CI: Add incremental mutation testing.

Note that this only runs on pull requests, because it needs to know a
base revision for the diff.

Also improve `cargo install` usage. We don't need to cache the builds
because `Swatinem/rust-cache` caches the binaries.

.github/workflows/ci.yml

@@ -138,17 +138,12 @@ jobs:
 
     - run: df -h .
 
-    - name: Install wasm-pack
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
+    - name: Install tools
+      # Note: Swatinem/rust-cache will cache these installed binaries, so we don't have to worry
+      # about caching the builds of them.
+      run: |
+        cargo install [email protected] [email protected] [email protected]
 
-    - name: Install cargo-about
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
 
     - run: df -h .
 
@@ -265,17 +260,10 @@ jobs:
     - name: Compile xtask
       run: cargo build --package xtask
 
-    - name: Install wasm-pack
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
-
-    - name: Install cargo-about
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
+    - name: Install tools
+      # These should already be installed but we might have a cache miss, so don't fail in that case.
+      run: |
+        cargo install [email protected] [email protected]
 
     - run: df -h .
 
@@ -294,7 +282,50 @@ jobs:
         name: cargo-timings features
         path: |
           target/cargo-timings/cargo-timing-*.html
+    
+  # Run mutation testing against the changed code
+  # Based on example from https://mutants.rs/pr-diff.html
+  incremental-mutants:
+    runs-on: ubuntu-latest
+    # Running on push would require more work to determine the base revision
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # Needed to ensure `main` branch is needed
+          fetch-depth: 0
+
+      # Load cache before doing any Rust builds
+      # Note: we aren't depending on the build job so this cache might be stale, but the
+      # shorter time-to-useful-results seems worth it.
+      - uses: Swatinem/[email protected]
+        with:
+          # Reuse the cache from the normal `build` job instead of creating an independent one,
+          # to reduce redundant work and save disk space — but don't *write* to that cache, so
+          # we don't bloat it.
+          prefix-key: "v1-rust-ubuntu-stable-locked"
+          shared-key: "build"
+          save-if: false
+
+      # Install cargo-mutants
+      # This should usually be already installed, but is present in case of cache miss
+      - run: cargo install --version 24.1.0 cargo-mutants
+
+      - name: Relative diff
+        run: |
+          git branch -av
+          git diff origin/${{ github.base_ref }}.. | tee git.diff
       
+      - name: Mutants
+        # See mutants.yml for notes on the exclusions
+        run: cargo mutants --in-diff git.diff --package all-is-cubes --exclude-re '::fmt ->' --exclude tests.rs -- --all-features
+
+      - uses: actions/upload-artifact@v3
+        if: always()
+        with:
+          name: mutants-incremental.out
+          path: mutants.out
+
   miri:
     runs-on: ubuntu-latest
 

.github/workflows/mutants.yml

@@ -1,3 +1,5 @@
+# Note that incremental mutation testing is done in ci.yml
+
 name: Mutation Testing
 
 permissions: {}
@@ -24,9 +26,8 @@ jobs:
 
       - name: Run mutation testing for all-is-cubes
         timeout-minutes: 60
-        # TODO: un-exclude src/vui eventually
         # TODO: maybe un-exclude fmt()
-        run: cargo mutants --package all-is-cubes --exclude-re '::fmt ->' --exclude tests.rs --exclude src/vui* -- --all-features
+        run: cargo mutants --package all-is-cubes --exclude-re '::fmt ->' --exclude tests.rs -- --all-features
         # TODO: Other crates' reasons for non-inclusion:
         #   all-is-cubes-content: insufficient test coverage
         #   all-is-cubes-desktop: insufficient test coverage, unclear if even feasible

.github/workflows/unstable-ci.yml

@@ -95,17 +95,11 @@ jobs:
         cargo xtask update "${{ matrix.depversions }}"
         cargo tree --all-features
 
-    - name: Install wasm-pack
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
-
-    - name: Install cargo-about
-      run: cargo install [email protected]
-      # Use workspace target dir for cargo install's build, so that the build will be cached.
-      env:
-        CARGO_TARGET_DIR: target/
+    - name: Install tools
+      # Note: Swatinem/rust-cache will cache these installed binaries, so we don't have to worry
+      # about caching the builds of them.
+      run: |
+        cargo install [email protected] [email protected]
 
     - name: Compile basic tests
       # compile is broken out so we have visibility into compile vs. run times