Verify the declared MSRV in CI, and record the all-features floor

.github/workflows/ci.yml

@@ -15,6 +15,52 @@ permissions:
   contents: read
 
 jobs:
+  msrv:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      with:
+        persist-credentials: false
+    - name: Find the declared MSRV
+      shell: bash  # Without this, the shell here is `bash` but without `-o pipefail`.
+      run: |
+        msrv="$(
+          cargo metadata --format-version 1 --no-deps |
+            jq -er '
+              .packages[]
+              | select(.name == "prodash")
+              | .rust_version
+            '
+        )"
+
+        # Expand to x.y.0, since rustup would otherwise install the newest x.y.z.
+        case "$msrv" in
+          *.*.*) ;;
+          *) msrv="$msrv.0" ;;
+        esac
+
+        tee -a "$GITHUB_ENV" <<<"MSRV=$msrv"
+    - name: Install the MSRV toolchain
+      run: rustup toolchain install "$MSRV" --profile minimal --no-self-update
+    - name: Check the base crate on the declared MSRV
+      run: cargo "+$MSRV" check --locked --lib
+
+  all-features-floor:
+    runs-on: ubuntu-latest
+    env:
+      # Not the MSRV. `rust-version` covers the base crate, while optional features can
+      # need newer compilers through their optional dependencies. Hard-coded, so that
+      # a rise in this floor fails here.
+      ALL_FEATURES_FLOOR: "1.88.0"
+    steps:
+    - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      with:
+        persist-credentials: false
+    - name: Install the all-features floor toolchain
+      run: rustup toolchain install "$ALL_FEATURES_FLOOR" --profile minimal --no-self-update
+    - name: Check every feature at once
+      run: cargo "+$ALL_FEATURES_FLOOR" check --locked --all-features
+
   build-and-test-linux:
     runs-on: ubuntu-latest
     steps: