Support Bazel 9

Bazel 9 is now released. The only Bzlmod blocker for p4c is that BCR
`p4runtime 1.5.0` declares `bazel_compatibility < 9.0.0`. Override it
with the tip of the `bazel9-support` branch which lifts that restriction.
Update `.bazelversion` so the default Bazel used locally and in CI is 9.

Also add a Bazel version matrix to CI (testing 7.x and 9.x) and macOS
build fixes: use Command Line Tools clang to avoid requiring a full Xcode
install, and raise `macos_minimum_os` to 10.15 which is the minimum for
`std::filesystem` support in libc++.

Signed-off-by: Steffen Smolka <steffen.smolka@gmail.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: smolkaj

.bazelrc

@@ -9,3 +9,9 @@ build --host_cxxopt=-std=c++20
 # Force the use of Clang for all builds.
 build --action_env=CC=clang
 build --action_env=CXX=clang++
+
+# macOS: use Command Line Tools clang (avoids requiring a full Xcode install)
+# and raise the deployment target to 10.15, which is the minimum for
+# std::filesystem support in libc++.
+build --repo_env=CC=/usr/bin/clang
+build --macos_minimum_os=10.15

.bazelversion

@@ -1 +1 @@
-8.5.0
+9.0.0

.github/workflows/ci-bazel.yml

@@ -36,6 +36,13 @@ jobs:
   build_direct: # Build p4c directly.
     needs: format_bazel_files
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        # Test oldest and newest supported Bazel; assume versions in between also work.
+        bazel_version: [7.x, 9.x]
+      fail-fast: false
+    env:
+      USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
     steps:
       - uses: actions/checkout@v6
         with:
@@ -45,9 +52,9 @@ jobs:
         uses: actions/cache/restore@v5
         with:
           path: ~/.cache/bazel
-          key: ${{ runner.os }}-bazel-direct-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}
+          key: ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-direct-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}
           restore-keys: |
-            ${{ runner.os }}-bazel-direct-
+            ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-direct-
 
       - name: Save start time
         run: echo "START_TIME=$(date +%s)" >> "$GITHUB_ENV"
@@ -68,11 +75,18 @@ jobs:
         if: always() && (github.ref_name == 'main' || env.duration > 180)
         with:
           path: ~/.cache/bazel
-          key: ${{ runner.os }}-bazel-direct-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}-${{ github.run_id }}
+          key: ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-direct-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}-${{ github.run_id }}
 
   build_indirect:  # Build 3rd party Bazel project depending on p4c as a subproject.
     needs: format_bazel_files
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        # Test oldest and newest supported Bazel; assume versions in between also work.
+        bazel_version: [7.x, 9.x]
+      fail-fast: false
+    env:
+      USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
     steps:
       - uses: actions/checkout@v6
         with:
@@ -82,9 +96,9 @@ jobs:
         uses: actions/cache/restore@v5
         with:
           path: ~/.cache/bazel
-          key: ${{ runner.os }}-bazel-indirect-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}
+          key: ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-indirect-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}
           restore-keys: |
-            ${{ runner.os }}-bazel-indirect-
+            ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-indirect-
 
       - name: Save start time
         run: echo "START_TIME=$(date +%s)" >> "$GITHUB_ENV"
@@ -107,4 +121,4 @@ jobs:
         if: always() && (github.ref_name == 'main' || env.duration > 180)
         with:
           path: ~/.cache/bazel
-          key: ${{ runner.os }}-bazel-indirect-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}-${{ github.run_id }}
+          key: ${{ runner.os }}-bazel-${{ matrix.bazel_version }}-indirect-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}-${{ github.run_id }}

MODULE.bazel

@@ -27,6 +27,11 @@ bazel_dep(name = "rules_m4", version = "0.3")
 bazel_dep(name = "rules_proto", version = "7.1.0")
 bazel_dep(name = "z3", version = "4.15.2")
 bazel_dep(name = "p4runtime", version = "1.5.0")
+git_override(
+    module_name = "p4runtime",
+    commit = "0599472658102ceb6853444f2614a443bf8f9bd8",
+    remote = "https://github.com/p4lang/p4runtime.git",
+)
 
 p4c_repositories = use_extension("//bazel:repositories.bzl", "p4c_repositories")
 use_repo(