ci: add cargo-public-api check for breaking API changes (#924)

* chore: bump Rust toolchain to 1.96

* ci: add cargo-public-api breaking-change check

Adds a release-type-aware `cargo-public-api` diff job that fails on
changed/removed public items unless the PR's commits mark a breaking
(major) release. This catches source-breaking API changes that
cargo-semver-checks cannot yet detect (e.g. function return-type or
field-type changes).

.github/workflows/ci.yml

@@ -112,6 +112,77 @@ jobs:
             --only-explicit-features \
             --features "$FEATURES"
 
+  public-api:
+    name: Public API Check
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      # cargo-public-api builds rustdoc JSON, which requires a nightly toolchain
+      # to be installed (it does not need to be the default; the tool invokes it
+      # via `cargo +nightly`).
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@nightly
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Install cargo-public-api
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-public-api
+
+      # Mirror the SemVer Check job's release-type detection: a breaking-change
+      # commit marker (`!:` or `BREAKING CHANGE:`) means a major release (any API
+      # change is allowed); otherwise a minor release (additions allowed, but
+      # changed/removed public items are denied). This catches breaking changes
+      # that cargo-semver-checks cannot yet detect, such as a change to a
+      # function's return type or a field's type.
+      # See https://github.com/obi1kenobi/cargo-semver-checks/issues/5
+      - name: Determine release type and deny flags
+        run: |
+          if git log --format=%B \
+            ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }} \
+            | grep -Eq '(^[A-Za-z0-9_-]+(\([^)]*\))?!:|^BREAKING[ -]CHANGE:)'; then
+            SEMVER_RELEASE_TYPE=major
+          else
+            SEMVER_RELEASE_TYPE=minor
+          fi
+          case "$SEMVER_RELEASE_TYPE" in
+            major) DENY="" ;;
+            patch) DENY="--deny added --deny changed --deny removed" ;;
+            *)     DENY="--deny changed --deny removed" ;;
+          esac
+          echo "SEMVER_RELEASE_TYPE=$SEMVER_RELEASE_TYPE" >> "$GITHUB_ENV"
+          echo "DENY=$DENY" >> "$GITHUB_ENV"
+
+      - name: Check rmcp (default features)
+        run: |
+          cargo public-api \
+            --package rmcp \
+            -ss \
+            diff \
+            $DENY \
+            --force \
+            ${{ github.event.pull_request.base.sha }}..${{ github.sha }}
+
+      - name: Check rmcp (all features except local)
+        run: |
+          FEATURES=$(cargo metadata --no-deps --format-version 1 \
+            | jq -r '[.packages[] | select(.name == "rmcp") | .features | keys[]
+                      | select(startswith("__") | not)
+                      | select(. != "local")] | join(",")')
+          cargo public-api \
+            --package rmcp \
+            --features "$FEATURES" \
+            -ss \
+            diff \
+            $DENY \
+            --force \
+            ${{ github.event.pull_request.base.sha }}..${{ github.sha }}
+
   spelling:
     name: spell check with typos
     runs-on: ubuntu-latest

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "1.92"
+channel = "1.96"
 components = ["rustc", "rust-std", "cargo", "clippy", "rustfmt", "rust-docs"]