Relax zerocopy-derive's MSRV policy

Fixes #1085
See also #1481

.github/workflows/ci.yml

@@ -120,6 +120,16 @@ jobs:
     - name: Populate cache
       uses: ./.github/actions/cache
 
+    # Ensure that Cargo resolves the minimum possible syn version so that if we
+    # accidentally make a change which depends upon features added in more
+    # recent versions of syn, we'll catch it in CI.
+    - name: Pin syn dependency
+      run: |
+        set -eo pipefail
+        # Override the exising `syn` dependency with one which requires an exact
+        # version.
+        cargo add -p zerocopy-derive 'syn@=2.0.31'
+
     - name: Configure environment variables
       run: |
         set -eo pipefail
@@ -438,28 +448,6 @@ jobs:
       - name: Check README.md
         run: ./ci/check_readme.sh
 
-  check_msrv:
-    needs: generate_cache
-    runs-on: ubuntu-latest
-    name: Check MSRVs match
-    steps:
-      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
-
-      - name: Populate cache
-        uses: ./.github/actions/cache
-
-      # Make sure that the MSRV in zerocopy's and zerocopy-derive's `Cargo.toml`
-      # files are the same. In CI, we test with a single MSRV (the one indicated
-      # in zerocopy's `Cargo.toml`), so it's important that:
-      # - zerocopy-derive's MSRV is not lower than zerocopy's (we don't test with
-      #   a lower MSRV in CI, so we couldn't guarantee that zerocopy-derive
-      #   actually built and ran on a lower MSRV)
-      # - zerocopy-derive's MSRV is not higher than zerocopy's (this would mean
-      #   that compiling zerocopy with the `derive` feature enabled would fail
-      #   on its own published MSRV)
-      - name: Check MSRVs match
-        run: ./ci/check_msrv.sh
-
   check_versions:
     needs: generate_cache
     runs-on: ubuntu-latest
@@ -487,6 +475,9 @@ jobs:
 
       - name: Download dependencies
         run: |
+          # See comment on "Pin syn dependency" job for why we do this.
+          cargo add -p zerocopy-derive 'syn@=2.0.31'
+
           # Ensure all dependencies are downloaded - both for our crates and for
           # tools we use in CI. We don't care about these tools succeeding for
           # two reasons: First, this entire job is best-effort since it's just a
@@ -559,7 +550,7 @@ jobs:
       # https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks
       if: failure()
       runs-on: ubuntu-latest
-      needs: [build_test, kani, check_fmt, check_readme, check_msrv, check_versions, generate_cache, check-all-toolchains-tested, check-job-dependencies, run-git-hooks]
+      needs: [build_test, kani, check_fmt, check_readme, check_versions, generate_cache, check-all-toolchains-tested, check-job-dependencies, run-git-hooks]
       steps:
         - name: Mark the job as failed
           run: exit 1

POLICIES.md

@@ -89,10 +89,21 @@ documented guarantees do not hold.
 
 ## MSRV
 
-Our minimum supported Rust version (MSRV) is encoded in our `Cargo.toml` file.
-We consider an increase in MSRV to be a semver-breaking change, and will only
-increase our MSRV during semver-breaking version changes (e.g., 0.1 -> 0.2, 1.0
--> 2.0, etc).
+<!-- Our policy used to be simply that MSRV was a breaking change in all
+circumstances. This implicitly relied on syn having the same MSRV policy, which
+it does not. See #1085 and #1088. -->
+
+Without the `derive` feature enabled, zerocopy's minimum supported Rust version
+(MSRV) is encoded the `package.rust-version` field in its `Cargo.toml` file. For
+zerocopy, we consider an increase in MSRV to be a semver-breaking change, and
+will only increase our MSRV during semver-breaking version changes (e.g., 0.1 ->
+0.2, 1.0 -> 2.0, etc).
+
+For zerocopy with the `derive` feature enabled, and for the zerocopy-derive
+crate, we inherit the MSRV of our sole external dependency, syn. As of this
+writing (2024-07-02), syn does *not* consider MSRV increases to be
+semver-breaking changes. Thus, using the `derive` feature may result in the
+effective MSRV increasing within a semver version train.
 
 ## Yanking
 

githooks/pre-push

@@ -17,7 +17,6 @@ echo "Running pre-push git hook: $0"
 ./ci/check_fmt.sh                              & FMT_PID=$!
 ./ci/check_all_toolchains_tested.sh >/dev/null & TOOLCHAINS_PID=$!
 ./ci/check_job_dependencies.sh      >/dev/null & JOB_DEPS_PID=$!
-./ci/check_msrv.sh                  >/dev/null & MSRV_PID=$!
 ./ci/check_readme.sh                >/dev/null & README_PID=$!
 ./ci/check_versions.sh              >/dev/null & VERSIONS_PID=$!
 
@@ -30,7 +29,6 @@ echo "Running pre-push git hook: $0"
 wait $FMT_PID
 wait $TOOLCHAINS_PID
 wait $JOB_DEPS_PID
-wait $MSRV_PID
 wait $README_PID
 wait $VERSIONS_PID
 

zerocopy-derive/Cargo.toml

@@ -14,7 +14,6 @@ authors = ["Joshua Liebow-Feeser <[email protected]>"]
 description = "Custom derive for traits from the zerocopy crate"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"
 repository = "https://github.com/google/zerocopy"
-rust-version = "1.56.0"
 
 # We prefer to include tests when publishing to crates.io so that Crater [1] can
 # detect regressions in our test suite. These two tests are excessively large,
@@ -29,10 +28,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0.1"
 quote = "1.0.10"
-# This pinned dependency is a temporary work-around for #1085. Per #1088, we
-# will not ship 0.8 until we've removed this work-around and replaced it with a
-# more permanent solution.
-syn = "=2.0.55"
+syn = "2.0.31"
 
 [dev-dependencies]
 # We don't use this directly, but trybuild does. On the MSRV toolchain, the