[CI] Use cargo-semver-checks to check compat in CI (google#351)

We run the checks in the matrix so that we can run once for each target
platform, although we don't yet actually make use of the target, so
we're just running the same job repeatedly for the native target (ie,
x86_64). I'm not yet sure how to run for different targets or whether
it's even supported (I've filed [1] to ask).

[1] obi1kenobi/cargo-semver-checks-action#54

.github/workflows/ci.yml

@@ -241,6 +241,28 @@ jobs:
       # `alloc` feature.
       if: matrix.features != '' && matrix.features != '--no-default-features'
 
+    # Check semver compatibility with the most recently-published version on
+    # crates.io. We do this in the matrix rather than in its own job so that it
+    # gets run on different targets. Some of our API is target-specific (e.g.,
+    # SIMD type impls), and so we need to run on each target.
+    #
+    # TODO(https://github.com/obi1kenobi/cargo-semver-checks-action/issues/54):
+    # Currently we don't actually do anything with `matrix.target`, so we're
+    # just duplicating work by running this job multiple times, each time
+    # targetting the host platform.
+    - name: Check semver compatibility
+      uses: obi1kenobi/cargo-semver-checks-action@v2
+      with:
+        # Don't semver check zerocopy-derive; as a proc macro, it doesn't have
+        # an API that cargo-semver-checks can understand.
+        package: zerocopy
+        feature-group: all-features
+        # TODO: Set this to the specific nightly we have pinned in CI. Not a big
+        # deal since this isn't affected by the trybuild stderr files, which is
+        # the reason we need to pin to a specific nightly.
+        rust-toolchain: nightly
+      if: matrix.crate == 'zerocopy' && matrix.features == '--all-features' && matrix.toolchain == 'nightly'
+
   check_fmt:
     runs-on: ubuntu-latest
     name: Check Rust formatting