ci(workflow): optimise CI runs using path filtering

Previously, the CI workflow ran all Rust checks (fmt, clippy, test, build)
on every push or pull request to the main branch, regardless of which
files were changed. This led to unnecessary resource consumption when
only documentation or CI configuration files were modified.

This change introduces path filtering using the `tj-actions/changed-files`
action. The workflow now identifies changes to Rust source files (`src/`,
`tests/`) or manifest files (`Cargo.toml`, `Cargo.lock`).

The `cargo fmt`, `cargo clippy`, `cargo test`, and `cargo build` steps
are now configured with `if` conditions to run only when relevant Rust
code or manifest files have been modified in the push or pull request.
This significantly speeds up CI runs for non-code changes.

Requires `pull-requests: read` permission for `tj-actions/changed-files`
and `fetch-depth: 0` during checkout to allow the action to compare
file changes accurately.

Author: Romelium

.github/workflows/ci.yml

@@ -8,6 +8,7 @@ on:
 
 permissions:
   contents: read
+  pull-requests: read
 
 env:
   CARGO_TERM_COLOR: always
@@ -23,25 +24,65 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
+        # Fetch depth 0 is needed for changed-files to correctly compare PR base/head or push history
+        with:
+          fetch-depth: 0
+
+      - name: Get changed files
+        id: changed_files
+        uses: tj-actions/changed-files@v44
+        with:
+          # Define file sets. Output will be like steps.changed_files.outputs.any_code_changed == 'true'
+          files: |
+            rust_code:
+              - src/**/*.rs
+              - tests/**/*.rs
+            manifest:
+              - Cargo.toml
+              - Cargo.lock
+            # Combined group for convenience
+            any_code:
+              - src/**/*.rs
+              - tests/**/*.rs
+              - Cargo.toml
+              - Cargo.lock
+            ci:
+              - .github/workflows/**
+              - .pre-commit-config.yaml
+            docs:
+              - README.md
+              - COMMIT.md
+              - LICENSE
 
       - name: Install Rust toolchain (stable)
         uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           toolchain: stable
           cache: 'cargo'
           components: clippy, rustfmt
+        # This step always runs, as caching makes it fast, and subsequent steps might need it.
 
       - name: Check Formatting (cargo fmt)
+        # Only run if Rust code or manifest files changed
+        if: steps.changed_files.outputs.any_code_changed == 'true'
         run: cargo fmt --all -- --check
 
       - name: Linting (cargo clippy)
+        if: steps.changed_files.outputs.any_code_changed == 'true'
         run: cargo clippy --all-targets -- -D warnings
 
       - name: Run Tests (cargo test)
+        if: steps.changed_files.outputs.any_code_changed == 'true'
         run: cargo test --all-targets --verbose
 
       - name: Build (Debug)
+        if: steps.changed_files.outputs.any_code_changed == 'true'
         run: cargo build --verbose
 
       - name: Build (Release)
+        if: steps.changed_files.outputs.any_code_changed == 'true'
         run: cargo build --release --verbose
+
+      - name: Indicate checks skipped (if applicable)
+        if: steps.changed_files.outputs.any_code_changed == 'false'
+        run: echo "Rust checks skipped as no relevant code changes were detected."