stackoverflow.com/questions/77186893/how-can-i-skip-the-whole-job-for-a-matrix-match-in-github-action
       - uses: actions/checkout@v4
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         with:
           ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.ref || '' }}
 
       - uses: prefix-dev/[email protected]
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         with:
           pixi-version: v0.41.4
           environments: cpp
 
       - name: Set up Rust
         uses: ./.github/actions/setup-rust
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         with:
           cache_key: ${{ matrix.cache_key }}
           # Cache will be produced by `reusable_checks/rs-lints`
@@ -82,26 +88,30 @@ jobs:
 
       # Workaround for ASAN issues on Github images https://github.com/actions/runner-images/issues/9491
       - name: Fix kernel mmap rnd bits
-        if: runner.os == 'Linux'
+        if: (${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}) && runner.os == 'Linux'
         # Asan in llvm 14 provided in ubuntu 22.04 is incompatible with
         # high-entropy ASLR in much newer kernels that GitHub runners are
         # using leading to random crashes: https://reviews.llvm.org/D148280
         run: sudo sysctl vm.mmap_rnd_bits=28
 
       - name: pixi run -e cpp cpp-clean
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         run: pixi run -e cpp cpp-clean
 
       - name: pixi run -e cpp cpp-build-all
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         run: ${{ matrix.extra_env_vars }} RERUN_WERROR=ON pixi run -e cpp cpp-build-all
 
       - name: pixi run -e cpp cpp-test
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         run: ${{ matrix.extra_env_vars }} RERUN_WERROR=ON pixi run -e cpp cpp-test
 
       - name: pixi run -e cpp cpp-build-all-shared-libs
         if: ${{ inputs.CHANNEL == 'nightly' }}
         run: ${{ matrix.extra_env_vars }} RERUN_WERROR=ON pixi run -e cpp cpp-build-all-shared-libs
 
       - name: additional_commands
+        if: ${{ github.event_name != 'pull_request' || matrix.pr_ci != false }}
         run: ${{ matrix.additional_commands }}
 
   cpp-formatting: